Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

77 files changed, 27346 insertions, 0 deletions

diff --git a/gfx/layers/apz/src/APZCTreeManager.cpp b/gfx/layers/apz/src/APZCTreeManager.cpp
new file mode 100644
index 0000000000..d28392b716
--- /dev/null
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -0,0 +1,3827 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <stack>
+#include <unordered_set>
+#include "APZCTreeManager.h"
+#include "AsyncPanZoomController.h"
+#include "Compositor.h"             // for Compositor
+#include "DragTracker.h"            // for DragTracker
+#include "GenericFlingAnimation.h"  // for FLING_LOG
+#include "HitTestingTreeNode.h"     // for HitTestingTreeNode
+#include "InputBlockState.h"        // for InputBlockState
+#include "InputData.h"              // for InputData, etc
+#include "WRHitTester.h"            // for WRHitTester
+#include "apz/src/APZUtils.h"
+#include "mozilla/RecursiveMutex.h"
+#include "mozilla/dom/MouseEventBinding.h"  // for MouseEvent constants
+#include "mozilla/dom/BrowserParent.h"      // for AreRecordReplayTabsActive
+#include "mozilla/dom/Touch.h"              // for Touch
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/gfx/LoggingConstants.h"
+#include "mozilla/gfx/Matrix.h"
+#include "mozilla/gfx/gfxVars.h"            // for gfxVars
+#include "mozilla/gfx/GPUParent.h"          // for GPUParent
+#include "mozilla/gfx/Logging.h"            // for gfx::TreeLog
+#include "mozilla/gfx/Point.h"              // for Point
+#include "mozilla/layers/APZSampler.h"      // for APZSampler
+#include "mozilla/layers/APZThreadUtils.h"  // for AssertOnControllerThread, etc
+#include "mozilla/layers/APZUpdater.h"      // for APZUpdater
+#include "mozilla/layers/APZUtils.h"        // for AsyncTransform
+#include "mozilla/layers/AsyncDragMetrics.h"        // for AsyncDragMetrics
+#include "mozilla/layers/CompositorBridgeParent.h"  // for CompositorBridgeParent, etc
+#include "mozilla/layers/DoubleTapToZoom.h"         // for ZoomTarget
+#include "mozilla/layers/MatrixMessage.h"
+#include "mozilla/layers/ScrollableLayerGuid.h"
+#include "mozilla/layers/UiCompositorControllerParent.h"
+#include "mozilla/layers/WebRenderScrollDataWrapper.h"
+#include "mozilla/MouseEvents.h"
+#include "mozilla/mozalloc.h"     // for operator new
+#include "mozilla/Preferences.h"  // for Preferences
+#include "mozilla/StaticPrefs_accessibility.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/ToString.h"
+#include "mozilla/TouchEvents.h"
+#include "mozilla/EventStateManager.h"  // for WheelPrefs
+#include "mozilla/webrender/WebRenderAPI.h"
+#include "mozilla/webrender/WebRenderTypes.h"
+#include "nsDebug.h"                 // for NS_WARNING
+#include "nsPoint.h"                 // for nsIntPoint
+#include "nsThreadUtils.h"           // for NS_IsMainThread
+#include "ScrollThumbUtils.h"        // for ComputeTransformForScrollThumb
+#include "OverscrollHandoffState.h"  // for OverscrollHandoffState
+#include "TreeTraversal.h"           // for ForEachNode, BreadthFirstSearch, etc
+#include "Units.h"                   // for ParentlayerPixel
+#include "GestureEventListener.h"  // for GestureEventListener::setLongTapEnabled
+#include "UnitTransforms.h"        // for ViewAs
+
+mozilla::LazyLogModule mozilla::layers::APZCTreeManager::sLog("apz.manager");
+#define APZCTM_LOG(...) \
+  MOZ_LOG(APZCTreeManager::sLog, LogLevel::Debug, (__VA_ARGS__))
+
+static mozilla::LazyLogModule sApzKeyLog("apz.key");
+#define APZ_KEY_LOG(...) MOZ_LOG(sApzKeyLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+using mozilla::gfx::CompositorHitTestFlags;
+using mozilla::gfx::CompositorHitTestInfo;
+using mozilla::gfx::CompositorHitTestInvisibleToHit;
+using mozilla::gfx::LOG_DEFAULT;
+
+typedef mozilla::gfx::Point Point;
+typedef mozilla::gfx::Point4D Point4D;
+typedef mozilla::gfx::Matrix4x4 Matrix4x4;
+
+typedef CompositorBridgeParent::LayerTreeState LayerTreeState;
+
+struct APZCTreeManager::TreeBuildingState {
+  TreeBuildingState(LayersId aRootLayersId, bool aIsFirstPaint,
+                    LayersId aOriginatingLayersId, APZTestData* aTestData,
+                    uint32_t aPaintSequence)
+      : mIsFirstPaint(aIsFirstPaint),
+        mOriginatingLayersId(aOriginatingLayersId),
+        mPaintLogger(aTestData, aPaintSequence) {
+    CompositorBridgeParent::CallWithIndirectShadowTree(
+        aRootLayersId, [this](LayerTreeState& aState) -> void {
+          mCompositorController = aState.GetCompositorController();
+        });
+  }
+
+  typedef std::unordered_map<AsyncPanZoomController*, gfx::Matrix4x4>
+      DeferredTransformMap;
+
+  // State that doesn't change as we recurse in the tree building
+  RefPtr<CompositorController> mCompositorController;
+  const bool mIsFirstPaint;
+  const LayersId mOriginatingLayersId;
+  const APZPaintLogHelper mPaintLogger;
+
+  // State that is updated as we perform the tree build
+
+  // A list of nodes that need to be destroyed at the end of the tree building.
+  // This is initialized with all nodes in the old tree, and nodes are removed
+  // from it as we reuse them in the new tree.
+  nsTArray<RefPtr<HitTestingTreeNode>> mNodesToDestroy;
+
+  // This map is populated as we place APZCs into the new tree. Its purpose is
+  // to facilitate re-using the same APZC for different layers that scroll
+  // together (and thus have the same ScrollableLayerGuid). The presShellId
+  // doesn't matter for this purpose, and we move the map to the APZCTreeManager
+  // after we're done building, so it's useful to have the presshell-ignoring
+  // map for that.
+  std::unordered_map<ScrollableLayerGuid, ApzcMapData,
+                     ScrollableLayerGuid::HashIgnoringPresShellFn,
+                     ScrollableLayerGuid::EqualIgnoringPresShellFn>
+      mApzcMap;
+
+  // This is populated with all the HitTestingTreeNodes that are scroll thumbs
+  // and have a scrollthumb animation id (which indicates that they need to be
+  // sampled for WebRender on the sampler thread).
+  std::vector<HitTestingTreeNode*> mScrollThumbs;
+  // This is populated with all the scroll target nodes. We use in conjunction
+  // with mScrollThumbs to build APZCTreeManager::mScrollThumbInfo.
+  std::unordered_map<ScrollableLayerGuid, HitTestingTreeNode*,
+                     ScrollableLayerGuid::HashIgnoringPresShellFn,
+                     ScrollableLayerGuid::EqualIgnoringPresShellFn>
+      mScrollTargets;
+
+  // During the tree building process, the perspective transform component
+  // of the ancestor transforms of some APZCs can be "deferred" to their
+  // children, meaning they are added to the children's ancestor transforms
+  // instead. Those deferred transforms are tracked here.
+  DeferredTransformMap mPerspectiveTransformsDeferredToChildren;
+
+  // As we recurse down through the tree, this picks up the zoom animation id
+  // from a node in the layer tree, and propagates it downwards to the nearest
+  // APZC instance that is for an RCD node. Generally it will be set on the
+  // root node of the layers (sub-)tree, which may not be same as the RCD node
+  // for the subtree, and so we need this mechanism to ensure it gets propagated
+  // to the RCD's APZC instance. Once it is set on the APZC instance, the value
+  // is cleared back to Nothing(). Note that this is only used in the WebRender
+  // codepath.
+  Maybe<uint64_t> mZoomAnimationId;
+
+  // See corresponding members of APZCTreeManager. These are the same thing, but
+  // on the tree-walking state. They are populated while walking the tree in
+  // a layers update, and then moved into APZCTreeManager.
+  std::vector<FixedPositionInfo> mFixedPositionInfo;
+  std::vector<RootScrollbarInfo> mRootScrollbarInfo;
+  std::vector<StickyPositionInfo> mStickyPositionInfo;
+
+  // As we recurse down through reflayers in the tree, this picks up the
+  // cumulative EventRegionsOverride flags from the reflayers, and is used to
+  // apply them to descendant layers.
+  std::stack<EventRegionsOverride> mOverrideFlags;
+};
+
+class APZCTreeManager::CheckerboardFlushObserver : public nsIObserver {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+
+  explicit CheckerboardFlushObserver(APZCTreeManager* aTreeManager)
+      : mTreeManager(aTreeManager) {
+    MOZ_ASSERT(NS_IsMainThread());
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    MOZ_ASSERT(obsSvc);
+    if (obsSvc) {
+      obsSvc->AddObserver(this, "APZ:FlushActiveCheckerboard", false);
+    }
+  }
+
+  void Unregister() {
+    MOZ_ASSERT(NS_IsMainThread());
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    if (obsSvc) {
+      obsSvc->RemoveObserver(this, "APZ:FlushActiveCheckerboard");
+    }
+    mTreeManager = nullptr;
+  }
+
+ protected:
+  virtual ~CheckerboardFlushObserver() = default;
+
+ private:
+  RefPtr<APZCTreeManager> mTreeManager;
+};
+
+NS_IMPL_ISUPPORTS(APZCTreeManager::CheckerboardFlushObserver, nsIObserver)
+
+NS_IMETHODIMP
+APZCTreeManager::CheckerboardFlushObserver::Observe(nsISupports* aSubject,
+                                                    const char* aTopic,
+                                                    const char16_t*) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mTreeManager.get());
+
+  RecursiveMutexAutoLock lock(mTreeManager->mTreeLock);
+  if (mTreeManager->mRootNode) {
+    ForEachNode<ReverseIterator>(
+        mTreeManager->mRootNode.get(), [](HitTestingTreeNode* aNode) {
+          if (aNode->IsPrimaryHolder()) {
+            MOZ_ASSERT(aNode->GetApzc());
+            aNode->GetApzc()->FlushActiveCheckerboardReport();
+          }
+        });
+  }
+  if (XRE_IsGPUProcess()) {
+    if (gfx::GPUParent* gpu = gfx::GPUParent::GetSingleton()) {
+      nsCString topic("APZ:FlushActiveCheckerboard:Done");
+      Unused << gpu->SendNotifyUiObservers(topic);
+    }
+  } else {
+    MOZ_ASSERT(XRE_IsParentProcess());
+    nsCOMPtr<nsIObserverService> obsSvc =
+        mozilla::services::GetObserverService();
+    if (obsSvc) {
+      obsSvc->NotifyObservers(nullptr, "APZ:FlushActiveCheckerboard:Done",
+                              nullptr);
+    }
+  }
+  return NS_OK;
+}
+
+/**
+ * A RAII class used for setting the focus sequence number on input events
+ * as they are being processed. Any input event is assumed to be potentially
+ * focus changing unless explicitly marked otherwise.
+ */
+class MOZ_RAII AutoFocusSequenceNumberSetter {
+ public:
+  AutoFocusSequenceNumberSetter(FocusState& aFocusState, InputData& aEvent)
+      : mFocusState(aFocusState), mEvent(aEvent), mMayChangeFocus(true) {}
+
+  void MarkAsNonFocusChanging() { mMayChangeFocus = false; }
+
+  ~AutoFocusSequenceNumberSetter() {
+    if (mMayChangeFocus) {
+      mFocusState.ReceiveFocusChangingEvent();
+
+      APZ_KEY_LOG(
+          "Marking input with type=%d as focus changing with seq=%" PRIu64 "\n",
+          static_cast<int>(mEvent.mInputType),
+          mFocusState.LastAPZProcessedEvent());
+    } else {
+      APZ_KEY_LOG(
+          "Marking input with type=%d as non focus changing with seq=%" PRIu64
+          "\n",
+          static_cast<int>(mEvent.mInputType),
+          mFocusState.LastAPZProcessedEvent());
+    }
+
+    mEvent.mFocusSequenceNumber = mFocusState.LastAPZProcessedEvent();
+  }
+
+ private:
+  FocusState& mFocusState;
+  InputData& mEvent;
+  bool mMayChangeFocus;
+};
+
+APZCTreeManager::APZCTreeManager(LayersId aRootLayersId,
+                                 UniquePtr<IAPZHitTester> aHitTester)
+    : mTestSampleTime(Nothing(), "APZCTreeManager::mTestSampleTime"),
+      mInputQueue(new InputQueue()),
+      mMapLock("APZCMapLock"),
+      mRootLayersId(aRootLayersId),
+      mSampler(nullptr),
+      mUpdater(nullptr),
+      mTreeLock("APZCTreeLock"),
+      mRetainedTouchIdentifier(-1),
+      mInScrollbarTouchDrag(false),
+      mCurrentMousePosition(ScreenPoint(),
+                            "APZCTreeManager::mCurrentMousePosition"),
+      mApzcTreeLog("apzctree"),
+      mTestDataLock("APZTestDataLock"),
+      mDPI(160.0),
+      mHitTester(std::move(aHitTester)),
+      mScrollGenerationLock("APZScrollGenerationLock") {
+  AsyncPanZoomController::InitializeGlobalState();
+  mApzcTreeLog.ConditionOnPrefFunction(StaticPrefs::apz_printtree);
+
+  if (!mHitTester) {
+    mHitTester = MakeUnique<WRHitTester>();
+  }
+  mHitTester->Initialize(this);
+}
+
+APZCTreeManager::~APZCTreeManager() = default;
+
+void APZCTreeManager::Init() {
+  RefPtr<APZCTreeManager> self(this);
+  NS_DispatchToMainThread(NS_NewRunnableFunction(
+      "layers::APZCTreeManager::Init",
+      [self] { self->mFlushObserver = new CheckerboardFlushObserver(self); }));
+}
+
+already_AddRefed<APZCTreeManager> APZCTreeManager::Create(
+    LayersId aRootLayersId, UniquePtr<IAPZHitTester> aHitTester) {
+  RefPtr<APZCTreeManager> manager =
+      new APZCTreeManager(aRootLayersId, std::move(aHitTester));
+  manager->Init();
+  return manager.forget();
+}
+
+void APZCTreeManager::SetSampler(APZSampler* aSampler) {
+  // We're either setting the sampler or clearing it
+  MOZ_ASSERT((mSampler == nullptr) != (aSampler == nullptr));
+  mSampler = aSampler;
+}
+
+void APZCTreeManager::SetUpdater(APZUpdater* aUpdater) {
+  // We're either setting the updater or clearing it
+  MOZ_ASSERT((mUpdater == nullptr) != (aUpdater == nullptr));
+  mUpdater = aUpdater;
+}
+
+void APZCTreeManager::NotifyLayerTreeAdopted(
+    LayersId aLayersId, const RefPtr<APZCTreeManager>& aOldApzcTreeManager) {
+  AssertOnUpdaterThread();
+
+  if (aOldApzcTreeManager) {
+    aOldApzcTreeManager->mFocusState.RemoveFocusTarget(aLayersId);
+    // While we could move the focus target information from the old APZC tree
+    // manager into this one, it's safer to not do that, as we'll probably have
+    // that information repopulated soon anyway (on the next layers update).
+  }
+
+  // There may be focus updates from the tab's content process in flight
+  // triggered by events that were processed by the old tree manager,
+  // which this tree manager does not expect.
+  // Resetting the focus state avoids this (the state will sync with content
+  // on the next focus update).
+  mFocusState.Reset();
+
+  UniquePtr<APZTestData> adoptedData;
+  if (aOldApzcTreeManager) {
+    MutexAutoLock lock(aOldApzcTreeManager->mTestDataLock);
+    auto it = aOldApzcTreeManager->mTestData.find(aLayersId);
+    if (it != aOldApzcTreeManager->mTestData.end()) {
+      adoptedData = std::move(it->second);
+      aOldApzcTreeManager->mTestData.erase(it);
+    }
+  }
+  if (adoptedData) {
+    MutexAutoLock lock(mTestDataLock);
+    mTestData[aLayersId] = std::move(adoptedData);
+  }
+}
+
+void APZCTreeManager::NotifyLayerTreeRemoved(LayersId aLayersId) {
+  AssertOnUpdaterThread();
+
+  mFocusState.RemoveFocusTarget(aLayersId);
+
+  {  // scope lock
+    MutexAutoLock lock(mTestDataLock);
+    mTestData.erase(aLayersId);
+  }
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::NewAPZCInstance(
+    LayersId aLayersId, GeckoContentController* aController) {
+  return MakeRefPtr<AsyncPanZoomController>(
+             aLayersId, this, mInputQueue, aController,
+             AsyncPanZoomController::USE_GESTURE_DETECTOR)
+      .forget();
+}
+
+void APZCTreeManager::SetTestSampleTime(const Maybe<TimeStamp>& aTime) {
+  auto testSampleTime = mTestSampleTime.Lock();
+  testSampleTime.ref() = aTime;
+}
+
+SampleTime APZCTreeManager::GetFrameTime() {
+  auto testSampleTime = mTestSampleTime.Lock();
+  if (testSampleTime.ref()) {
+    return SampleTime::FromTest(*testSampleTime.ref());
+  }
+  return SampleTime::FromNow();
+}
+
+void APZCTreeManager::SetAllowedTouchBehavior(
+    uint64_t aInputBlockId, const nsTArray<TouchBehaviorFlags>& aValues) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<uint64_t,
+                          StoreCopyPassByLRef<nsTArray<TouchBehaviorFlags>>>(
+            "layers::APZCTreeManager::SetAllowedTouchBehavior", this,
+            &APZCTreeManager::SetAllowedTouchBehavior, aInputBlockId,
+            aValues.Clone()));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+
+  mInputQueue->SetAllowedTouchBehavior(aInputBlockId, aValues);
+}
+
+void APZCTreeManager::SetBrowserGestureResponse(
+    uint64_t aInputBlockId, BrowserGestureResponse aResponse) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<uint64_t, BrowserGestureResponse>(
+            "layers::APZCTreeManager::SetBrowserGestureResponse", this,
+            &APZCTreeManager::SetBrowserGestureResponse, aInputBlockId,
+            aResponse));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+
+  mInputQueue->SetBrowserGestureResponse(aInputBlockId, aResponse);
+}
+
+void APZCTreeManager::UpdateHitTestingTree(
+    const WebRenderScrollDataWrapper& aRoot, bool aIsFirstPaint,
+    LayersId aOriginatingLayersId, uint32_t aPaintSequenceNumber) {
+  AssertOnUpdaterThread();
+
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  // For testing purposes, we log some data to the APZTestData associated with
+  // the layers id that originated this update.
+  APZTestData* testData = nullptr;
+  if (StaticPrefs::apz_test_logging_enabled()) {
+    MutexAutoLock lock(mTestDataLock);
+    UniquePtr<APZTestData> ptr = MakeUnique<APZTestData>();
+    auto result =
+        mTestData.insert(std::make_pair(aOriginatingLayersId, std::move(ptr)));
+    testData = result.first->second.get();
+    testData->StartNewPaint(aPaintSequenceNumber);
+  }
+
+  TreeBuildingState state(mRootLayersId, aIsFirstPaint, aOriginatingLayersId,
+                          testData, aPaintSequenceNumber);
+
+  // We do this business with collecting the entire tree into an array because
+  // otherwise it's very hard to determine which APZC instances need to be
+  // destroyed. In the worst case, there are two scenarios: (a) a layer with an
+  // APZC is removed from the layer tree and (b) a layer with an APZC is moved
+  // in the layer tree from one place to a completely different place. In
+  // scenario (a) we would want to destroy the APZC while walking the layer tree
+  // and noticing that the layer/APZC is no longer there. But if we do that then
+  // we run into a problem in scenario (b) because we might encounter that layer
+  // later during the walk. To handle both of these we have to 'remember' that
+  // the layer was not found, and then do the destroy only at the end of the
+  // tree walk after we are sure that the layer was removed and not just
+  // transplanted elsewhere. Doing that as part of a recursive tree walk is hard
+  // and so maintaining a list and removing APZCs that are still alive is much
+  // simpler.
+  ForEachNode<ReverseIterator>(mRootNode.get(),
+                               [&state](HitTestingTreeNode* aNode) {
+                                 state.mNodesToDestroy.AppendElement(aNode);
+                               });
+  mRootNode = nullptr;
+  Maybe<LayersId> asyncZoomContainerSubtree = Nothing();
+  int asyncZoomContainerNestingDepth = 0;
+  bool haveNestedAsyncZoomContainers = false;
+  nsTArray<LayersId> subtreesWithRootContentOutsideAsyncZoomContainer;
+
+  if (aRoot) {
+    std::unordered_set<LayersId, LayersId::HashFn> seenLayersIds;
+    std::stack<gfx::TreeAutoIndent<gfx::LOG_CRITICAL>> indents;
+    std::stack<AncestorTransform> ancestorTransforms;
+    HitTestingTreeNode* parent = nullptr;
+    HitTestingTreeNode* next = nullptr;
+    LayersId layersId = mRootLayersId;
+    seenLayersIds.insert(mRootLayersId);
+    ancestorTransforms.push(AncestorTransform());
+    state.mOverrideFlags.push(EventRegionsOverride::NoOverride);
+    nsTArray<Maybe<ZoomConstraints>> zoomConstraintsStack;
+
+    // push a nothing to be used for anything outside an async zoom container
+    zoomConstraintsStack.AppendElement(Nothing());
+
+    mApzcTreeLog << "[start]\n";
+    mTreeLock.AssertCurrentThreadIn();
+
+    ForEachNode<ReverseIterator>(
+        aRoot,
+        [&](ScrollNode aLayerMetrics) {
+          if (auto asyncZoomContainerId =
+                  aLayerMetrics.GetAsyncZoomContainerId()) {
+            if (asyncZoomContainerNestingDepth > 0) {
+              haveNestedAsyncZoomContainers = true;
+            }
+            asyncZoomContainerSubtree = Some(layersId);
+            ++asyncZoomContainerNestingDepth;
+
+            auto it = mZoomConstraints.find(
+                ScrollableLayerGuid(layersId, 0, *asyncZoomContainerId));
+            if (it != mZoomConstraints.end()) {
+              zoomConstraintsStack.AppendElement(Some(it->second));
+            } else {
+              zoomConstraintsStack.AppendElement(Nothing());
+            }
+          }
+
+          if (aLayerMetrics.Metrics().IsRootContent()) {
+            MutexAutoLock lock(mMapLock);
+            mGeckoFixedLayerMargins =
+                aLayerMetrics.Metrics().GetFixedLayerMargins();
+          } else {
+            MOZ_ASSERT(aLayerMetrics.Metrics().GetFixedLayerMargins() ==
+                           ScreenMargin(),
+                       "fixed-layer-margins should be 0 on non-root layer");
+          }
+
+          // Note that this check happens after the potential increment of
+          // asyncZoomContainerNestingDepth, to allow the root content
+          // metadata to be on the same node as the async zoom container.
+          if (aLayerMetrics.Metrics().IsRootContent() &&
+              asyncZoomContainerNestingDepth == 0) {
+            subtreesWithRootContentOutsideAsyncZoomContainer.AppendElement(
+                layersId);
+          }
+
+          HitTestingTreeNode* node = PrepareNodeForLayer(
+              lock, aLayerMetrics, aLayerMetrics.Metrics(), layersId,
+              zoomConstraintsStack.LastElement(), ancestorTransforms.top(),
+              parent, next, state);
+          MOZ_ASSERT(node);
+          AsyncPanZoomController* apzc = node->GetApzc();
+          aLayerMetrics.SetApzc(apzc);
+
+          // GetScrollbarAnimationId is only set when webrender is enabled,
+          // which limits the extra thumb mapping work to the webrender-enabled
+          // case where it is needed.
+          // Note also that when webrender is enabled, a "valid" animation id
+          // is always nonzero, so we don't need to worry about handling the
+          // case where WR is enabled and the animation id is zero.
+          if (node->GetScrollbarAnimationId()) {
+            if (node->IsScrollThumbNode()) {
+              state.mScrollThumbs.push_back(node);
+            } else if (node->IsScrollbarContainerNode()) {
+              // Only scrollbar containers for the root have an animation id.
+              state.mRootScrollbarInfo.emplace_back(
+                  *(node->GetScrollbarAnimationId()),
+                  node->GetScrollbarDirection());
+            }
+          }
+
+          // GetFixedPositionAnimationId is only set when webrender is enabled.
+          if (node->GetFixedPositionAnimationId().isSome()) {
+            state.mFixedPositionInfo.emplace_back(node);
+          }
+          // GetStickyPositionAnimationId is only set when webrender is enabled.
+          if (node->GetStickyPositionAnimationId().isSome()) {
+            state.mStickyPositionInfo.emplace_back(node);
+          }
+          if (apzc && node->IsPrimaryHolder()) {
+            state.mScrollTargets[apzc->GetGuid()] = node;
+          }
+
+          // Accumulate the CSS transform between layers that have an APZC.
+          // In the terminology of the big comment above
+          // APZCTreeManager::GetScreenToApzcTransform, if we are at layer M,
+          // then aAncestorTransform is NC * OC * PC, and we left-multiply MC
+          // and compute ancestorTransform to be MC * NC * OC * PC. This gets
+          // passed down as the ancestor transform to layer L when we recurse
+          // into the children below. If we are at a layer with an APZC, such as
+          // P, then we reset the ancestorTransform to just PC, to start the new
+          // accumulation as we go down.
+          AncestorTransform currentTransform{
+              aLayerMetrics.GetTransform(),
+              aLayerMetrics.TransformIsPerspective()};
+          if (!apzc) {
+            currentTransform = currentTransform * ancestorTransforms.top();
+          }
+          ancestorTransforms.push(currentTransform);
+
+          // Note that |node| at this point will not have any children,
+          // otherwise we we would have to set next to node->GetFirstChild().
+          MOZ_ASSERT(!node->GetFirstChild());
+          parent = node;
+          next = nullptr;
+
+          // Update the layersId if we have a new one
+          if (Maybe<LayersId> newLayersId = aLayerMetrics.GetReferentId()) {
+            layersId = *newLayersId;
+            seenLayersIds.insert(layersId);
+
+            // Propagate any event region override flags down into all
+            // descendant nodes from the reflayer that has the flag. This is an
+            // optimization to avoid having to walk up the tree to check the
+            // override flags. Note that we don't keep the flags on the reflayer
+            // itself, because the semantics of the flags are that they apply
+            // to all content in the layer subtree being referenced. This
+            // matters with the WR hit-test codepath, because this reflayer may
+            // be just one of many nodes associated with a particular APZC, and
+            // calling GetTargetNode with a guid may return any one of the
+            // nodes. If different nodes have different flags on them that can
+            // make the WR hit-test result incorrect, but being strict about
+            // only putting the flags on descendant layers avoids this problem.
+            state.mOverrideFlags.push(state.mOverrideFlags.top() |
+                                      aLayerMetrics.GetEventRegionsOverride());
+          }
+
+          indents.push(gfx::TreeAutoIndent<gfx::LOG_CRITICAL>(mApzcTreeLog));
+        },
+        [&](ScrollNode aLayerMetrics) {
+          if (aLayerMetrics.GetAsyncZoomContainerId()) {
+            --asyncZoomContainerNestingDepth;
+            zoomConstraintsStack.RemoveLastElement();
+          }
+          if (aLayerMetrics.GetReferentId()) {
+            state.mOverrideFlags.pop();
+          }
+
+          next = parent;
+          parent = parent->GetParent();
+          layersId = next->GetLayersId();
+          ancestorTransforms.pop();
+          indents.pop();
+        });
+
+    mApzcTreeLog << "[end]\n";
+
+    MOZ_ASSERT(
+        !asyncZoomContainerSubtree ||
+            !subtreesWithRootContentOutsideAsyncZoomContainer.Contains(
+                *asyncZoomContainerSubtree),
+        "If there is an async zoom container, all scroll nodes with root "
+        "content scroll metadata should be inside it");
+    MOZ_ASSERT(!haveNestedAsyncZoomContainers,
+               "Should not have nested async zoom container");
+
+    // If we have perspective transforms deferred to children, do another
+    // walk of the tree and actually apply them to the children.
+    // We can't do this "as we go" in the previous traversal, because by the
+    // time we realize we need to defer a perspective transform for an APZC,
+    // we may already have processed a previous layer (including children
+    // found in its subtree) that shares that APZC.
+    if (!state.mPerspectiveTransformsDeferredToChildren.empty()) {
+      ForEachNode<ReverseIterator>(
+          mRootNode.get(), [&state](HitTestingTreeNode* aNode) {
+            AsyncPanZoomController* apzc = aNode->GetApzc();
+            if (!apzc) {
+              return;
+            }
+            if (!aNode->IsPrimaryHolder()) {
+              return;
+            }
+
+            AsyncPanZoomController* parent = apzc->GetParent();
+            if (!parent) {
+              return;
+            }
+
+            auto it =
+                state.mPerspectiveTransformsDeferredToChildren.find(parent);
+            if (it != state.mPerspectiveTransformsDeferredToChildren.end()) {
+              apzc->SetAncestorTransform(AncestorTransform{
+                  it->second * apzc->GetAncestorTransform(), false});
+            }
+          });
+    }
+
+    // Remove any layers ids for which we no longer have content from
+    // mDetachedLayersIds.
+    for (auto iter = mDetachedLayersIds.begin();
+         iter != mDetachedLayersIds.end();) {
+      // unordered_set::erase() invalidates the iterator pointing to the
+      // element being erased, but returns an iterator to the next element.
+      if (seenLayersIds.find(*iter) == seenLayersIds.end()) {
+        iter = mDetachedLayersIds.erase(iter);
+      } else {
+        ++iter;
+      }
+    }
+  }
+
+  // We do not support tree structures where the root node has siblings.
+  MOZ_ASSERT(!(mRootNode && mRootNode->GetPrevSibling()));
+
+  {  // scope lock and update our mApzcMap before we destroy all the unused
+    // APZC instances
+    MutexAutoLock lock(mMapLock);
+    mApzcMap = std::move(state.mApzcMap);
+
+    for (auto& mapping : mApzcMap) {
+      AsyncPanZoomController* parent = mapping.second.apzc->GetParent();
+      mapping.second.parent = parent ? Some(parent->GetGuid()) : Nothing();
+    }
+
+    mScrollThumbInfo.clear();
+    // For non-webrender, state.mScrollThumbs will be empty so this will be a
+    // no-op.
+    for (HitTestingTreeNode* thumb : state.mScrollThumbs) {
+      MOZ_ASSERT(thumb->IsScrollThumbNode());
+      ScrollableLayerGuid targetGuid(thumb->GetLayersId(), 0,
+                                     thumb->GetScrollTargetId());
+      auto it = state.mScrollTargets.find(targetGuid);
+      if (it == state.mScrollTargets.end()) {
+        // It could be that |thumb| is a scrollthumb for content which didn't
+        // have an APZC, for example if the content isn't layerized. Regardless,
+        // we can't async-scroll it so we don't need to worry about putting it
+        // in mScrollThumbInfo.
+        continue;
+      }
+      HitTestingTreeNode* target = it->second;
+      mScrollThumbInfo.emplace_back(
+          *(thumb->GetScrollbarAnimationId()), thumb->GetTransform(),
+          thumb->GetScrollbarData(), targetGuid, target->GetTransform(),
+          target->IsAncestorOf(thumb));
+    }
+
+    mRootScrollbarInfo = std::move(state.mRootScrollbarInfo);
+    mFixedPositionInfo = std::move(state.mFixedPositionInfo);
+    mStickyPositionInfo = std::move(state.mStickyPositionInfo);
+  }
+
+  for (size_t i = 0; i < state.mNodesToDestroy.Length(); i++) {
+    APZCTM_LOG("Destroying node at %p with APZC %p\n",
+               state.mNodesToDestroy[i].get(),
+               state.mNodesToDestroy[i]->GetApzc());
+    state.mNodesToDestroy[i]->Destroy();
+  }
+
+  APZCTM_LOG("APZCTreeManager (%p)\n", this);
+  if (mRootNode && MOZ_LOG_TEST(sLog, LogLevel::Debug)) {
+    mRootNode->Dump("  ");
+  }
+  SendSubtreeTransformsToChromeMainThread(nullptr);
+}
+
+void APZCTreeManager::UpdateFocusState(LayersId aRootLayerTreeId,
+                                       LayersId aOriginatingLayersId,
+                                       const FocusTarget& aFocusTarget) {
+  AssertOnUpdaterThread();
+
+  if (!StaticPrefs::apz_keyboard_enabled_AtStartup()) {
+    return;
+  }
+
+  mFocusState.Update(aRootLayerTreeId, aOriginatingLayersId, aFocusTarget);
+}
+
+void APZCTreeManager::SampleForWebRender(const Maybe<VsyncId>& aVsyncId,
+                                         wr::TransactionWrapper& aTxn,
+                                         const SampleTime& aSampleTime) {
+  AssertOnSamplerThread();
+  MutexAutoLock lock(mMapLock);
+
+  RefPtr<WebRenderBridgeParent> wrBridgeParent;
+  RefPtr<CompositorController> controller;
+  CompositorBridgeParent::CallWithIndirectShadowTree(
+      mRootLayersId, [&](LayerTreeState& aState) -> void {
+        controller = aState.GetCompositorController();
+        wrBridgeParent = aState.mWrBridge;
+      });
+
+  const bool activeAnimations = AdvanceAnimationsInternal(lock, aSampleTime);
+  if (activeAnimations && controller) {
+    controller->ScheduleRenderOnCompositorThread(
+        wr::RenderReasons::ANIMATED_PROPERTY);
+  }
+  APZCTM_LOG(
+      "APZCTreeManager(%p)::SampleForWebRender, want more composites: %d\n",
+      this, (activeAnimations && controller));
+
+  nsTArray<wr::WrTransformProperty> transforms;
+
+  // Sample async transforms on scrollable layers.
+  for (const auto& [_, mapData] : mApzcMap) {
+    AsyncPanZoomController* apzc = mapData.apzc;
+
+    if (Maybe<CompositionPayload> payload = apzc->NotifyScrollSampling()) {
+      if (wrBridgeParent && aVsyncId) {
+        wrBridgeParent->AddPendingScrollPayload(*payload, *aVsyncId);
+      }
+    }
+
+    if (StaticPrefs::apz_test_logging_enabled()) {
+      MutexAutoLock lock(mTestDataLock);
+
+      ScrollableLayerGuid guid = apzc->GetGuid();
+      auto it = mTestData.find(guid.mLayersId);
+      if (it != mTestData.end()) {
+        it->second->RecordSampledResult(
+            apzc->GetCurrentAsyncVisualViewport(
+                    AsyncPanZoomController::eForCompositing)
+                .TopLeft(),
+            (aSampleTime.Time() - TimeStamp::ProcessCreation())
+                .ToMicroseconds(),
+            guid.mLayersId, guid.mScrollId);
+      }
+    }
+
+    wr::LayoutTransform zoomForMinimap =
+        wr::ToLayoutTransform(gfx::Matrix4x4());
+    if (Maybe<uint64_t> zoomAnimationId = apzc->GetZoomAnimationId()) {
+      // for now we only support zooming on root content APZCs
+      MOZ_ASSERT(apzc->IsRootContent());
+
+      LayoutDeviceToParentLayerScale zoom = apzc->GetCurrentPinchZoomScale(
+          AsyncPanZoomController::eForCompositing);
+
+      AsyncTransform asyncVisualTransform = apzc->GetCurrentAsyncTransform(
+          AsyncPanZoomController::eForCompositing,
+          AsyncTransformComponents{AsyncTransformComponent::eVisual});
+
+      wr::WrTransformProperty zoomTransform = wr::ToWrTransformProperty(
+          *zoomAnimationId, LayoutDeviceToParentLayerMatrix4x4::Scaling(
+                                zoom.scale, zoom.scale, 1.0f) *
+                                AsyncTransformComponentMatrix::Translation(
+                                    asyncVisualTransform.mTranslation));
+
+      // Store the zoom transform to be added to the minimap data so that we
+      // can take it into account correctly during minimap rendering.
+      zoomForMinimap = zoomTransform.value;
+
+      transforms.AppendElement(zoomTransform);
+      aTxn.UpdateIsTransformAsyncZooming(*zoomAnimationId,
+                                         apzc->IsAsyncZooming());
+    }
+
+    nsTArray<wr::SampledScrollOffset> sampledOffsets =
+        apzc->GetSampledScrollOffsets();
+    wr::ExternalScrollId scrollId{apzc->GetGuid().mScrollId,
+                                  wr::AsPipelineId(apzc->GetGuid().mLayersId)};
+    aTxn.UpdateScrollPosition(scrollId, sampledOffsets);
+
+    if (StaticPrefs::apz_minimap_enabled()) {
+      wr::MinimapData minimapData = apzc->GetMinimapData();
+      minimapData.zoom_transform = zoomForMinimap;
+      // If this APZC is inside the subtree of a root content APZC, find the
+      // ID of that root content APZC.
+      ScrollableLayerGuid enclosingRootContentId;
+      ApzcMapData currentEntry = mapData;
+      AsyncPanZoomController* current = currentEntry.apzc;
+      while (current) {
+        if (current->IsRootContent()) {
+          enclosingRootContentId = current->GetGuid();
+          break;
+        }
+        // We can't call AsyncPanZoomController::GetParent(), since that
+        // requires acquiring the tree lock, and doing that on the sampler
+        // thread would violate the lock ordering. Instead, get the parent
+        // from the mApzcMap entry.
+        if (auto parentGuid = currentEntry.parent) {
+          auto iter = mApzcMap.find(*parentGuid);
+          if (iter != mApzcMap.end()) {
+            currentEntry = iter->second;
+            current = currentEntry.apzc;
+            continue;
+          }
+        }
+        break;
+      }
+      minimapData.root_content_pipeline_id =
+          wr::AsPipelineId(enclosingRootContentId.mLayersId);
+      minimapData.root_content_scroll_id = enclosingRootContentId.mScrollId;
+      aTxn.AddMinimapData(scrollId, minimapData);
+    }
+
+#if defined(MOZ_WIDGET_ANDROID)
+    // Send the root frame metrics to java through the UIController
+    RefPtr<UiCompositorControllerParent> uiController =
+        UiCompositorControllerParent::GetFromRootLayerTreeId(mRootLayersId);
+    if (uiController &&
+        apzc->UpdateRootFrameMetricsIfChanged(mLastRootMetrics)) {
+      uiController->NotifyUpdateScreenMetrics(mLastRootMetrics);
+    }
+#endif
+  }
+
+  // Now collect all the async transforms needed for the scrollthumbs.
+  for (const ScrollThumbInfo& info : mScrollThumbInfo) {
+    auto it = mApzcMap.find(info.mTargetGuid);
+    if (it == mApzcMap.end()) {
+      // It could be that |info| is a scrollthumb for content which didn't
+      // have an APZC, for example if the content isn't layerized. Regardless,
+      // we can't async-scroll it so we don't need to worry about putting it
+      // in mScrollThumbInfo.
+      continue;
+    }
+    AsyncPanZoomController* scrollTargetApzc = it->second.apzc;
+    MOZ_ASSERT(scrollTargetApzc);
+    LayerToParentLayerMatrix4x4 transform =
+        scrollTargetApzc->CallWithLastContentPaintMetrics(
+            [&](const FrameMetrics& aMetrics) {
+              return ComputeTransformForScrollThumb(
+                  info.mThumbTransform * AsyncTransformMatrix(),
+                  info.mTargetTransform.ToUnknownMatrix(), scrollTargetApzc,
+                  aMetrics, info.mThumbData, info.mTargetIsAncestor);
+            });
+    transforms.AppendElement(
+        wr::ToWrTransformProperty(info.mThumbAnimationId, transform));
+  }
+
+  // Move the root scrollbar in response to the dynamic toolbar transition.
+  for (const RootScrollbarInfo& info : mRootScrollbarInfo) {
+    // We only care about the horizontal scrollbar.
+    if (info.mScrollDirection == ScrollDirection::eHorizontal) {
+      ScreenPoint translation =
+          apz::ComputeFixedMarginsOffset(GetCompositorFixedLayerMargins(lock),
+                                         SideBits::eBottom, ScreenMargin());
+
+      LayerToParentLayerMatrix4x4 transform =
+          LayerToParentLayerMatrix4x4::Translation(ViewAs<ParentLayerPixel>(
+              translation, PixelCastJustification::ScreenIsParentLayerForRoot));
+
+      transforms.AppendElement(
+          wr::ToWrTransformProperty(info.mScrollbarAnimationId, transform));
+    }
+  }
+
+  for (const FixedPositionInfo& info : mFixedPositionInfo) {
+    MOZ_ASSERT(info.mFixedPositionAnimationId.isSome());
+    if (!IsFixedToRootContent(info, lock)) {
+      continue;
+    }
+
+    ScreenPoint translation = apz::ComputeFixedMarginsOffset(
+        GetCompositorFixedLayerMargins(lock), info.mFixedPosSides,
+        mGeckoFixedLayerMargins);
+
+    LayerToParentLayerMatrix4x4 transform =
+        LayerToParentLayerMatrix4x4::Translation(ViewAs<ParentLayerPixel>(
+            translation, PixelCastJustification::ScreenIsParentLayerForRoot));
+
+    transforms.AppendElement(
+        wr::ToWrTransformProperty(*info.mFixedPositionAnimationId, transform));
+  }
+
+  for (const StickyPositionInfo& info : mStickyPositionInfo) {
+    MOZ_ASSERT(info.mStickyPositionAnimationId.isSome());
+    SideBits sides = SidesStuckToRootContent(
+        info, AsyncTransformConsumer::eForCompositing, lock);
+    if (sides == SideBits::eNone) {
+      continue;
+    }
+
+    ScreenPoint translation = apz::ComputeFixedMarginsOffset(
+        GetCompositorFixedLayerMargins(lock), sides,
+        // For sticky layers, we don't need to factor
+        // mGeckoFixedLayerMargins because Gecko doesn't shift the
+        // position of sticky elements for dynamic toolbar movements.
+        ScreenMargin());
+
+    LayerToParentLayerMatrix4x4 transform =
+        LayerToParentLayerMatrix4x4::Translation(ViewAs<ParentLayerPixel>(
+            translation, PixelCastJustification::ScreenIsParentLayerForRoot));
+
+    transforms.AppendElement(
+        wr::ToWrTransformProperty(*info.mStickyPositionAnimationId, transform));
+  }
+
+  aTxn.AppendTransformProperties(transforms);
+}
+
+ParentLayerRect APZCTreeManager::ComputeClippedCompositionBounds(
+    const MutexAutoLock& aProofOfMapLock, ClippedCompositionBoundsMap& aDestMap,
+    ScrollableLayerGuid aGuid) {
+  if (auto iter = aDestMap.find(aGuid); iter != aDestMap.end()) {
+    // We already computed it for this one, early-exit. This might happen
+    // because on a later iteration of mApzcMap we might encounter an ancestor
+    // of an APZC that we processed on an earlier iteration. In this case we
+    // would have computed the ancestor's clipped composition bounds when
+    // recursing up on the earlier iteration.
+    return iter->second;
+  }
+
+  ParentLayerRect bounds = mApzcMap[aGuid].apzc->GetCompositionBounds();
+  const auto& mapEntry = mApzcMap.find(aGuid);
+  MOZ_ASSERT(mapEntry != mApzcMap.end());
+  if (mapEntry->second.parent.isNothing()) {
+    // Recursion base case, where the APZC with guid `aGuid` has no parent.
+    // In this case, we don't need to clip `bounds` any further and can just
+    // early exit.
+    aDestMap.emplace(aGuid, bounds);
+    return bounds;
+  }
+
+  ScrollableLayerGuid parentGuid = mapEntry->second.parent.value();
+  auto parentBoundsEntry = aDestMap.find(parentGuid);
+  // If aDestMap doesn't contain the parent entry yet, we recurse to compute
+  // that one first.
+  ParentLayerRect parentClippedBounds =
+      (parentBoundsEntry == aDestMap.end())
+          ? ComputeClippedCompositionBounds(aProofOfMapLock, aDestMap,
+                                            parentGuid)
+          : parentBoundsEntry->second;
+
+  // The parent layer's async transform applies to the current layer to take
+  // `bounds` into the same coordinate space as `parentClippedBounds`. However,
+  // we're going to do the inverse operation and unapply this transform to
+  // `parentClippedBounds` to bring it into the same coordinate space as
+  // `bounds`.
+  AsyncTransform appliesToLayer =
+      mApzcMap[parentGuid].apzc->GetCurrentAsyncTransform(
+          AsyncPanZoomController::eForCompositing);
+
+  // Do the unapplication
+  LayerRect parentClippedBoundsInParentLayerSpace =
+      (parentClippedBounds - appliesToLayer.mTranslation) /
+      appliesToLayer.mScale;
+
+  // And then clip `bounds` by the parent's comp bounds in the current space.
+  bounds = bounds.Intersect(
+      ViewAs<ParentLayerPixel>(parentClippedBoundsInParentLayerSpace,
+                               PixelCastJustification::MovingDownToChildren));
+
+  // Done!
+  aDestMap.emplace(aGuid, bounds);
+  return bounds;
+}
+
+bool APZCTreeManager::AdvanceAnimationsInternal(
+    const MutexAutoLock& aProofOfMapLock, const SampleTime& aSampleTime) {
+  ClippedCompositionBoundsMap clippedCompBounds;
+  bool activeAnimations = false;
+  for (const auto& mapping : mApzcMap) {
+    AsyncPanZoomController* apzc = mapping.second.apzc;
+    // Note that this call is recursive, but it early-exits if called again
+    // with the same guid. So this loop is still amortized O(n) with respect to
+    // the number of APZCs.
+    ParentLayerRect clippedBounds = ComputeClippedCompositionBounds(
+        aProofOfMapLock, clippedCompBounds, mapping.first);
+
+    apzc->ReportCheckerboard(aSampleTime, clippedBounds);
+    activeAnimations |= apzc->AdvanceAnimations(aSampleTime);
+  }
+  return activeAnimations;
+}
+
+void APZCTreeManager::PrintLayerInfo(const ScrollNode& aLayer) {
+  if (StaticPrefs::apz_printtree() && aLayer.Dump(mApzcTreeLog) > 0) {
+    mApzcTreeLog << "\n";
+  }
+}
+
+// mTreeLock is held, and checked with static analysis
+void APZCTreeManager::AttachNodeToTree(HitTestingTreeNode* aNode,
+                                       HitTestingTreeNode* aParent,
+                                       HitTestingTreeNode* aNextSibling) {
+  if (aNextSibling) {
+    aNextSibling->SetPrevSibling(aNode);
+  } else if (aParent) {
+    aParent->SetLastChild(aNode);
+  } else {
+    MOZ_ASSERT(!mRootNode);
+    mRootNode = aNode;
+    aNode->MakeRoot();
+  }
+}
+
+already_AddRefed<HitTestingTreeNode> APZCTreeManager::RecycleOrCreateNode(
+    const RecursiveMutexAutoLock& aProofOfTreeLock, TreeBuildingState& aState,
+    AsyncPanZoomController* aApzc, LayersId aLayersId) {
+  // Find a node without an APZC and return it. Note that unless the layer tree
+  // actually changes, this loop should generally do an early-return on the
+  // first iteration, so it should be cheap in the common case.
+  for (int32_t i = aState.mNodesToDestroy.Length() - 1; i >= 0; i--) {
+    RefPtr<HitTestingTreeNode> node = aState.mNodesToDestroy[i];
+    if (node->IsRecyclable(aProofOfTreeLock)) {
+      aState.mNodesToDestroy.RemoveElementAt(i);
+      node->RecycleWith(aProofOfTreeLock, aApzc, aLayersId);
+      return node.forget();
+    }
+  }
+  RefPtr<HitTestingTreeNode> node =
+      new HitTestingTreeNode(aApzc, false, aLayersId);
+  return node.forget();
+}
+
+void APZCTreeManager::StartScrollbarDrag(const ScrollableLayerGuid& aGuid,
+                                         const AsyncDragMetrics& aDragMetrics) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<ScrollableLayerGuid, AsyncDragMetrics>(
+            "layers::APZCTreeManager::StartScrollbarDrag", this,
+            &APZCTreeManager::StartScrollbarDrag, aGuid, aDragMetrics));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+
+  RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aGuid);
+  if (!apzc) {
+    NotifyScrollbarDragRejected(aGuid);
+    return;
+  }
+
+  uint64_t inputBlockId = aDragMetrics.mDragStartSequenceNumber;
+  mInputQueue->ConfirmDragBlock(inputBlockId, apzc, aDragMetrics);
+}
+
+bool APZCTreeManager::StartAutoscroll(const ScrollableLayerGuid& aGuid,
+                                      const ScreenPoint& aAnchorLocation) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aGuid);
+  if (!apzc) {
+    if (XRE_IsGPUProcess()) {
+      // If we're in the compositor process, the "return false" will be
+      // ignored because the query comes over the PAPZCTreeManager protocol
+      // via an async message. In this case, send an explicit rejection
+      // message to content.
+      NotifyAutoscrollRejected(aGuid);
+    }
+    return false;
+  }
+
+  apzc->StartAutoscroll(aAnchorLocation);
+  return true;
+}
+
+void APZCTreeManager::StopAutoscroll(const ScrollableLayerGuid& aGuid) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  if (RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aGuid)) {
+    apzc->StopAutoscroll();
+  }
+}
+
+void APZCTreeManager::NotifyScrollbarDragInitiated(
+    uint64_t aDragBlockId, const ScrollableLayerGuid& aGuid,
+    ScrollDirection aDirection) const {
+  RefPtr<GeckoContentController> controller =
+      GetContentController(aGuid.mLayersId);
+  if (controller) {
+    controller->NotifyAsyncScrollbarDragInitiated(aDragBlockId, aGuid.mScrollId,
+                                                  aDirection);
+  }
+}
+
+void APZCTreeManager::NotifyScrollbarDragRejected(
+    const ScrollableLayerGuid& aGuid) const {
+  RefPtr<GeckoContentController> controller =
+      GetContentController(aGuid.mLayersId);
+  if (controller) {
+    controller->NotifyAsyncScrollbarDragRejected(aGuid.mScrollId);
+  }
+}
+
+void APZCTreeManager::NotifyAutoscrollRejected(
+    const ScrollableLayerGuid& aGuid) const {
+  RefPtr<GeckoContentController> controller =
+      GetContentController(aGuid.mLayersId);
+  MOZ_ASSERT(controller);
+  controller->NotifyAsyncAutoscrollRejected(aGuid.mScrollId);
+}
+
+void SetHitTestData(HitTestingTreeNode* aNode,
+                    const WebRenderScrollDataWrapper& aLayer,
+                    const EventRegionsOverride& aOverrideFlags) {
+  aNode->SetHitTestData(aLayer.GetVisibleRect(), aLayer.GetRemoteDocumentSize(),
+                        aLayer.GetTransformTyped(), aOverrideFlags,
+                        aLayer.GetAsyncZoomContainerId());
+}
+
+HitTestingTreeNode* APZCTreeManager::PrepareNodeForLayer(
+    const RecursiveMutexAutoLock& aProofOfTreeLock, const ScrollNode& aLayer,
+    const FrameMetrics& aMetrics, LayersId aLayersId,
+    const Maybe<ZoomConstraints>& aZoomConstraints,
+    const AncestorTransform& aAncestorTransform, HitTestingTreeNode* aParent,
+    HitTestingTreeNode* aNextSibling, TreeBuildingState& aState) {
+  mTreeLock.AssertCurrentThreadIn();  // for static analysis
+  bool needsApzc = true;
+  if (!aMetrics.IsScrollable()) {
+    needsApzc = false;
+  }
+
+  // XXX: As a future optimization we can probably stick these things on the
+  // TreeBuildingState, and update them as we change layers id during the
+  // traversal
+  RefPtr<GeckoContentController> geckoContentController;
+  CompositorBridgeParent::CallWithIndirectShadowTree(
+      aLayersId, [&](LayerTreeState& lts) -> void {
+        geckoContentController = lts.mController;
+      });
+
+  if (!geckoContentController) {
+    needsApzc = false;
+  }
+
+  if (Maybe<uint64_t> zoomAnimationId = aLayer.GetZoomAnimationId()) {
+    aState.mZoomAnimationId = zoomAnimationId;
+  }
+
+  RefPtr<HitTestingTreeNode> node = nullptr;
+  if (!needsApzc) {
+    // Note: if layer properties must be propagated to nodes, RecvUpdate in
+    // LayerTransactionParent.cpp must ensure that APZ will be notified
+    // when those properties change.
+    node = RecycleOrCreateNode(aProofOfTreeLock, aState, nullptr, aLayersId);
+    AttachNodeToTree(node, aParent, aNextSibling);
+    SetHitTestData(node, aLayer, aState.mOverrideFlags.top());
+    node->SetScrollbarData(aLayer.GetScrollbarAnimationId(),
+                           aLayer.GetScrollbarData());
+    node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId(),
+                          aLayer.GetFixedPositionSides(),
+                          aLayer.GetFixedPositionAnimationId());
+    node->SetStickyPosData(aLayer.GetStickyScrollContainerId(),
+                           aLayer.GetStickyScrollRangeOuter(),
+                           aLayer.GetStickyScrollRangeInner(),
+                           aLayer.GetStickyPositionAnimationId());
+    PrintLayerInfo(aLayer);
+    return node;
+  }
+
+  RefPtr<AsyncPanZoomController> apzc;
+  // If we get here, aLayer is a scrollable layer and somebody
+  // has registered a GeckoContentController for it, so we need to ensure
+  // it has an APZC instance to manage its scrolling.
+
+  // aState.mApzcMap allows reusing the exact same APZC instance for different
+  // layers with the same FrameMetrics data. This is needed because in some
+  // cases content that is supposed to scroll together is split into multiple
+  // layers because of e.g. non-scrolling content interleaved in z-index order.
+  ScrollableLayerGuid guid(aLayersId, aMetrics.GetPresShellId(),
+                           aMetrics.GetScrollId());
+  auto insertResult = aState.mApzcMap.insert(std::make_pair(
+      guid,
+      ApzcMapData{static_cast<AsyncPanZoomController*>(nullptr), Nothing()}));
+  if (!insertResult.second) {
+    apzc = insertResult.first->second.apzc;
+    PrintLayerInfo(aLayer);
+  }
+  APZCTM_LOG(
+      "Found APZC %p for layer %p with identifiers %" PRIx64 " %" PRId64 "\n",
+      apzc.get(), aLayer.GetLayer(), uint64_t(guid.mLayersId), guid.mScrollId);
+
+  // If we haven't encountered a layer already with the same metrics, then we
+  // need to do the full reuse-or-make-an-APZC algorithm, which is contained
+  // inside the block below.
+  if (apzc == nullptr) {
+    apzc = aLayer.GetApzc();
+
+    // If the content represented by the scrollable layer has changed (which may
+    // be possible because of DLBI heuristics) then we don't want to keep using
+    // the same old APZC for the new content. Also, when reparenting a tab into
+    // a new window a layer might get moved to a different layer tree with a
+    // different APZCTreeManager. In these cases we don't want to reuse the same
+    // APZC, so null it out so we run through the code to find another one or
+    // create one.
+    if (apzc && (!apzc->Matches(guid) || !apzc->HasTreeManager(this))) {
+      apzc = nullptr;
+    }
+
+    // See if we can find an APZC from the previous tree that matches the
+    // ScrollableLayerGuid from this layer. If there is one, then we know that
+    // the layout of the page changed causing the layer tree to be rebuilt, but
+    // the underlying content for the APZC is still there somewhere. Therefore,
+    // we want to find the APZC instance and continue using it here.
+    //
+    // We particularly want to find the primary-holder node from the previous
+    // tree that matches, because we don't want that node to get destroyed. If
+    // it does get destroyed, then the APZC will get destroyed along with it by
+    // definition, but we want to keep that APZC around in the new tree.
+    // We leave non-primary-holder nodes in the destroy list because we don't
+    // care about those nodes getting destroyed.
+    for (size_t i = 0; i < aState.mNodesToDestroy.Length(); i++) {
+      RefPtr<HitTestingTreeNode> n = aState.mNodesToDestroy[i];
+      if (n->IsPrimaryHolder() && n->GetApzc() && n->GetApzc()->Matches(guid)) {
+        node = n;
+        if (apzc != nullptr) {
+          // If there is an APZC already then it should match the one from the
+          // old primary-holder node
+          MOZ_ASSERT(apzc == node->GetApzc());
+        }
+        apzc = node->GetApzc();
+        break;
+      }
+    }
+
+    // The APZC we get off the layer may have been destroyed previously if the
+    // layer was inactive or omitted from the layer tree for whatever reason
+    // from a layers update. If it later comes back it will have a reference to
+    // a destroyed APZC and so we need to throw that out and make a new one.
+    bool newApzc = (apzc == nullptr || apzc->IsDestroyed());
+    if (newApzc) {
+      apzc = NewAPZCInstance(aLayersId, geckoContentController);
+      apzc->SetCompositorController(aState.mCompositorController.get());
+      MOZ_ASSERT(node == nullptr);
+      node = new HitTestingTreeNode(apzc, true, aLayersId);
+    } else {
+      // If we are re-using a node for this layer clear the tree pointers
+      // so that it doesn't continue pointing to nodes that might no longer
+      // be in the tree. These pointers will get reset properly as we continue
+      // building the tree. Also remove it from the set of nodes that are going
+      // to be destroyed, because it's going to remain active.
+      aState.mNodesToDestroy.RemoveElement(node);
+      node->SetPrevSibling(nullptr);
+      node->SetLastChild(nullptr);
+    }
+
+    if (aMetrics.IsRootContent()) {
+      apzc->SetZoomAnimationId(aState.mZoomAnimationId);
+      aState.mZoomAnimationId = Nothing();
+    }
+
+    APZCTM_LOG("Using APZC %p for layer %p with identifiers %" PRIx64
+               " %" PRId64 "\n",
+               apzc.get(), aLayer.GetLayer(), uint64_t(aLayersId),
+               aMetrics.GetScrollId());
+
+    apzc->NotifyLayersUpdated(aLayer.Metadata(), aState.mIsFirstPaint,
+                              aLayersId == aState.mOriginatingLayersId);
+
+    // Since this is the first time we are encountering an APZC with this guid,
+    // the node holding it must be the primary holder. It may be newly-created
+    // or not, depending on whether it went through the newApzc branch above.
+    MOZ_ASSERT(node->IsPrimaryHolder() && node->GetApzc() &&
+               node->GetApzc()->Matches(guid));
+
+    SetHitTestData(node, aLayer, aState.mOverrideFlags.top());
+    apzc->SetAncestorTransform(aAncestorTransform);
+
+    PrintLayerInfo(aLayer);
+
+    // Bind the APZC instance into the tree of APZCs
+    AttachNodeToTree(node, aParent, aNextSibling);
+
+    // For testing, log the parent scroll id of every APZC that has a
+    // parent. This allows test code to reconstruct the APZC tree.
+    // Note that we currently only do this for APZCs in the layer tree
+    // that originated the update, because the only identifying information
+    // we are logging about APZCs is the scroll id, and otherwise we could
+    // confuse APZCs from different layer trees with the same scroll id.
+    if (aLayersId == aState.mOriginatingLayersId) {
+      if (apzc->HasNoParentWithSameLayersId()) {
+        aState.mPaintLogger.LogTestData(aMetrics.GetScrollId(),
+                                        "hasNoParentWithSameLayersId", true);
+      } else {
+        MOZ_ASSERT(apzc->GetParent());
+        aState.mPaintLogger.LogTestData(aMetrics.GetScrollId(),
+                                        "parentScrollId",
+                                        apzc->GetParent()->GetGuid().mScrollId);
+      }
+      if (aMetrics.IsRootContent()) {
+        aState.mPaintLogger.LogTestData(aMetrics.GetScrollId(), "isRootContent",
+                                        true);
+      }
+      // Note that the async scroll offset is in ParentLayer pixels
+      aState.mPaintLogger.LogTestData(
+          aMetrics.GetScrollId(), "asyncScrollOffset",
+          apzc->GetCurrentAsyncScrollOffset(
+              AsyncPanZoomController::eForEventHandling));
+      aState.mPaintLogger.LogTestData(aMetrics.GetScrollId(),
+                                      "hasAsyncKeyScrolled",
+                                      apzc->TestHasAsyncKeyScrolled());
+    }
+
+    // We must update the zoom constraints even if the apzc isn't new because it
+    // might have moved.
+    if (node->IsPrimaryHolder()) {
+      if (aZoomConstraints) {
+        apzc->UpdateZoomConstraints(*aZoomConstraints);
+
+#ifdef DEBUG
+        auto it = mZoomConstraints.find(guid);
+        if (it != mZoomConstraints.end()) {
+          MOZ_ASSERT(it->second == *aZoomConstraints);
+        }
+      } else {
+        // We'd like to assert these things (if the first doesn't hold then at
+        // least the second) but neither are not true because xul root content
+        // gets zoomable zoom constraints, but which is not zoomable because it
+        // doesn't have a root scroll frame.
+        // clang-format off
+        // MOZ_ASSERT(mZoomConstraints.find(guid) == mZoomConstraints.end());
+        // auto it = mZoomConstraints.find(guid);
+        // if (it != mZoomConstraints.end()) {
+        //   MOZ_ASSERT(!it->second.mAllowZoom && !it->second.mAllowDoubleTapZoom);
+        // }
+        // clang-format on
+#endif
+      }
+    }
+
+    // Add a guid -> APZC mapping for the newly created APZC.
+    insertResult.first->second.apzc = apzc;
+  } else {
+    // We already built an APZC earlier in this tree walk, but we have another
+    // layer now that will also be using that APZC. The hit-test region on the
+    // APZC needs to be updated to deal with the new layer's hit region.
+
+    node = RecycleOrCreateNode(aProofOfTreeLock, aState, apzc, aLayersId);
+    AttachNodeToTree(node, aParent, aNextSibling);
+
+    // Even though different layers associated with a given APZC may be at
+    // different levels in the layer tree (e.g. one being an uncle of another),
+    // we require from Layout that the CSS transforms up to their common
+    // ancestor be roughly the same. There are cases in which the transforms
+    // are not exactly the same, for example if the parent is container layer
+    // for an opacity, and this container layer has a resolution-induced scale
+    // as its base transform and a prescale that is supposed to undo that scale.
+    // Due to floating point inaccuracies those transforms can end up not quite
+    // canceling each other. That's why we're using a fuzzy comparison here
+    // instead of an exact one.
+    // In addition, two ancestor transforms are allowed to differ if one of
+    // them contains a perspective transform component and the other does not.
+    // This represents situations where some content in a scrollable frame
+    // is subject to a perspective transform and other content does not.
+    // In such cases, go with the one that does not include the perspective
+    // component; the perspective transform is remembered and applied to the
+    // children instead.
+    auto ancestorTransform = aAncestorTransform.CombinedTransform();
+    auto existingAncestorTransform = apzc->GetAncestorTransform();
+    if (!ancestorTransform.FuzzyEqualsMultiplicative(
+            existingAncestorTransform)) {
+      typedef TreeBuildingState::DeferredTransformMap::value_type PairType;
+      if (!aAncestorTransform.ContainsPerspectiveTransform() &&
+          !apzc->AncestorTransformContainsPerspective()) {
+        // If this content is being presented in a paginated fashion (e.g.
+        // print preview), the multiple layers may reflect multiple instances
+        // of the same display item being rendered on different pages. In such
+        // cases, it's expected that different instances can have different
+        // transforms, since each page renders a different part of the item.
+        if (!aLayer.Metadata().IsPaginatedPresentation()) {
+          if (ancestorTransform.IsFinite() &&
+              existingAncestorTransform.IsFinite()) {
+            // Log separately from the assert because assert doesn't allow
+            // printf-style arguments, but it's important for debugging that the
+            // log identifies *which* scroll frame violated the condition.
+            MOZ_LOG(sLog, LogLevel::Error,
+                    ("Two layers that scroll together have different ancestor "
+                     "transforms (guid=%s)",
+                     ToString(apzc->GetGuid()).c_str()));
+            MOZ_ASSERT(false,
+                       "Two layers that scroll together have different "
+                       "ancestor transforms");
+          } else {
+            MOZ_ASSERT(ancestorTransform.IsFinite() ==
+                       existingAncestorTransform.IsFinite());
+          }
+        }
+      } else if (!aAncestorTransform.ContainsPerspectiveTransform()) {
+        aState.mPerspectiveTransformsDeferredToChildren.insert(
+            PairType{apzc, apzc->GetAncestorTransformPerspective()});
+        apzc->SetAncestorTransform(aAncestorTransform);
+      } else {
+        aState.mPerspectiveTransformsDeferredToChildren.insert(
+            PairType{apzc, aAncestorTransform.GetPerspectiveTransform()});
+      }
+    }
+
+    SetHitTestData(node, aLayer, aState.mOverrideFlags.top());
+  }
+
+  // Note: if layer properties must be propagated to nodes, RecvUpdate in
+  // LayerTransactionParent.cpp must ensure that APZ will be notified
+  // when those properties change.
+  node->SetScrollbarData(aLayer.GetScrollbarAnimationId(),
+                         aLayer.GetScrollbarData());
+  node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId(),
+                        aLayer.GetFixedPositionSides(),
+                        aLayer.GetFixedPositionAnimationId());
+  node->SetStickyPosData(aLayer.GetStickyScrollContainerId(),
+                         aLayer.GetStickyScrollRangeOuter(),
+                         aLayer.GetStickyScrollRangeInner(),
+                         aLayer.GetStickyPositionAnimationId());
+  return node;
+}
+
+template <typename PanGestureOrScrollWheelInput>
+static bool WillHandleInput(const PanGestureOrScrollWheelInput& aPanInput) {
+  if (!XRE_IsParentProcess() || !NS_IsMainThread()) {
+    return true;
+  }
+
+  WidgetWheelEvent wheelEvent = aPanInput.ToWidgetEvent(nullptr);
+  return APZInputBridge::ActionForWheelEvent(&wheelEvent).isSome();
+}
+
+/*static*/
+void APZCTreeManager::FlushApzRepaints(LayersId aLayersId) {
+  // Previously, paints were throttled and therefore this method was used to
+  // ensure any pending paints were flushed. Now, paints are flushed
+  // immediately, so it is safe to simply send a notification now.
+  APZCTM_LOG("Flushing repaints for layers id 0x%" PRIx64 "\n",
+             uint64_t(aLayersId));
+  RefPtr<GeckoContentController> controller = GetContentController(aLayersId);
+#ifndef MOZ_WIDGET_ANDROID
+  // On Android, this code is run in production and may actually get a nullptr
+  // controller here. On other platforms this code is test-only and should never
+  // get a nullptr.
+  MOZ_ASSERT(controller);
+#endif
+  if (controller) {
+    controller->DispatchToRepaintThread(NewRunnableMethod(
+        "layers::GeckoContentController::NotifyFlushComplete", controller,
+        &GeckoContentController::NotifyFlushComplete));
+  }
+}
+
+void APZCTreeManager::MarkAsDetached(LayersId aLayersId) {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  mDetachedLayersIds.insert(aLayersId);
+}
+
+static bool HasNonLockModifier(Modifiers aModifiers) {
+  return (aModifiers &
+          (MODIFIER_ALT | MODIFIER_ALTGRAPH | MODIFIER_CONTROL | MODIFIER_FN |
+           MODIFIER_META | MODIFIER_SHIFT | MODIFIER_SYMBOL)) != 0;
+}
+
+APZEventResult APZCTreeManager::ReceiveInputEvent(
+    InputData& aEvent, InputBlockCallback&& aCallback) {
+  APZThreadUtils::AssertOnControllerThread();
+  InputHandlingState state{aEvent};
+
+  // Use a RAII class for updating the focus sequence number of this event
+  AutoFocusSequenceNumberSetter focusSetter(mFocusState, aEvent);
+
+  switch (aEvent.mInputType) {
+    case MULTITOUCH_INPUT: {
+      MultiTouchInput& touchInput = aEvent.AsMultiTouchInput();
+      ProcessTouchInput(state, touchInput);
+      break;
+    }
+    case MOUSE_INPUT: {
+      MouseInput& mouseInput = aEvent.AsMouseInput();
+      mouseInput.mHandledByAPZ = true;
+
+      SetCurrentMousePosition(mouseInput.mOrigin);
+
+      bool startsDrag = DragTracker::StartsDrag(mouseInput);
+      if (startsDrag) {
+        // If this is the start of a drag we need to unambiguously know if it's
+        // going to land on a scrollbar or not. We can't apply an untransform
+        // here without knowing that, so we need to ensure the untransform is
+        // a no-op.
+        FlushRepaintsToClearScreenToGeckoTransform();
+      }
+
+      // TODO(botond): Is it necessary to do a hit test on every mouse-move?
+      state.mHit = GetTargetAPZC(mouseInput.mOrigin);
+      bool hitScrollbar = (bool)state.mHit.mScrollbarNode;
+
+      // When the mouse is outside the window we still want to handle dragging
+      // but we won't find an APZC. Fallback to root APZC then.
+      {  // scope lock
+        RecursiveMutexAutoLock lock(mTreeLock);
+        if (!state.mHit.mTargetApzc && mRootNode) {
+          state.mHit.mTargetApzc = mRootNode->GetApzc();
+        }
+      }
+
+      if (state.mHit.mTargetApzc) {
+        if (StaticPrefs::apz_test_logging_enabled() &&
+            mouseInput.mType == MouseInput::MOUSE_HITTEST) {
+          ScrollableLayerGuid guid = state.mHit.mTargetApzc->GetGuid();
+
+          MutexAutoLock lock(mTestDataLock);
+          auto it = mTestData.find(guid.mLayersId);
+          MOZ_ASSERT(it != mTestData.end());
+          it->second->RecordHitResult(mouseInput.mOrigin, state.mHit.mHitResult,
+                                      guid.mLayersId, guid.mScrollId);
+        }
+
+        TargetConfirmationFlags confFlags{state.mHit.mHitResult};
+        state.mResult = mInputQueue->ReceiveInputEvent(state.mHit.mTargetApzc,
+                                                       confFlags, mouseInput);
+
+        // If we're starting an async scrollbar drag
+        bool apzDragEnabled = StaticPrefs::apz_drag_enabled();
+        if (apzDragEnabled && startsDrag && state.mHit.mScrollbarNode &&
+            state.mHit.mScrollbarNode->IsScrollThumbNode() &&
+            state.mHit.mScrollbarNode->GetScrollbarData()
+                .mThumbIsAsyncDraggable) {
+          SetupScrollbarDrag(mouseInput, state.mHit.mScrollbarNode,
+                             state.mHit.mTargetApzc.get());
+        }
+
+        if (state.mResult.GetStatus() == nsEventStatus_eConsumeDoDefault) {
+          // This input event is part of a drag block, so whether or not it is
+          // directed at a scrollbar depends on whether the drag block started
+          // on a scrollbar.
+          hitScrollbar = mInputQueue->IsDragOnScrollbar(hitScrollbar);
+        }
+
+        if (!hitScrollbar) {
+          // The input was not targeted at a scrollbar, so we untransform it
+          // like we do for other content. Scrollbars are "special" because they
+          // have special handling in AsyncCompositionManager when resolution is
+          // applied. TODO: we should find a better way to deal with this.
+          ScreenToParentLayerMatrix4x4 transformToApzc =
+              GetScreenToApzcTransform(state.mHit.mTargetApzc);
+          ParentLayerToScreenMatrix4x4 transformToGecko =
+              GetApzcToGeckoTransformForHit(state.mHit);
+          ScreenToScreenMatrix4x4 outTransform =
+              transformToApzc * transformToGecko;
+          Maybe<ScreenPoint> untransformedRefPoint =
+              UntransformBy(outTransform, mouseInput.mOrigin);
+          if (untransformedRefPoint) {
+            mouseInput.mOrigin = *untransformedRefPoint;
+          }
+        } else {
+          // Likewise, if the input was targeted at a scrollbar, we don't want
+          // to apply the callback transform in the main thread, so we remove
+          // the scrollid from the guid. We need to keep the layersId intact so
+          // that the response from the child process doesn't get discarded.
+          state.mResult.mTargetGuid.mScrollId =
+              ScrollableLayerGuid::NULL_SCROLL_ID;
+        }
+      }
+      break;
+    }
+    case SCROLLWHEEL_INPUT: {
+      FlushRepaintsToClearScreenToGeckoTransform();
+
+      // Do this before early return for Fission hit testing.
+      ScrollWheelInput& wheelInput = aEvent.AsScrollWheelInput();
+      state.mHit = GetTargetAPZC(wheelInput.mOrigin);
+
+      wheelInput.mHandledByAPZ = WillHandleInput(wheelInput);
+      if (!wheelInput.mHandledByAPZ) {
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      mOvershootDetector.Update(wheelInput);
+
+      if (state.mHit.mTargetApzc) {
+        MOZ_ASSERT(state.mHit.mHitResult != CompositorHitTestInvisibleToHit);
+
+        if (wheelInput.mAPZAction == APZWheelAction::PinchZoom) {
+          // The mousewheel may have hit a subframe, but we want to send the
+          // pinch-zoom events to the root-content APZC.
+          {
+            RecursiveMutexAutoLock lock(mTreeLock);
+            state.mHit.mTargetApzc = FindRootContentApzcForLayersId(
+                state.mHit.mTargetApzc->GetLayersId());
+          }
+          if (state.mHit.mTargetApzc) {
+            SynthesizePinchGestureFromMouseWheel(wheelInput,
+                                                 state.mHit.mTargetApzc);
+          }
+          state.mResult.SetStatusAsConsumeNoDefault();
+          return state.Finish(*this, std::move(aCallback));
+        }
+
+        MOZ_ASSERT(wheelInput.mAPZAction == APZWheelAction::Scroll);
+
+        // For wheel events, the call to ReceiveInputEvent below may result in
+        // scrolling, which changes the async transform. However, the event we
+        // want to pass to gecko should be the pre-scroll event coordinates,
+        // transformed into the gecko space. (pre-scroll because the mouse
+        // cursor is stationary during wheel scrolling, unlike touchmove
+        // events). Since we just flushed the pending repaints the transform to
+        // gecko space should only consist of overscroll-cancelling transforms.
+        ScreenToScreenMatrix4x4 transformToGecko =
+            GetScreenToApzcTransform(state.mHit.mTargetApzc) *
+            GetApzcToGeckoTransformForHit(state.mHit);
+        Maybe<ScreenPoint> untransformedOrigin =
+            UntransformBy(transformToGecko, wheelInput.mOrigin);
+
+        if (!untransformedOrigin) {
+          return state.Finish(*this, std::move(aCallback));
+        }
+
+        state.mResult = mInputQueue->ReceiveInputEvent(
+            state.mHit.mTargetApzc,
+            TargetConfirmationFlags{state.mHit.mHitResult}, wheelInput);
+
+        // Update the out-parameters so they are what the caller expects.
+        wheelInput.mOrigin = *untransformedOrigin;
+      }
+      break;
+    }
+    case PANGESTURE_INPUT: {
+      FlushRepaintsToClearScreenToGeckoTransform();
+
+      // Do this before early return for Fission hit testing.
+      PanGestureInput& panInput = aEvent.AsPanGestureInput();
+      state.mHit = GetTargetAPZC(panInput.mPanStartPoint);
+
+      panInput.mHandledByAPZ = WillHandleInput(panInput);
+      if (!panInput.mHandledByAPZ) {
+        if (mInputQueue->GetCurrentPanGestureBlock()) {
+          if (state.mHit.mTargetApzc &&
+              (panInput.mType == PanGestureInput::PANGESTURE_END ||
+               panInput.mType == PanGestureInput::PANGESTURE_CANCELLED)) {
+            // If we've already been processing a pan gesture in an APZC but
+            // fall into this _if_ branch, which means this pan-end or
+            // pan-cancelled event will not be proccessed in the APZC, send a
+            // pan-interrupted event to stop any on-going work for the pan
+            // gesture, otherwise we will get stuck at an intermidiate state
+            // becasue we might no longer receive any events which will be
+            // handled by the APZC.
+            PanGestureInput panInterrupted(
+                PanGestureInput::PANGESTURE_INTERRUPTED, panInput.mTimeStamp,
+                panInput.mPanStartPoint, panInput.mPanDisplacement,
+                panInput.modifiers);
+            Unused << mInputQueue->ReceiveInputEvent(
+                state.mHit.mTargetApzc,
+                TargetConfirmationFlags{state.mHit.mHitResult}, panInterrupted);
+          }
+        }
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      // If/when we enable support for pan inputs off-main-thread, we'll need
+      // to duplicate this EventStateManager code or something. See the call to
+      // GetUserPrefsForWheelEvent in APZInputBridge.cpp for why these fields
+      // are stored separately.
+      MOZ_ASSERT(NS_IsMainThread());
+      WidgetWheelEvent wheelEvent = panInput.ToWidgetEvent(nullptr);
+      EventStateManager::GetUserPrefsForWheelEvent(
+          &wheelEvent, &panInput.mUserDeltaMultiplierX,
+          &panInput.mUserDeltaMultiplierY);
+
+      if (state.mHit.mTargetApzc) {
+        MOZ_ASSERT(state.mHit.mHitResult != CompositorHitTestInvisibleToHit);
+
+        // For pan gesture events, the call to ReceiveInputEvent below may
+        // result in scrolling, which changes the async transform. However, the
+        // event we want to pass to gecko should be the pre-scroll event
+        // coordinates, transformed into the gecko space. (pre-scroll because
+        // the mouse cursor is stationary during pan gesture scrolling, unlike
+        // touchmove events). Since we just flushed the pending repaints the
+        // transform to gecko space should only consist of overscroll-cancelling
+        // transforms.
+        ScreenToScreenMatrix4x4 transformToGecko =
+            GetScreenToApzcTransform(state.mHit.mTargetApzc) *
+            GetApzcToGeckoTransformForHit(state.mHit);
+        Maybe<ScreenPoint> untransformedStartPoint =
+            UntransformBy(transformToGecko, panInput.mPanStartPoint);
+        Maybe<ScreenPoint> untransformedDisplacement =
+            UntransformVector(transformToGecko, panInput.mPanDisplacement,
+                              panInput.mPanStartPoint);
+
+        if (!untransformedStartPoint || !untransformedDisplacement) {
+          return state.Finish(*this, std::move(aCallback));
+        }
+
+        panInput.mOverscrollBehaviorAllowsSwipe =
+            state.mHit.mTargetApzc->OverscrollBehaviorAllowsSwipe();
+
+        state.mResult = mInputQueue->ReceiveInputEvent(
+            state.mHit.mTargetApzc,
+            TargetConfirmationFlags{state.mHit.mHitResult}, panInput);
+
+        // Update the out-parameters so they are what the caller expects.
+        panInput.mPanStartPoint = *untransformedStartPoint;
+        panInput.mPanDisplacement = *untransformedDisplacement;
+      }
+      break;
+    }
+    case PINCHGESTURE_INPUT: {
+      PinchGestureInput& pinchInput = aEvent.AsPinchGestureInput();
+      if (HasNonLockModifier(pinchInput.modifiers)) {
+        APZCTM_LOG("Discarding pinch input due to modifiers 0x%x\n",
+                   pinchInput.modifiers);
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      state.mHit = GetTargetAPZC(pinchInput.mFocusPoint);
+
+      // We always handle pinch gestures as pinch zooms.
+      pinchInput.mHandledByAPZ = true;
+
+      if (state.mHit.mTargetApzc) {
+        MOZ_ASSERT(state.mHit.mHitResult != CompositorHitTestInvisibleToHit);
+
+        if (!state.mHit.mTargetApzc->IsRootContent()) {
+          state.mHit.mTargetApzc = FindZoomableApzc(state.mHit.mTargetApzc);
+        }
+      }
+
+      if (state.mHit.mTargetApzc) {
+        ScreenToScreenMatrix4x4 outTransform =
+            GetScreenToApzcTransform(state.mHit.mTargetApzc) *
+            GetApzcToGeckoTransformForHit(state.mHit);
+        Maybe<ScreenPoint> untransformedFocusPoint =
+            UntransformBy(outTransform, pinchInput.mFocusPoint);
+
+        if (!untransformedFocusPoint) {
+          return state.Finish(*this, std::move(aCallback));
+        }
+
+        state.mResult = mInputQueue->ReceiveInputEvent(
+            state.mHit.mTargetApzc,
+            TargetConfirmationFlags{state.mHit.mHitResult}, pinchInput);
+
+        // Update the out-parameters so they are what the caller expects.
+        pinchInput.mFocusPoint = *untransformedFocusPoint;
+      }
+      break;
+    }
+    case TAPGESTURE_INPUT: {  // note: no one currently sends these
+      TapGestureInput& tapInput = aEvent.AsTapGestureInput();
+      state.mHit = GetTargetAPZC(tapInput.mPoint);
+
+      if (state.mHit.mTargetApzc) {
+        MOZ_ASSERT(state.mHit.mHitResult != CompositorHitTestInvisibleToHit);
+
+        ScreenToScreenMatrix4x4 outTransform =
+            GetScreenToApzcTransform(state.mHit.mTargetApzc) *
+            GetApzcToGeckoTransformForHit(state.mHit);
+        Maybe<ScreenIntPoint> untransformedPoint =
+            UntransformBy(outTransform, tapInput.mPoint);
+
+        if (!untransformedPoint) {
+          return state.Finish(*this, std::move(aCallback));
+        }
+
+        // Tap gesture events are not grouped into input blocks, and they're
+        // never queued in InputQueue, but processed right away. So, we only
+        // need to set |mTapGestureHitResult| for the duration of the
+        // InputQueue::ReceiveInputEvent() call.
+        {
+          RecursiveMutexAutoLock lock(mTreeLock);
+          mTapGestureHitResult =
+              mHitTester->CloneHitTestResult(lock, state.mHit);
+        }
+
+        state.mResult = mInputQueue->ReceiveInputEvent(
+            state.mHit.mTargetApzc,
+            TargetConfirmationFlags{state.mHit.mHitResult}, tapInput);
+
+        mTapGestureHitResult = HitTestResult();
+
+        // Update the out-parameters so they are what the caller expects.
+        tapInput.mPoint = *untransformedPoint;
+      }
+      break;
+    }
+    case KEYBOARD_INPUT: {
+      // Disable async keyboard scrolling when accessibility.browsewithcaret is
+      // enabled
+      if (!StaticPrefs::apz_keyboard_enabled_AtStartup() ||
+          StaticPrefs::accessibility_browsewithcaret()) {
+        APZ_KEY_LOG("Skipping key input from invalid prefs\n");
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      KeyboardInput& keyInput = aEvent.AsKeyboardInput();
+
+      // Try and find a matching shortcut for this keyboard input
+      Maybe<KeyboardShortcut> shortcut = mKeyboardMap.FindMatch(keyInput);
+
+      if (!shortcut) {
+        APZ_KEY_LOG("Skipping key input with no shortcut\n");
+
+        // If we don't have a shortcut for this key event, then we can keep our
+        // focus only if we know there are no key event listeners for this
+        // target
+        if (mFocusState.CanIgnoreKeyboardShortcutMisses()) {
+          focusSetter.MarkAsNonFocusChanging();
+        }
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      // Check if this shortcut needs to be dispatched to content. Anything
+      // matching this is assumed to be able to change focus.
+      if (shortcut->mDispatchToContent) {
+        APZ_KEY_LOG("Skipping key input with dispatch-to-content shortcut\n");
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      // We know we have an action to execute on whatever is the current focus
+      // target
+      const KeyboardScrollAction& action = shortcut->mAction;
+
+      // The current focus target depends on which direction the scroll is to
+      // happen
+      Maybe<ScrollableLayerGuid> targetGuid;
+      switch (action.mType) {
+        case KeyboardScrollAction::eScrollCharacter: {
+          targetGuid = mFocusState.GetHorizontalTarget();
+          break;
+        }
+        case KeyboardScrollAction::eScrollLine:
+        case KeyboardScrollAction::eScrollPage:
+        case KeyboardScrollAction::eScrollComplete: {
+          targetGuid = mFocusState.GetVerticalTarget();
+          break;
+        }
+      }
+
+      // If we don't have a scroll target then either we have a stale focus
+      // target, the focused element has event listeners, or the focused element
+      // doesn't have a layerized scroll frame. In any case we need to dispatch
+      // to content.
+      if (!targetGuid) {
+        APZ_KEY_LOG("Skipping key input with no current focus target\n");
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      RefPtr<AsyncPanZoomController> targetApzc =
+          GetTargetAPZC(targetGuid->mLayersId, targetGuid->mScrollId);
+
+      if (!targetApzc) {
+        APZ_KEY_LOG("Skipping key input with focus target but no APZC\n");
+        return state.Finish(*this, std::move(aCallback));
+      }
+
+      // Attach the keyboard scroll action to the input event for processing
+      // by the input queue.
+      keyInput.mAction = action;
+
+      APZ_KEY_LOG("Dispatching key input with apzc=%p\n", targetApzc.get());
+
+      // Dispatch the event to the input queue.
+      state.mResult = mInputQueue->ReceiveInputEvent(
+          targetApzc, TargetConfirmationFlags{true}, keyInput);
+
+      // Any keyboard event that is dispatched to the input queue at this point
+      // should have been consumed
+      MOZ_ASSERT(state.mResult.GetStatus() == nsEventStatus_eConsumeDoDefault ||
+                 state.mResult.GetStatus() == nsEventStatus_eConsumeNoDefault);
+
+      keyInput.mHandledByAPZ = true;
+      focusSetter.MarkAsNonFocusChanging();
+
+      break;
+    }
+  }
+  return state.Finish(*this, std::move(aCallback));
+}
+
+static TouchBehaviorFlags ConvertToTouchBehavior(
+    const CompositorHitTestInfo& info) {
+  TouchBehaviorFlags result = AllowedTouchBehavior::UNKNOWN;
+  if (info == CompositorHitTestInvisibleToHit) {
+    result = AllowedTouchBehavior::NONE;
+  } else if (info.contains(CompositorHitTestFlags::eIrregularArea)) {
+    // Note that eApzAwareListeners and eInactiveScrollframe are similar
+    // to eIrregularArea in some respects, but are not relevant for the
+    // purposes of this function, which deals specifically with touch-action.
+    result = AllowedTouchBehavior::UNKNOWN;
+  } else {
+    result = AllowedTouchBehavior::VERTICAL_PAN |
+             AllowedTouchBehavior::HORIZONTAL_PAN |
+             AllowedTouchBehavior::PINCH_ZOOM |
+             AllowedTouchBehavior::ANIMATING_ZOOM;
+    if (info.contains(CompositorHitTestFlags::eTouchActionPanXDisabled)) {
+      result &= ~AllowedTouchBehavior::HORIZONTAL_PAN;
+    }
+    if (info.contains(CompositorHitTestFlags::eTouchActionPanYDisabled)) {
+      result &= ~AllowedTouchBehavior::VERTICAL_PAN;
+    }
+    if (info.contains(CompositorHitTestFlags::eTouchActionPinchZoomDisabled)) {
+      result &= ~AllowedTouchBehavior::PINCH_ZOOM;
+    }
+    if (info.contains(
+            CompositorHitTestFlags::eTouchActionAnimatingZoomDisabled)) {
+      result &= ~AllowedTouchBehavior::ANIMATING_ZOOM;
+    }
+  }
+  return result;
+}
+
+APZCTreeManager::HitTestResult APZCTreeManager::GetTouchInputBlockAPZC(
+    const MultiTouchInput& aEvent,
+    nsTArray<TouchBehaviorFlags>* aOutTouchBehaviors) {
+  HitTestResult hit;
+  if (aEvent.mTouches.Length() == 0) {
+    return hit;
+  }
+
+  FlushRepaintsToClearScreenToGeckoTransform();
+
+  hit = GetTargetAPZC(aEvent.mTouches[0].mScreenPoint);
+  // Don't set a layers id on multi-touch events.
+  if (aEvent.mTouches.Length() != 1) {
+    hit.mLayersId = LayersId{0};
+  }
+
+  if (aOutTouchBehaviors) {
+    aOutTouchBehaviors->AppendElement(ConvertToTouchBehavior(hit.mHitResult));
+  }
+  for (size_t i = 1; i < aEvent.mTouches.Length(); i++) {
+    HitTestResult hit2 = GetTargetAPZC(aEvent.mTouches[i].mScreenPoint);
+    if (aOutTouchBehaviors) {
+      aOutTouchBehaviors->AppendElement(
+          ConvertToTouchBehavior(hit2.mHitResult));
+    }
+    hit.mTargetApzc = GetZoomableTarget(hit.mTargetApzc, hit2.mTargetApzc);
+    APZCTM_LOG("Using APZC %p as the root APZC for multi-touch\n",
+               hit.mTargetApzc.get());
+    // A multi-touch gesture will not be a scrollbar drag, even if the
+    // first touch point happened to hit a scrollbar.
+    hit.mScrollbarNode.Clear();
+
+    // XXX we should probably be combining the hit results from the different
+    // touch points somehow, instead of just using the last one.
+    hit.mHitResult = hit2.mHitResult;
+  }
+
+  return hit;
+}
+
+APZEventResult APZCTreeManager::InputHandlingState::Finish(
+    APZCTreeManager& aTreeManager, InputBlockCallback&& aCallback) {
+  // The validity check here handles both the case where mHit was
+  // never populated (because this event did not trigger a hit-test),
+  // and the case where it was populated with an invalid LayersId
+  // (which can happen e.g. for multi-touch events).
+  if (mHit.mLayersId.IsValid()) {
+    mEvent.mLayersId = mHit.mLayersId;
+  }
+
+  // Absorb events that are in targetted at a position in the gutter,
+  // unless they are fixed position elements.
+  if (mHit.mHitOverscrollGutter && mHit.mFixedPosSides == SideBits::eNone) {
+    mResult.SetStatusAsConsumeNoDefault();
+  }
+
+  // If the event will have a delayed result then add the callback to the
+  // APZCTreeManager.
+  if (aCallback && mResult.WillHaveDelayedResult()) {
+    aTreeManager.AddInputBlockCallback(
+        mResult.mInputBlockId, {mResult.GetStatus(), std::move(aCallback)});
+  }
+
+  return mResult;
+}
+
+void APZCTreeManager::ProcessTouchInput(InputHandlingState& aState,
+                                        MultiTouchInput& aInput) {
+  aInput.mHandledByAPZ = true;
+  nsTArray<TouchBehaviorFlags> touchBehaviors;
+  HitTestingTreeNodeAutoLock hitScrollbarNode;
+  if (aInput.mType == MultiTouchInput::MULTITOUCH_START) {
+    // If we are panned into overscroll and a second finger goes down,
+    // ignore that second touch point completely. The touch-start for it is
+    // dropped completely; subsequent touch events until the touch-end for it
+    // will have this touch point filtered out.
+    // (By contrast, if we're in overscroll but not panning, such as after
+    // putting two fingers down during an overscroll animation, we process the
+    // second touch and proceed to pinch.)
+    if (mTouchBlockHitResult.mTargetApzc &&
+        mTouchBlockHitResult.mTargetApzc->IsInPanningState() &&
+        BuildOverscrollHandoffChain(mTouchBlockHitResult.mTargetApzc)
+            ->HasOverscrolledApzc()) {
+      if (mRetainedTouchIdentifier == -1) {
+        mRetainedTouchIdentifier =
+            mTouchBlockHitResult.mTargetApzc->GetLastTouchIdentifier();
+      }
+
+      aState.mResult.SetStatusAsConsumeNoDefault();
+      return;
+    }
+
+    aState.mHit = GetTouchInputBlockAPZC(aInput, &touchBehaviors);
+    RecursiveMutexAutoLock lock(mTreeLock);
+    // Repopulate mTouchBlockHitResult from the input state.
+    mTouchBlockHitResult = mHitTester->CloneHitTestResult(lock, aState.mHit);
+    hitScrollbarNode = std::move(aState.mHit.mScrollbarNode);
+
+    // Check if this event starts a scrollbar touch-drag. The conditions
+    // checked are similar to the ones we check for MOUSE_INPUT starting
+    // a scrollbar mouse-drag.
+    mInScrollbarTouchDrag =
+        StaticPrefs::apz_drag_enabled() &&
+        StaticPrefs::apz_drag_touch_enabled() && hitScrollbarNode &&
+        hitScrollbarNode->IsScrollThumbNode() &&
+        hitScrollbarNode->GetScrollbarData().mThumbIsAsyncDraggable;
+
+    MOZ_ASSERT(touchBehaviors.Length() == aInput.mTouches.Length());
+    for (size_t i = 0; i < touchBehaviors.Length(); i++) {
+      APZCTM_LOG("Touch point has allowed behaviours 0x%02x\n",
+                 touchBehaviors[i]);
+      if (touchBehaviors[i] == AllowedTouchBehavior::UNKNOWN) {
+        // If there's any unknown items in the list, throw it out and we'll
+        // wait for the main thread to send us a notification.
+        touchBehaviors.Clear();
+        break;
+      }
+    }
+  } else if (mTouchBlockHitResult.mTargetApzc) {
+    APZCTM_LOG("Re-using APZC %p as continuation of event block\n",
+               mTouchBlockHitResult.mTargetApzc.get());
+    RecursiveMutexAutoLock lock(mTreeLock);
+    aState.mHit = mHitTester->CloneHitTestResult(lock, mTouchBlockHitResult);
+  }
+
+  if (mInScrollbarTouchDrag) {
+    aState.mResult = ProcessTouchInputForScrollbarDrag(
+        aInput, hitScrollbarNode, mTouchBlockHitResult.mHitResult);
+  } else {
+    // If we receive a touch-cancel, it means all touches are finished, so we
+    // can stop ignoring any that we were ignoring.
+    if (aInput.mType == MultiTouchInput::MULTITOUCH_CANCEL) {
+      mRetainedTouchIdentifier = -1;
+    }
+
+    // If we are currently ignoring any touch points, filter them out from the
+    // set of touch points included in this event. Note that we modify aInput
+    // itself, so that the touch points are also filtered out when the caller
+    // passes the event on to content.
+    if (mRetainedTouchIdentifier != -1) {
+      for (size_t j = 0; j < aInput.mTouches.Length(); ++j) {
+        if (aInput.mTouches[j].mIdentifier != mRetainedTouchIdentifier) {
+          aInput.mTouches.RemoveElementAt(j);
+          if (!touchBehaviors.IsEmpty()) {
+            MOZ_ASSERT(touchBehaviors.Length() > j);
+            touchBehaviors.RemoveElementAt(j);
+          }
+          --j;
+        }
+      }
+      if (aInput.mTouches.IsEmpty()) {
+        aState.mResult.SetStatusAsConsumeNoDefault();
+        return;
+      }
+    }
+
+    if (mTouchBlockHitResult.mTargetApzc) {
+      MOZ_ASSERT(mTouchBlockHitResult.mHitResult !=
+                 CompositorHitTestInvisibleToHit);
+
+      aState.mResult = mInputQueue->ReceiveInputEvent(
+          mTouchBlockHitResult.mTargetApzc,
+          TargetConfirmationFlags{mTouchBlockHitResult.mHitResult}, aInput,
+          touchBehaviors.IsEmpty() ? Nothing()
+                                   : Some(std::move(touchBehaviors)));
+
+      // For computing the event to pass back to Gecko, use up-to-date
+      // transforms (i.e. not anything cached in an input block). This ensures
+      // that transformToApzc and transformToGecko are in sync.
+      // Note: we are not using ConvertToGecko() here, because we don't
+      // want to multiply transformToApzc and transformToGecko once
+      // for each touch point.
+      ScreenToParentLayerMatrix4x4 transformToApzc =
+          GetScreenToApzcTransform(mTouchBlockHitResult.mTargetApzc);
+      ParentLayerToScreenMatrix4x4 transformToGecko =
+          GetApzcToGeckoTransformForHit(mTouchBlockHitResult);
+      ScreenToScreenMatrix4x4 outTransform = transformToApzc * transformToGecko;
+
+      for (size_t i = 0; i < aInput.mTouches.Length(); i++) {
+        SingleTouchData& touchData = aInput.mTouches[i];
+        Maybe<ScreenIntPoint> untransformedScreenPoint =
+            UntransformBy(outTransform, touchData.mScreenPoint);
+        if (!untransformedScreenPoint) {
+          aState.mResult.SetStatusAsIgnore();
+          return;
+        }
+        touchData.mScreenPoint = *untransformedScreenPoint;
+        AdjustEventPointForDynamicToolbar(touchData.mScreenPoint,
+                                          mTouchBlockHitResult);
+      }
+    }
+  }
+
+  mTouchCounter.Update(aInput);
+
+  // If it's the end of the touch sequence then clear out variables so we
+  // don't keep dangling references and leak things.
+  if (mTouchCounter.GetActiveTouchCount() == 0) {
+    mTouchBlockHitResult = HitTestResult();
+    mRetainedTouchIdentifier = -1;
+    mInScrollbarTouchDrag = false;
+  }
+}
+
+void APZCTreeManager::AdjustEventPointForDynamicToolbar(
+    ScreenIntPoint& aEventPoint, const HitTestResult& aHit) {
+  if (aHit.mFixedPosSides != SideBits::eNone) {
+    MutexAutoLock lock(mMapLock);
+    aEventPoint -= RoundedToInt(apz::ComputeFixedMarginsOffset(
+        GetCompositorFixedLayerMargins(lock), aHit.mFixedPosSides,
+        mGeckoFixedLayerMargins));
+  } else if (aHit.mNode && aHit.mNode->GetStickyPositionAnimationId()) {
+    SideBits sideBits = SideBits::eNone;
+    {
+      RecursiveMutexAutoLock lock(mTreeLock);
+      sideBits = SidesStuckToRootContent(
+          aHit.mNode.Get(lock), AsyncTransformConsumer::eForEventHandling);
+    }
+    MutexAutoLock lock(mMapLock);
+    aEventPoint -= RoundedToInt(apz::ComputeFixedMarginsOffset(
+        GetCompositorFixedLayerMargins(lock), sideBits, ScreenMargin()));
+  }
+}
+
+static MouseInput::MouseType MultiTouchTypeToMouseType(
+    MultiTouchInput::MultiTouchType aType) {
+  switch (aType) {
+    case MultiTouchInput::MULTITOUCH_START:
+      return MouseInput::MOUSE_DOWN;
+    case MultiTouchInput::MULTITOUCH_MOVE:
+      return MouseInput::MOUSE_MOVE;
+    case MultiTouchInput::MULTITOUCH_END:
+    case MultiTouchInput::MULTITOUCH_CANCEL:
+      return MouseInput::MOUSE_UP;
+  }
+  MOZ_ASSERT_UNREACHABLE("Invalid multi-touch type");
+  return MouseInput::MOUSE_NONE;
+}
+
+APZEventResult APZCTreeManager::ProcessTouchInputForScrollbarDrag(
+    MultiTouchInput& aTouchInput,
+    const HitTestingTreeNodeAutoLock& aScrollThumbNode,
+    const gfx::CompositorHitTestInfo& aHitInfo) {
+  MOZ_ASSERT(mRetainedTouchIdentifier == -1);
+  MOZ_ASSERT(mTouchBlockHitResult.mTargetApzc);
+  MOZ_ASSERT(aTouchInput.mTouches.Length() == 1);
+
+  // Synthesize a mouse event based on the touch event, so that we can
+  // reuse code in InputQueue and APZC for handling scrollbar mouse-drags.
+  MouseInput mouseInput{MultiTouchTypeToMouseType(aTouchInput.mType),
+                        MouseInput::PRIMARY_BUTTON,
+                        dom::MouseEvent_Binding::MOZ_SOURCE_TOUCH,
+                        MouseButtonsFlag::ePrimaryFlag,
+                        aTouchInput.mTouches[0].mScreenPoint,
+                        aTouchInput.mTimeStamp,
+                        aTouchInput.modifiers};
+  mouseInput.mHandledByAPZ = true;
+
+  TargetConfirmationFlags targetConfirmed{aHitInfo};
+  APZEventResult result;
+  result = mInputQueue->ReceiveInputEvent(mTouchBlockHitResult.mTargetApzc,
+                                          targetConfirmed, mouseInput);
+
+  // |aScrollThumbNode| is non-null iff. this is the event that starts the drag.
+  // If so, set up the drag.
+  if (aScrollThumbNode) {
+    SetupScrollbarDrag(mouseInput, aScrollThumbNode,
+                       mTouchBlockHitResult.mTargetApzc.get());
+  }
+
+  // Since the input was targeted at a scrollbar:
+  //    - The original touch event (which will be sent on to content) will
+  //      not be untransformed.
+  //    - We don't want to apply the callback transform in the main thread,
+  //      so we remove the scrollid from the guid.
+  // Both of these match the behaviour of mouse events that target a scrollbar;
+  // see the code for handling mouse events in ReceiveInputEvent() for
+  // additional explanation.
+  result.mTargetGuid.mScrollId = ScrollableLayerGuid::NULL_SCROLL_ID;
+
+  return result;
+}
+
+void APZCTreeManager::SetupScrollbarDrag(
+    MouseInput& aMouseInput, const HitTestingTreeNodeAutoLock& aScrollThumbNode,
+    AsyncPanZoomController* aApzc) {
+  DragBlockState* dragBlock = mInputQueue->GetCurrentDragBlock();
+  if (!dragBlock) {
+    return;
+  }
+
+  const ScrollbarData& thumbData = aScrollThumbNode->GetScrollbarData();
+  MOZ_ASSERT(thumbData.mDirection.isSome());
+
+  // Record the thumb's position at the start of the drag.
+  // We snap back to this position if, during the drag, the mouse
+  // gets sufficiently far away from the scrollbar.
+  dragBlock->SetInitialThumbPos(thumbData.mThumbStart);
+
+  // Under some conditions, we can confirm the drag block right away.
+  // Otherwise, we have to wait for a main-thread confirmation.
+  if (StaticPrefs::apz_drag_initial_enabled() &&
+      // check that the scrollbar's target scroll frame is layerized
+      aScrollThumbNode->GetScrollTargetId() == aApzc->GetGuid().mScrollId &&
+      !aApzc->IsScrollInfoLayer()) {
+    uint64_t dragBlockId = dragBlock->GetBlockId();
+    // AsyncPanZoomController::HandleInputEvent() will call
+    // TransformToLocal() on the event, but we need its mLocalOrigin now
+    // to compute a drag start offset for the AsyncDragMetrics.
+    aMouseInput.TransformToLocal(aApzc->GetTransformToThis());
+    OuterCSSCoord dragStart =
+        aApzc->ConvertScrollbarPoint(aMouseInput.mLocalOrigin, thumbData);
+    // ConvertScrollbarPoint() got the drag start offset relative to
+    // the scroll track. Now get it relative to the thumb.
+    // ScrollThumbData::mThumbStart stores the offset of the thumb
+    // relative to the scroll track at the time of the last paint.
+    // Since that paint, the thumb may have acquired an async transform
+    // due to async scrolling, so look that up and apply it.
+    LayerToParentLayerMatrix4x4 thumbTransform;
+    {
+      RecursiveMutexAutoLock lock(mTreeLock);
+      thumbTransform =
+          ComputeTransformForScrollThumbNode(aScrollThumbNode.Get(lock));
+    }
+    // Only consider the translation, since we do not support both
+    // zooming and scrollbar dragging on any platform.
+    // FIXME: We do now.
+    OuterCSSCoord thumbStart =
+        thumbData.mThumbStart +
+        ((*thumbData.mDirection == ScrollDirection::eHorizontal)
+             ? thumbTransform._41
+             : thumbTransform._42);
+    dragStart -= thumbStart;
+
+    // Content can't prevent scrollbar dragging with preventDefault(),
+    // so we don't need to wait for a content response. It's important
+    // to do this before calling ConfirmDragBlock() since that can
+    // potentially process and consume the block.
+    dragBlock->SetContentResponse(false);
+
+    NotifyScrollbarDragInitiated(dragBlockId, aApzc->GetGuid(),
+                                 *thumbData.mDirection);
+
+    mInputQueue->ConfirmDragBlock(
+        dragBlockId, aApzc,
+        AsyncDragMetrics(aApzc->GetGuid().mScrollId,
+                         aApzc->GetGuid().mPresShellId, dragBlockId, dragStart,
+                         *thumbData.mDirection));
+  }
+}
+
+void APZCTreeManager::SynthesizePinchGestureFromMouseWheel(
+    const ScrollWheelInput& aWheelInput,
+    const RefPtr<AsyncPanZoomController>& aTarget) {
+  MOZ_ASSERT(aTarget);
+
+  ScreenPoint focusPoint = aWheelInput.mOrigin;
+
+  // Compute span values based on the wheel delta.
+  ScreenCoord oldSpan = 100;
+  ScreenCoord newSpan = oldSpan + aWheelInput.mDeltaY;
+
+  // There's no ambiguity as to the target for pinch gesture events.
+  TargetConfirmationFlags confFlags{true};
+
+  PinchGestureInput pinchStart{PinchGestureInput::PINCHGESTURE_START,
+                               PinchGestureInput::MOUSEWHEEL,
+                               aWheelInput.mTimeStamp,
+                               ExternalPoint(0, 0),
+                               focusPoint,
+                               oldSpan,
+                               oldSpan,
+                               aWheelInput.modifiers};
+  PinchGestureInput pinchScale1{PinchGestureInput::PINCHGESTURE_SCALE,
+                                PinchGestureInput::MOUSEWHEEL,
+                                aWheelInput.mTimeStamp,
+                                ExternalPoint(0, 0),
+                                focusPoint,
+                                oldSpan,
+                                oldSpan,
+                                aWheelInput.modifiers};
+  PinchGestureInput pinchScale2{PinchGestureInput::PINCHGESTURE_SCALE,
+                                PinchGestureInput::MOUSEWHEEL,
+                                aWheelInput.mTimeStamp,
+                                ExternalPoint(0, 0),
+                                focusPoint,
+                                oldSpan,
+                                newSpan,
+                                aWheelInput.modifiers};
+  PinchGestureInput pinchEnd{PinchGestureInput::PINCHGESTURE_END,
+                             PinchGestureInput::MOUSEWHEEL,
+                             aWheelInput.mTimeStamp,
+                             ExternalPoint(0, 0),
+                             focusPoint,
+                             newSpan,
+                             newSpan,
+                             aWheelInput.modifiers};
+
+  mInputQueue->ReceiveInputEvent(aTarget, confFlags, pinchStart);
+  mInputQueue->ReceiveInputEvent(aTarget, confFlags, pinchScale1);
+  mInputQueue->ReceiveInputEvent(aTarget, confFlags, pinchScale2);
+  mInputQueue->ReceiveInputEvent(aTarget, confFlags, pinchEnd);
+}
+
+void APZCTreeManager::UpdateWheelTransaction(
+    LayoutDeviceIntPoint aRefPoint, EventMessage aEventMessage,
+    const Maybe<ScrollableLayerGuid>& aTargetGuid) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  WheelBlockState* txn = mInputQueue->GetActiveWheelTransaction();
+  if (!txn) {
+    return;
+  }
+
+  // If the transaction has simply timed out, we don't need to do anything
+  // else.
+  if (txn->MaybeTimeout(TimeStamp::Now())) {
+    return;
+  }
+
+  switch (aEventMessage) {
+    case eMouseMove:
+    case eDragOver: {
+      ScreenIntPoint point = ViewAs<ScreenPixel>(
+          aRefPoint,
+          PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent);
+
+      txn->OnMouseMove(point, aTargetGuid);
+
+      return;
+    }
+    case eKeyPress:
+    case eKeyUp:
+    case eKeyDown:
+    case eMouseUp:
+    case eMouseDown:
+    case eMouseDoubleClick:
+    case eMouseAuxClick:
+    case eMouseClick:
+    case eContextMenu:
+    case eDrop:
+      txn->EndTransaction();
+      return;
+    default:
+      break;
+  }
+}
+
+void APZCTreeManager::ProcessUnhandledEvent(LayoutDeviceIntPoint* aRefPoint,
+                                            ScrollableLayerGuid* aOutTargetGuid,
+                                            uint64_t* aOutFocusSequenceNumber,
+                                            LayersId* aOutLayersId) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  // Transform the aRefPoint.
+  // If the event hits an overscrolled APZC, instruct the caller to ignore it.
+  PixelCastJustification LDIsScreen =
+      PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent;
+  ScreenIntPoint refPointAsScreen = ViewAs<ScreenPixel>(*aRefPoint, LDIsScreen);
+  HitTestResult hit = GetTargetAPZC(refPointAsScreen);
+  if (aOutLayersId) {
+    *aOutLayersId = hit.mLayersId;
+  }
+  if (hit.mTargetApzc) {
+    MOZ_ASSERT(hit.mHitResult != CompositorHitTestInvisibleToHit);
+    hit.mTargetApzc->GetGuid(aOutTargetGuid);
+    ScreenToParentLayerMatrix4x4 transformToApzc =
+        GetScreenToApzcTransform(hit.mTargetApzc);
+    ParentLayerToScreenMatrix4x4 transformToGecko =
+        GetApzcToGeckoTransformForHit(hit);
+    ScreenToScreenMatrix4x4 outTransform = transformToApzc * transformToGecko;
+    Maybe<ScreenIntPoint> untransformedRefPoint =
+        UntransformBy(outTransform, refPointAsScreen);
+    if (untransformedRefPoint) {
+      *aRefPoint =
+          ViewAs<LayoutDevicePixel>(*untransformedRefPoint, LDIsScreen);
+    }
+  }
+
+  // Update the focus sequence number and attach it to the event
+  mFocusState.ReceiveFocusChangingEvent();
+  *aOutFocusSequenceNumber = mFocusState.LastAPZProcessedEvent();
+}
+
+void APZCTreeManager::SetKeyboardMap(const KeyboardMap& aKeyboardMap) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(NewRunnableMethod<KeyboardMap>(
+        "layers::APZCTreeManager::SetKeyboardMap", this,
+        &APZCTreeManager::SetKeyboardMap, aKeyboardMap));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+
+  mKeyboardMap = aKeyboardMap;
+}
+
+void APZCTreeManager::ZoomToRect(const ScrollableLayerGuid& aGuid,
+                                 const ZoomTarget& aZoomTarget,
+                                 const uint32_t aFlags) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<ScrollableLayerGuid, ZoomTarget, uint32_t>(
+            "layers::APZCTreeManager::ZoomToRect", this,
+            &APZCTreeManager::ZoomToRect, aGuid, aZoomTarget, aFlags));
+    return;
+  }
+
+  // We could probably move this to run on the updater thread if needed, but
+  // either way we should restrict it to a single thread. For now let's use the
+  // controller thread.
+  APZThreadUtils::AssertOnControllerThread();
+
+  RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aGuid);
+  if (aFlags & ZOOM_TO_FOCUSED_INPUT) {
+    // In the case of ZoomToFocusInput the targetRect is in the in-process root
+    // content coordinates. We need to convert it to the top level content
+    // coordiates.
+    if (apzc) {
+      CSSRect transformedRect =
+          ConvertRectInApzcToRoot(apzc, aZoomTarget.targetRect);
+
+      transformedRect.Inflate(15.0f, 0.0f);
+      // Note that ZoomToFocusedInput doesn't use the other fields of
+      // ZoomTarget.
+      ZoomTarget zoomTarget{transformedRect};
+
+      apzc = FindZoomableApzc(apzc);
+      if (apzc) {
+        apzc->ZoomToRect(zoomTarget, aFlags);
+      }
+    }
+    return;
+  }
+
+  if (apzc) {
+    apzc = FindZoomableApzc(apzc);
+    if (apzc) {
+      apzc->ZoomToRect(aZoomTarget, aFlags);
+    }
+  }
+}
+
+void APZCTreeManager::ContentReceivedInputBlock(uint64_t aInputBlockId,
+                                                bool aPreventDefault) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(NewRunnableMethod<uint64_t, bool>(
+        "layers::APZCTreeManager::ContentReceivedInputBlock", this,
+        &APZCTreeManager::ContentReceivedInputBlock, aInputBlockId,
+        aPreventDefault));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+
+  mInputQueue->ContentReceivedInputBlock(aInputBlockId, aPreventDefault);
+}
+
+void APZCTreeManager::SetTargetAPZC(
+    uint64_t aInputBlockId, const nsTArray<ScrollableLayerGuid>& aTargets) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<uint64_t,
+                          StoreCopyPassByRRef<nsTArray<ScrollableLayerGuid>>>(
+            "layers::APZCTreeManager::SetTargetAPZC", this,
+            &layers::APZCTreeManager::SetTargetAPZC, aInputBlockId,
+            aTargets.Clone()));
+    return;
+  }
+
+  RefPtr<AsyncPanZoomController> target = nullptr;
+  if (aTargets.Length() > 0) {
+    target = GetTargetAPZC(aTargets[0]);
+  }
+  for (size_t i = 1; i < aTargets.Length(); i++) {
+    RefPtr<AsyncPanZoomController> apzc = GetTargetAPZC(aTargets[i]);
+    target = GetZoomableTarget(target, apzc);
+  }
+  if (InputBlockState* block = mInputQueue->GetBlockForId(aInputBlockId)) {
+    if (block->AsPinchGestureBlock() && aTargets.Length() == 1) {
+      target = FindZoomableApzc(target);
+    }
+  }
+  mInputQueue->SetConfirmedTargetApzc(aInputBlockId, target);
+}
+
+void APZCTreeManager::UpdateZoomConstraints(
+    const ScrollableLayerGuid& aGuid,
+    const Maybe<ZoomConstraints>& aConstraints) {
+  if (!GetUpdater()->IsUpdaterThread()) {
+    // This can happen if we're in the UI process and got a call directly from
+    // nsBaseWidget or from a content process over PAPZCTreeManager. In that
+    // case we get this call on the compositor thread, which may be different
+    // from the updater thread. It can also happen in the GPU process if that is
+    // enabled, since the call will go over PAPZCTreeManager and arrive on the
+    // compositor thread in the GPU process.
+    GetUpdater()->RunOnUpdaterThread(
+        aGuid.mLayersId,
+        NewRunnableMethod<ScrollableLayerGuid, Maybe<ZoomConstraints>>(
+            "APZCTreeManager::UpdateZoomConstraints", this,
+            &APZCTreeManager::UpdateZoomConstraints, aGuid, aConstraints));
+    return;
+  }
+
+  AssertOnUpdaterThread();
+
+  // Propagate the zoom constraints down to the subtree, stopping at APZCs
+  // which have their own zoom constraints or are in a different layers id.
+  if (aConstraints) {
+    APZCTM_LOG("Recording constraints %s for guid %s\n",
+               ToString(aConstraints.value()).c_str(), ToString(aGuid).c_str());
+    mZoomConstraints[aGuid] = aConstraints.ref();
+  } else {
+    APZCTM_LOG("Removing constraints for guid %s\n", ToString(aGuid).c_str());
+    mZoomConstraints.erase(aGuid);
+  }
+
+  RecursiveMutexAutoLock lock(mTreeLock);
+  RefPtr<HitTestingTreeNode> node = DepthFirstSearchPostOrder<ReverseIterator>(
+      mRootNode.get(), [&aGuid](HitTestingTreeNode* aNode) {
+        bool matches = false;
+        if (auto zoomId = aNode->GetAsyncZoomContainerId()) {
+          matches = ScrollableLayerGuid::EqualsIgnoringPresShell(
+              aGuid, ScrollableLayerGuid(aNode->GetLayersId(), 0, *zoomId));
+        }
+        return matches;
+      });
+
+  // This does not hold because we can get zoom constraints updates before the
+  // layer tree update with the async zoom container (I assume).
+  // clang-format off
+  // MOZ_ASSERT(node || aConstraints.isNothing() ||
+  //           (!aConstraints->mAllowZoom && !aConstraints->mAllowDoubleTapZoom));
+  // clang-format on
+
+  // If there is no async zoom container then the zoom constraints should not
+  // allow zooming and building the HTT should have handled clearing the zoom
+  // constraints from all nodes so we don't have to handle doing anything in
+  // case there is no async zoom container.
+
+  if (node && aConstraints) {
+    ForEachNode<ReverseIterator>(node.get(), [&aConstraints, &node, &aGuid,
+                                              this](HitTestingTreeNode* aNode) {
+      if (aNode != node) {
+        // don't go into other async zoom containers
+        if (auto zoomId = aNode->GetAsyncZoomContainerId()) {
+          MOZ_ASSERT(!ScrollableLayerGuid::EqualsIgnoringPresShell(
+              aGuid, ScrollableLayerGuid(aNode->GetLayersId(), 0, *zoomId)));
+          return TraversalFlag::Skip;
+        }
+        if (AsyncPanZoomController* childApzc = aNode->GetApzc()) {
+          if (!ScrollableLayerGuid::EqualsIgnoringPresShell(
+                  aGuid, childApzc->GetGuid())) {
+            // We can have subtrees with their own zoom constraints - leave
+            // these alone.
+            if (this->mZoomConstraints.find(childApzc->GetGuid()) !=
+                this->mZoomConstraints.end()) {
+              return TraversalFlag::Skip;
+            }
+          }
+        }
+      }
+      if (aNode->IsPrimaryHolder()) {
+        MOZ_ASSERT(aNode->GetApzc());
+        aNode->GetApzc()->UpdateZoomConstraints(aConstraints.ref());
+      }
+      return TraversalFlag::Continue;
+    });
+  }
+}
+
+void APZCTreeManager::FlushRepaintsToClearScreenToGeckoTransform() {
+  // As the name implies, we flush repaint requests for the entire APZ tree in
+  // order to clear the screen-to-gecko transform (aka the "untransform" applied
+  // to incoming input events before they can be passed on to Gecko).
+  //
+  // The primary reason we do this is to avoid the problem where input events,
+  // after being untransformed, end up hit-testing differently in Gecko. This
+  // might happen in cases where the input event lands on content that is async-
+  // scrolled into view, but Gecko still thinks it is out of view given the
+  // visible area of a scrollframe.
+  //
+  // Another reason we want to clear the untransform is that if our APZ hit-test
+  // hits a dispatch-to-content region then that's an ambiguous result and we
+  // need to ask Gecko what actually got hit. In order to do this we need to
+  // untransform the input event into Gecko space - but to do that we need to
+  // know which APZC got hit! This leads to a circular dependency; the only way
+  // to get out of it is to make sure that the untransform for all the possible
+  // matched APZCs is the same. It is simplest to ensure that by flushing the
+  // pending repaint requests, which makes all of the untransforms empty (and
+  // therefore equal).
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  ForEachNode<ReverseIterator>(mRootNode.get(), [](HitTestingTreeNode* aNode) {
+    if (aNode->IsPrimaryHolder()) {
+      MOZ_ASSERT(aNode->GetApzc());
+      aNode->GetApzc()->FlushRepaintForNewInputBlock();
+    }
+  });
+}
+
+void APZCTreeManager::ClearTree() {
+  AssertOnUpdaterThread();
+
+  // Ensure that no references to APZCs are alive in any lingering input
+  // blocks. This breaks cycles from InputBlockState::mTargetApzc back to
+  // the InputQueue.
+  APZThreadUtils::RunOnControllerThread(NewRunnableMethod(
+      "layers::InputQueue::Clear", mInputQueue, &InputQueue::Clear));
+
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  // Collect the nodes into a list, and then destroy each one.
+  // We can't destroy them as we collect them, because ForEachNode()
+  // does a pre-order traversal of the tree, and Destroy() nulls out
+  // the fields needed to reach the children of the node.
+  nsTArray<RefPtr<HitTestingTreeNode>> nodesToDestroy;
+  ForEachNode<ReverseIterator>(mRootNode.get(),
+                               [&nodesToDestroy](HitTestingTreeNode* aNode) {
+                                 nodesToDestroy.AppendElement(aNode);
+                               });
+
+  for (size_t i = 0; i < nodesToDestroy.Length(); i++) {
+    nodesToDestroy[i]->Destroy();
+  }
+  mRootNode = nullptr;
+
+  {
+    // Also remove references to APZC instances in the map
+    MutexAutoLock lock(mMapLock);
+    mApzcMap.clear();
+  }
+
+  RefPtr<APZCTreeManager> self(this);
+  NS_DispatchToMainThread(
+      NS_NewRunnableFunction("layers::APZCTreeManager::ClearTree", [self] {
+        self->mFlushObserver->Unregister();
+        self->mFlushObserver = nullptr;
+      }));
+}
+
+RefPtr<HitTestingTreeNode> APZCTreeManager::GetRootNode() const {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  return mRootNode;
+}
+
+/**
+ * Transform a displacement from the ParentLayer coordinates of a source APZC
+ * to the ParentLayer coordinates of a target APZC.
+ * @param aTreeManager the tree manager for the APZC tree containing |aSource|
+ *                     and |aTarget|
+ * @param aSource the source APZC
+ * @param aTarget the target APZC
+ * @param aStartPoint the start point of the displacement
+ * @param aEndPoint the end point of the displacement
+ * @return true on success, false if aStartPoint or aEndPoint cannot be
+ * transformed into target's coordinate space
+ */
+static bool TransformDisplacement(APZCTreeManager* aTreeManager,
+                                  AsyncPanZoomController* aSource,
+                                  AsyncPanZoomController* aTarget,
+                                  ParentLayerPoint& aStartPoint,
+                                  ParentLayerPoint& aEndPoint) {
+  if (aSource == aTarget) {
+    return true;
+  }
+
+  // Convert start and end points to Screen coordinates.
+  ParentLayerToScreenMatrix4x4 untransformToApzc =
+      aTreeManager->GetScreenToApzcTransform(aSource).Inverse();
+  ScreenPoint screenStart = TransformBy(untransformToApzc, aStartPoint);
+  ScreenPoint screenEnd = TransformBy(untransformToApzc, aEndPoint);
+
+  // Convert start and end points to aTarget's ParentLayer coordinates.
+  ScreenToParentLayerMatrix4x4 transformToApzc =
+      aTreeManager->GetScreenToApzcTransform(aTarget);
+  Maybe<ParentLayerPoint> startPoint =
+      UntransformBy(transformToApzc, screenStart);
+  Maybe<ParentLayerPoint> endPoint = UntransformBy(transformToApzc, screenEnd);
+  if (!startPoint || !endPoint) {
+    return false;
+  }
+  aEndPoint = *endPoint;
+  aStartPoint = *startPoint;
+
+  return true;
+}
+
+bool APZCTreeManager::DispatchScroll(
+    AsyncPanZoomController* aPrev, ParentLayerPoint& aStartPoint,
+    ParentLayerPoint& aEndPoint,
+    OverscrollHandoffState& aOverscrollHandoffState) {
+  const OverscrollHandoffChain& overscrollHandoffChain =
+      aOverscrollHandoffState.mChain;
+  uint32_t overscrollHandoffChainIndex = aOverscrollHandoffState.mChainIndex;
+  RefPtr<AsyncPanZoomController> next;
+  // If we have reached the end of the overscroll handoff chain, there is
+  // nothing more to scroll, so we ignore the rest of the pan gesture.
+  if (overscrollHandoffChainIndex >= overscrollHandoffChain.Length()) {
+    // Nothing more to scroll - ignore the rest of the pan gesture.
+    return false;
+  }
+
+  next = overscrollHandoffChain.GetApzcAtIndex(overscrollHandoffChainIndex);
+
+  if (next == nullptr || next->IsDestroyed()) {
+    return false;
+  }
+
+  // Convert the start and end points from |aPrev|'s coordinate space to
+  // |next|'s coordinate space.
+  if (!TransformDisplacement(this, aPrev, next, aStartPoint, aEndPoint)) {
+    return false;
+  }
+
+  // Scroll |next|. If this causes overscroll, it will call DispatchScroll()
+  // again with an incremented index.
+  if (!next->AttemptScroll(aStartPoint, aEndPoint, aOverscrollHandoffState)) {
+    // Transform |aStartPoint| and |aEndPoint| (which now represent the
+    // portion of the displacement that wasn't consumed by APZCs later
+    // in the handoff chain) back into |aPrev|'s coordinate space. This
+    // allows the caller (which is |aPrev|) to interpret the unconsumed
+    // displacement in its own coordinate space, and make use of it
+    // (e.g. by going into overscroll).
+    if (!TransformDisplacement(this, next, aPrev, aStartPoint, aEndPoint)) {
+      NS_WARNING("Failed to untransform scroll points during dispatch");
+    }
+    return false;
+  }
+
+  // Return true to indicate the scroll was consumed entirely.
+  return true;
+}
+
+ParentLayerPoint APZCTreeManager::DispatchFling(
+    AsyncPanZoomController* aPrev, const FlingHandoffState& aHandoffState) {
+  // If immediate handoff is disallowed, do not allow handoff beyond the
+  // single APZC that's scrolled by the input block that triggered this fling.
+  if (aHandoffState.mIsHandoff && !StaticPrefs::apz_allow_immediate_handoff() &&
+      aHandoffState.mScrolledApzc == aPrev) {
+    FLING_LOG("APZCTM dropping handoff due to disallowed immediate handoff\n");
+    return aHandoffState.mVelocity;
+  }
+
+  const OverscrollHandoffChain* chain = aHandoffState.mChain;
+  RefPtr<AsyncPanZoomController> current;
+  uint32_t overscrollHandoffChainLength = chain->Length();
+  uint32_t startIndex;
+
+  // The fling's velocity needs to be transformed from the screen coordinates
+  // of |aPrev| to the screen coordinates of |next|. To transform a velocity
+  // correctly, we need to convert it to a displacement. For now, we do this
+  // by anchoring it to a start point of (0, 0).
+  // TODO: For this to be correct in the presence of 3D transforms, we should
+  // use the end point of the touch that started the fling as the start point
+  // rather than (0, 0).
+  ParentLayerPoint startPoint;  // (0, 0)
+  ParentLayerPoint endPoint;
+
+  if (aHandoffState.mIsHandoff) {
+    startIndex = chain->IndexOf(aPrev) + 1;
+
+    // IndexOf will return aOverscrollHandoffChain->Length() if
+    // |aPrev| is not found.
+    if (startIndex >= overscrollHandoffChainLength) {
+      return aHandoffState.mVelocity;
+    }
+  } else {
+    startIndex = 0;
+  }
+
+  // This will store any velocity left over after the entire handoff.
+  ParentLayerPoint finalResidualVelocity = aHandoffState.mVelocity;
+
+  ParentLayerPoint currentVelocity = aHandoffState.mVelocity;
+  for (; startIndex < overscrollHandoffChainLength; startIndex++) {
+    current = chain->GetApzcAtIndex(startIndex);
+
+    // Make sure the apzc about to be handled can be handled
+    if (current == nullptr || current->IsDestroyed()) {
+      break;
+    }
+
+    endPoint = startPoint + currentVelocity;
+
+    RefPtr<AsyncPanZoomController> prevApzc =
+        (startIndex > 0) ? chain->GetApzcAtIndex(startIndex - 1) : nullptr;
+
+    // Only transform when current apzc can be transformed with previous
+    if (prevApzc) {
+      if (!TransformDisplacement(this, prevApzc, current, startPoint,
+                                 endPoint)) {
+        break;
+      }
+    }
+
+    ParentLayerPoint availableVelocity = (endPoint - startPoint);
+    ParentLayerPoint residualVelocity;
+
+    FlingHandoffState transformedHandoffState = aHandoffState;
+    transformedHandoffState.mVelocity = availableVelocity;
+
+    // Obey overscroll-behavior.
+    if (prevApzc) {
+      residualVelocity += prevApzc->AdjustHandoffVelocityForOverscrollBehavior(
+          transformedHandoffState.mVelocity);
+    }
+
+    residualVelocity += current->AttemptFling(transformedHandoffState);
+
+    // If there's no residual velocity, there's nothing more to hand off.
+    if (current->IsZero(residualVelocity)) {
+      return ParentLayerPoint();
+    }
+
+    // If any of the velocity available to be handed off was consumed,
+    // subtract the proportion of consumed velocity from finalResidualVelocity.
+    // Note: it's important to compare |residualVelocity| to |availableVelocity|
+    // here and not to |transformedHandoffState.mVelocity|, since the latter
+    // may have been modified by AdjustHandoffVelocityForOverscrollBehavior().
+    if (!current->IsZero(availableVelocity.x - residualVelocity.x)) {
+      finalResidualVelocity.x *= (residualVelocity.x / availableVelocity.x);
+    }
+    if (!current->IsZero(availableVelocity.y - residualVelocity.y)) {
+      finalResidualVelocity.y *= (residualVelocity.y / availableVelocity.y);
+    }
+
+    currentVelocity = residualVelocity;
+  }
+
+  // Return any residual velocity left over after the entire handoff process.
+  return finalResidualVelocity;
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::GetTargetAPZC(
+    const ScrollableLayerGuid& aGuid) {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  RefPtr<HitTestingTreeNode> node = GetTargetNode(aGuid, nullptr);
+  MOZ_ASSERT(!node || node->GetApzc());  // any node returned must have an APZC
+  RefPtr<AsyncPanZoomController> apzc = node ? node->GetApzc() : nullptr;
+  return apzc.forget();
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::GetTargetAPZC(
+    const LayersId& aLayersId,
+    const ScrollableLayerGuid::ViewID& aScrollId) const {
+  MutexAutoLock lock(mMapLock);
+  return GetTargetAPZC(aLayersId, aScrollId, lock);
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::GetTargetAPZC(
+    const LayersId& aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+    const MutexAutoLock& aProofOfMapLock) const {
+  ScrollableLayerGuid guid(aLayersId, 0, aScrollId);
+  auto it = mApzcMap.find(guid);
+  RefPtr<AsyncPanZoomController> apzc =
+      (it != mApzcMap.end() ? it->second.apzc : nullptr);
+  return apzc.forget();
+}
+
+already_AddRefed<HitTestingTreeNode> APZCTreeManager::GetTargetNode(
+    const ScrollableLayerGuid& aGuid, GuidComparator aComparator) const {
+  mTreeLock.AssertCurrentThreadIn();
+  RefPtr<HitTestingTreeNode> target =
+      DepthFirstSearchPostOrder<ReverseIterator>(
+          mRootNode.get(), [&aGuid, &aComparator](HitTestingTreeNode* node) {
+            bool matches = false;
+            if (node->GetApzc()) {
+              if (aComparator) {
+                matches = aComparator(aGuid, node->GetApzc()->GetGuid());
+              } else {
+                matches = node->GetApzc()->Matches(aGuid);
+              }
+            }
+            return matches;
+          });
+  return target.forget();
+}
+
+APZCTreeManager::HitTestResult APZCTreeManager::GetTargetAPZC(
+    const ScreenPoint& aPoint) {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  MOZ_ASSERT(mHitTester);
+  return mHitTester->GetAPZCAtPoint(aPoint, lock);
+}
+
+APZCTreeManager::TargetApzcForNodeResult APZCTreeManager::FindHandoffParent(
+    const AsyncPanZoomController* aApzc) {
+  RefPtr<HitTestingTreeNode> node = GetTargetNode(aApzc->GetGuid(), nullptr);
+  while (node) {
+    auto result = GetTargetApzcForNode(node->GetParent());
+    if (result.mApzc) {
+      // avoid infinite recursion in the overscroll handoff chain.
+      if (result.mApzc != aApzc) {
+        return result;
+      }
+    }
+    node = node->GetParent();
+  }
+
+  return {nullptr, false};
+}
+
+RefPtr<const OverscrollHandoffChain>
+APZCTreeManager::BuildOverscrollHandoffChain(
+    const RefPtr<AsyncPanZoomController>& aInitialTarget) {
+  // Scroll grabbing is a mechanism that allows content to specify that
+  // the initial target of a pan should be not the innermost scrollable
+  // frame at the touch point (which is what GetTargetAPZC finds), but
+  // something higher up in the tree.
+  // It's not sufficient to just find the initial target, however, as
+  // overscroll can be handed off to another APZC. Without scroll grabbing,
+  // handoff just occurs from child to parent. With scroll grabbing, the
+  // handoff order can be different, so we build a chain of APZCs in the
+  // order in which scroll will be handed off to them.
+
+  // Grab tree lock since we'll be walking the APZC tree.
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  // Build the chain. If there is a scroll parent link, we use that. This is
+  // needed to deal with scroll info layers, because they participate in handoff
+  // but do not follow the expected layer tree structure. If there are no
+  // scroll parent links we just walk up the tree to find the scroll parent.
+  OverscrollHandoffChain* result = new OverscrollHandoffChain;
+  AsyncPanZoomController* apzc = aInitialTarget;
+  while (apzc != nullptr) {
+    result->Add(apzc);
+
+    APZCTreeManager::TargetApzcForNodeResult handoffResult =
+        FindHandoffParent(apzc);
+
+    if (!handoffResult.mIsFixed && !apzc->IsRootForLayersId() &&
+        apzc->GetScrollHandoffParentId() ==
+            ScrollableLayerGuid::NULL_SCROLL_ID) {
+      // This probably indicates a bug or missed case in layout code
+      NS_WARNING("Found a non-root APZ with no handoff parent");
+    }
+
+    // If `apzc` is inside fixed content, we want to hand off to the document's
+    // root APZC next. The scroll parent id wouldn't give us this because it's
+    // based on ASRs.
+    if (handoffResult.mIsFixed || apzc->GetScrollHandoffParentId() ==
+                                      ScrollableLayerGuid::NULL_SCROLL_ID) {
+      apzc = handoffResult.mApzc;
+      continue;
+    }
+
+    // Guard against a possible infinite-loop condition. If we hit this, the
+    // layout code that generates the handoff parents did something wrong.
+    MOZ_ASSERT(apzc->GetScrollHandoffParentId() != apzc->GetGuid().mScrollId);
+    RefPtr<AsyncPanZoomController> scrollParent = GetTargetAPZC(
+        apzc->GetGuid().mLayersId, apzc->GetScrollHandoffParentId());
+    apzc = scrollParent.get();
+  }
+
+  // Now adjust the chain to account for scroll grabbing. Sorting is a bit
+  // of an overkill here, but scroll grabbing will likely be generalized
+  // to scroll priorities, so we might as well do it this way.
+  result->SortByScrollPriority();
+
+  // Print the overscroll chain for debugging.
+  for (uint32_t i = 0; i < result->Length(); ++i) {
+    APZCTM_LOG("OverscrollHandoffChain[%d] = %p\n", i,
+               result->GetApzcAtIndex(i).get());
+  }
+
+  return result;
+}
+
+void APZCTreeManager::SetLongTapEnabled(bool aLongTapEnabled) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(NewRunnableMethod<bool>(
+        "layers::APZCTreeManager::SetLongTapEnabled", this,
+        &APZCTreeManager::SetLongTapEnabled, aLongTapEnabled));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+  GestureEventListener::SetLongTapEnabled(aLongTapEnabled);
+}
+
+void APZCTreeManager::AddInputBlockCallback(
+    uint64_t aInputBlockId, InputBlockCallbackInfo&& aCallbackInfo) {
+  APZThreadUtils::AssertOnControllerThread();
+  mInputQueue->AddInputBlockCallback(aInputBlockId, std::move(aCallbackInfo));
+}
+
+void APZCTreeManager::FindScrollThumbNode(
+    const AsyncDragMetrics& aDragMetrics, LayersId aLayersId,
+    HitTestingTreeNodeAutoLock& aOutThumbNode) {
+  if (!aDragMetrics.mDirection) {
+    // The AsyncDragMetrics has not been initialized yet - there will be
+    // no matching node, so don't bother searching the tree.
+    return;
+  }
+
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  RefPtr<HitTestingTreeNode> result = DepthFirstSearch<ReverseIterator>(
+      mRootNode.get(), [&aDragMetrics, &aLayersId](HitTestingTreeNode* aNode) {
+        return aNode->MatchesScrollDragMetrics(aDragMetrics, aLayersId);
+      });
+  if (result) {
+    aOutThumbNode.Initialize(lock, result.forget(), mTreeLock);
+  }
+}
+
+APZCTreeManager::TargetApzcForNodeResult APZCTreeManager::GetTargetApzcForNode(
+    const HitTestingTreeNode* aNode) {
+  for (const HitTestingTreeNode* n = aNode;
+       n && n->GetLayersId() == aNode->GetLayersId(); n = n->GetParent()) {
+    // For a fixed node, GetApzc() may return an APZC for content in the
+    // enclosing document, so we need to check GetFixedPosTarget() before
+    // GetApzc().
+    if (n->GetFixedPosTarget() != ScrollableLayerGuid::NULL_SCROLL_ID) {
+      RefPtr<AsyncPanZoomController> fpTarget =
+          GetTargetAPZC(n->GetLayersId(), n->GetFixedPosTarget());
+      APZCTM_LOG("Found target APZC %p using fixed-pos lookup on %" PRIu64 "\n",
+                 fpTarget.get(), n->GetFixedPosTarget());
+      return {fpTarget.get(), true};
+    }
+    if (n->GetApzc()) {
+      APZCTM_LOG("Found target %p using ancestor lookup\n", n->GetApzc());
+      return {n->GetApzc(), false};
+    }
+  }
+  return {nullptr, false};
+}
+
+HitTestingTreeNode* APZCTreeManager::FindRootNodeForLayersId(
+    LayersId aLayersId) const {
+  mTreeLock.AssertCurrentThreadIn();
+
+  HitTestingTreeNode* resultNode = BreadthFirstSearch<ReverseIterator>(
+      mRootNode.get(), [aLayersId](HitTestingTreeNode* aNode) {
+        AsyncPanZoomController* apzc = aNode->GetApzc();
+        return apzc && apzc->GetLayersId() == aLayersId &&
+               apzc->IsRootForLayersId();
+      });
+  return resultNode;
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::FindZoomableApzc(
+    AsyncPanZoomController* aStart) const {
+  return GetZoomableTarget(aStart, aStart);
+}
+
+ScreenMargin APZCTreeManager::GetCompositorFixedLayerMargins() const {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  return mCompositorFixedLayerMargins;
+}
+
+AsyncPanZoomController* APZCTreeManager::FindRootApzcFor(
+    LayersId aLayersId) const {
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  HitTestingTreeNode* resultNode = FindRootNodeForLayersId(aLayersId);
+  return resultNode ? resultNode->GetApzc() : nullptr;
+}
+
+AsyncPanZoomController* APZCTreeManager::FindRootContentApzcForLayersId(
+    LayersId aLayersId) const {
+  mTreeLock.AssertCurrentThreadIn();
+
+  HitTestingTreeNode* resultNode = BreadthFirstSearch<ReverseIterator>(
+      mRootNode.get(), [aLayersId](HitTestingTreeNode* aNode) {
+        AsyncPanZoomController* apzc = aNode->GetApzc();
+        return apzc && apzc->GetLayersId() == aLayersId &&
+               apzc->IsRootContent();
+      });
+  return resultNode ? resultNode->GetApzc() : nullptr;
+}
+
+// clang-format off
+/* The methods GetScreenToApzcTransform() and GetApzcToGeckoTransform() return
+   some useful transformations that input events may need applied. This is best
+   illustrated with an example. Consider a chain of layers, L, M, N, O, P, Q, R. Layer L
+   is the layer that corresponds to the argument |aApzc|, and layer R is the root
+   of the layer tree. Layer M is the parent of L, N is the parent of M, and so on.
+   When layer L is displayed to the screen by the compositor, the set of transforms that
+   are applied to L are (in order from top to bottom):
+
+        L's CSS transform                   (hereafter referred to as transform matrix LC)
+        L's async transform                 (hereafter referred to as transform matrix LA)
+        M's CSS transform                   (hereafter referred to as transform matrix MC)
+        M's async transform                 (hereafter referred to as transform matrix MA)
+        ...
+        R's CSS transform                   (hereafter referred to as transform matrix RC)
+        R's async transform                 (hereafter referred to as transform matrix RA)
+
+   If we want user input to modify L's async transform, we have to first convert
+   user input from screen space to the coordinate space of L's async transform. Doing
+   this involves applying the following transforms (in order from top to bottom):
+        RA.Inverse()
+        RC.Inverse()
+        ...
+        MA.Inverse()
+        MC.Inverse()
+   This combined transformation is returned by GetScreenToApzcTransform().
+
+   Next, if we want user inputs sent to gecko for event-dispatching, we will need to strip
+   out all of the async transforms that are involved in this chain. This is because async
+   transforms are stored only in the compositor and gecko does not account for them when
+   doing display-list-based hit-testing for event dispatching.
+   Furthermore, because these input events are processed by Gecko in a FIFO queue that
+   includes other things (specifically paint requests), it is possible that by time the
+   input event reaches gecko, it will have painted something else. Therefore, we need to
+   apply another transform to the input events to account for the possible disparity between
+   what we know gecko last painted and the last paint request we sent to gecko. Let this
+   transform be represented by LD, MD, ... RD.
+   Therefore, given a user input in screen space, the following transforms need to be applied
+   (in order from top to bottom):
+        RT.Inverse()
+        RN.Inverse()
+        RC.Inverse()
+        ...
+        MT.Inverse()
+        MN.Inverse()
+        MC.Inverse()
+        LT.Inverse()
+        LN.Inverse()
+        LC.Inverse()
+        LC
+        LD
+        MC
+        MD
+        ...
+        RC
+        RD
+   This sequence can be simplified and refactored to the following:
+        GetScreenToApzcTransform()
+        LA.Inverse()
+        LD
+        MC
+        MD
+        ...
+        RC
+        RD
+   Since GetScreenToApzcTransform() can be obtained by calling that function, GetApzcToGeckoTransform()
+   returns the remaining transforms (LA.Inverse() * LD * ... * RD), so that the caller code can
+   combine it with GetScreenToApzcTransform() to get the final transform required in this case.
+
+   Note that for many of these layers, there will be no AsyncPanZoomController attached, and
+   so the async transform will be the identity transform. So, in the example above, if layers
+   L and P have APZC instances attached, MT, MN, MD, NT, NN, ND, OT, ON, OD, QT, QN, QD, RT,
+   RN and RD will be identity transforms.
+   Additionally, for space-saving purposes, each APZC instance stores its layer's individual
+   CSS transform and the accumulation of CSS transforms to its parent APZC. So the APZC for
+   layer L would store LC and (MC * NC * OC), and the layer P would store PC and (QC * RC).
+   The APZC instances track the last dispatched paint request and so are able to calculate LD and
+   PD using those internally stored values.
+   The APZCs also obviously have LT, LN, PT, and PN, so all of the above transformation combinations
+   required can be generated.
+ */
+// clang-format on
+
+/*
+ * See the long comment above for a detailed explanation of this function.
+ */
+ScreenToParentLayerMatrix4x4 APZCTreeManager::GetScreenToApzcTransform(
+    const AsyncPanZoomController* aApzc) const {
+  Matrix4x4 result;
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  // The comments below assume there is a chain of layers L..R with L and P
+  // having APZC instances as explained in the comment above. This function is
+  // called with aApzc at L, and the loop below performs one iteration, where
+  // parent is at P. The comments explain what values are stored in the
+  // variables at these two levels. All the comments use standard matrix
+  // notation where the leftmost matrix in a multiplication is applied first.
+
+  // ancestorUntransform is PC.Inverse() * OC.Inverse() * NC.Inverse() *
+  // MC.Inverse()
+  Matrix4x4 ancestorUntransform = aApzc->GetAncestorTransform().Inverse();
+
+  // result is initialized to PC.Inverse() * OC.Inverse() * NC.Inverse() *
+  // MC.Inverse()
+  result = ancestorUntransform;
+
+  for (AsyncPanZoomController* parent = aApzc->GetParent(); parent;
+       parent = parent->GetParent()) {
+    // ancestorUntransform is updated to RC.Inverse() * QC.Inverse() when parent
+    // == P
+    ancestorUntransform = parent->GetAncestorTransform().Inverse();
+    // asyncUntransform is updated to PA.Inverse() when parent == P
+    Matrix4x4 asyncUntransform = parent
+                                     ->GetAsyncTransformForInputTransformation(
+                                         LayoutAndVisual, aApzc->GetLayersId())
+                                     .Inverse()
+                                     .ToUnknownMatrix();
+    // untransformSinceLastApzc is RC.Inverse() * QC.Inverse() * PA.Inverse()
+    Matrix4x4 untransformSinceLastApzc = ancestorUntransform * asyncUntransform;
+
+    // result is RC.Inverse() * QC.Inverse() * PA.Inverse() * PC.Inverse() *
+    // OC.Inverse() * NC.Inverse() * MC.Inverse()
+    result = untransformSinceLastApzc * result;
+
+    // The above value for result when parent == P matches the required output
+    // as explained in the comment above this method. Note that any missing
+    // terms are guaranteed to be identity transforms.
+  }
+
+  return ViewAs<ScreenToParentLayerMatrix4x4>(result);
+}
+
+/*
+ * See the long comment above GetScreenToApzcTransform() for a detailed
+ * explanation of this function.
+ */
+ParentLayerToParentLayerMatrix4x4 APZCTreeManager::GetApzcToApzcTransform(
+    const AsyncPanZoomController* aStartApzc,
+    const AsyncPanZoomController* aStopApzc,
+    const AsyncTransformComponents& aComponents) const {
+  Matrix4x4 result;
+  RecursiveMutexAutoLock lock(mTreeLock);
+
+  // The comments below assume there is a chain of layers L..R with L and P
+  // having APZC instances as explained in the comment above, and |aStopApzc| is
+  // nullptr. This function is called with aStartApzc at L, and the loop below
+  // performs one iteration, where parent is at P. The comments explain what
+  // values are stored in the variables at these two levels. All the comments
+  // use standard matrix notation where the leftmost matrix in a multiplication
+  // is applied first.
+
+  // asyncUntransform is LA.Inverse()
+  Matrix4x4 asyncUntransform = aStartApzc
+                                   ->GetAsyncTransformForInputTransformation(
+                                       aComponents, aStartApzc->GetLayersId())
+                                   .Inverse()
+                                   .ToUnknownMatrix();
+
+  // result is initialized to LA.Inverse() * LD * MC * NC * OC * PC
+  result = asyncUntransform *
+           aStartApzc->GetTransformToLastDispatchedPaint(
+               aComponents, aStartApzc->GetLayersId()) *
+           aStartApzc->GetAncestorTransform();
+
+  for (AsyncPanZoomController* parent = aStartApzc->GetParent();
+       parent && parent != aStopApzc; parent = parent->GetParent()) {
+    // result is LA.Inverse() * LD * MC * NC * OC * PC * PD * QC * RC
+    //
+    // Note: Do not pass the async transform components for the current target
+    // to the parent.
+    result = result *
+             parent->GetTransformToLastDispatchedPaint(
+                 LayoutAndVisual, aStartApzc->GetLayersId()) *
+             parent->GetAncestorTransform();
+
+    // The above value for result when parent == P matches the required output
+    // as explained in the comment above this method. Note that any missing
+    // terms are guaranteed to be identity transforms.
+  }
+
+  return ViewAs<ParentLayerToParentLayerMatrix4x4>(result);
+}
+
+ParentLayerToScreenMatrix4x4 APZCTreeManager::GetApzcToGeckoTransform(
+    const AsyncPanZoomController* aApzc,
+    const AsyncTransformComponents& aComponents) const {
+  return ViewAs<ParentLayerToScreenMatrix4x4>(
+      GetApzcToApzcTransform(aApzc, nullptr, aComponents),
+      PixelCastJustification::ScreenIsParentLayerForRoot);
+}
+
+ParentLayerToScreenMatrix4x4 APZCTreeManager::GetApzcToGeckoTransformForHit(
+    HitTestResult& aHitResult) const {
+  // Fixed content is only subject to the visual component of the async
+  // transform.
+  AsyncTransformComponents components =
+      aHitResult.mFixedPosSides == SideBits::eNone
+          ? LayoutAndVisual
+          : AsyncTransformComponents{AsyncTransformComponent::eVisual};
+  return GetApzcToGeckoTransform(aHitResult.mTargetApzc, components);
+}
+
+CSSToCSSMatrix4x4 APZCTreeManager::GetOopifToRootContentTransform(
+    AsyncPanZoomController* aApzc) const {
+  MOZ_ASSERT(aApzc->IsRootForLayersId());
+
+  RefPtr<AsyncPanZoomController> rootContentApzc = FindZoomableApzc(aApzc);
+  MOZ_ASSERT(aApzc->GetLayersId() != rootContentApzc->GetLayersId(),
+             "aApzc must be out-of-process of the rootContentApzc");
+  if (!rootContentApzc || rootContentApzc == aApzc ||
+      rootContentApzc->GetLayersId() == aApzc->GetLayersId()) {
+    return CSSToCSSMatrix4x4();
+  }
+  ParentLayerToParentLayerMatrix4x4 result =
+      GetApzcToApzcTransform(aApzc, rootContentApzc,
+                             AsyncTransformComponent::eLayout) *
+      // We need to multiply by the root content APZC's
+      // GetPaintedResolutionTransform() here; See
+      // https://phabricator.services.mozilla.com/D184440?vs=755900&id=757585#6173584
+      // for the details.
+      ViewAs<AsyncTransformComponentMatrix>(
+          rootContentApzc->GetPaintedResolutionTransform());
+
+  CSSToParentLayerScale rootZoom = rootContentApzc->GetZoom();
+
+  if (rootZoom == CSSToParentLayerScale(0)) {
+    rootZoom = CSSToParentLayerScale(1.0f);
+  }
+
+  CSSPoint rootScrollPosition = rootContentApzc->GetLayoutScrollOffset();
+
+  return result.PreScale(aApzc->GetZoom())
+      .PostScale(rootZoom.Inverse())
+      .PostTranslate(rootScrollPosition);
+}
+
+CSSRect APZCTreeManager::ConvertRectInApzcToRoot(AsyncPanZoomController* aApzc,
+                                                 const CSSRect& aRect) const {
+  MOZ_ASSERT(aApzc->IsRootForLayersId());
+  RefPtr<AsyncPanZoomController> rootContentApzc = FindZoomableApzc(aApzc);
+  if (!rootContentApzc) {
+    return aRect;
+  }
+
+  if (rootContentApzc == aApzc) {
+    return aRect + rootContentApzc->GetLayoutScrollOffset();
+  }
+
+  return GetOopifToRootContentTransform(aApzc).TransformBounds(aRect);
+}
+
+ScreenPoint APZCTreeManager::GetCurrentMousePosition() const {
+  auto pos = mCurrentMousePosition.Lock();
+  return pos.ref();
+}
+
+void APZCTreeManager::SetCurrentMousePosition(const ScreenPoint& aNewPos) {
+  auto pos = mCurrentMousePosition.Lock();
+  pos.ref() = aNewPos;
+}
+
+static AsyncPanZoomController* GetApzcWithDifferentLayersIdByWalkingParents(
+    AsyncPanZoomController* aApzc) {
+  if (!aApzc) {
+    return nullptr;
+  }
+  AsyncPanZoomController* parent = aApzc->GetParent();
+  while (parent && (parent->GetLayersId() == aApzc->GetLayersId())) {
+    parent = parent->GetParent();
+  }
+  return parent;
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::GetZoomableTarget(
+    AsyncPanZoomController* aApzc1, AsyncPanZoomController* aApzc2) const {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  RefPtr<AsyncPanZoomController> apzc;
+  // For now, we only ever want to do pinching on the root-content APZC for
+  // a given layers id.
+  if (aApzc1 && aApzc2 && aApzc1->GetLayersId() == aApzc2->GetLayersId()) {
+    // If the two APZCs have the same layers id, find the root-content APZC
+    // for that layers id. Don't call CommonAncestor() because there may not
+    // be a common ancestor for the layers id (e.g. if one APZCs is inside a
+    // fixed-position element).
+    apzc = FindRootContentApzcForLayersId(aApzc1->GetLayersId());
+    if (apzc) {
+      return apzc.forget();
+    }
+  }
+
+  // Otherwise, find the common ancestor (to reach a common layers id), and then
+  // walk up the apzc tree until we find a root-content APZC.
+  apzc = CommonAncestor(aApzc1, aApzc2);
+  RefPtr<AsyncPanZoomController> zoomable;
+  while (apzc && !zoomable) {
+    zoomable = FindRootContentApzcForLayersId(apzc->GetLayersId());
+    apzc = GetApzcWithDifferentLayersIdByWalkingParents(apzc);
+  }
+
+  return zoomable.forget();
+}
+
+Maybe<ScreenIntPoint> APZCTreeManager::ConvertToGecko(
+    const ScreenIntPoint& aPoint, AsyncPanZoomController* aApzc) {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  // TODO: The current check assumes that a touch gesture and a touchpad tap
+  // gesture can't both be active at the same time. If we turn on double-tap-
+  // to-zoom on a touchscreen platform like Windows or Linux, this assumption
+  // would no longer be valid, and we'd have to instead have TapGestureInput
+  // track and inform this function whether it was created from touch events.
+  const HitTestResult& hit = mInputQueue->GetCurrentTouchBlock()
+                                 ? mTouchBlockHitResult
+                                 : mTapGestureHitResult;
+  AsyncTransformComponents components =
+      hit.mFixedPosSides == SideBits::eNone
+          ? LayoutAndVisual
+          : AsyncTransformComponents{AsyncTransformComponent::eVisual};
+  ScreenToScreenMatrix4x4 transformScreenToGecko =
+      GetScreenToApzcTransform(aApzc) *
+      GetApzcToGeckoTransform(aApzc, components);
+  Maybe<ScreenIntPoint> geckoPoint =
+      UntransformBy(transformScreenToGecko, aPoint);
+  if (geckoPoint) {
+    AdjustEventPointForDynamicToolbar(*geckoPoint, hit);
+  }
+  return geckoPoint;
+}
+
+already_AddRefed<AsyncPanZoomController> APZCTreeManager::CommonAncestor(
+    AsyncPanZoomController* aApzc1, AsyncPanZoomController* aApzc2) const {
+  mTreeLock.AssertCurrentThreadIn();
+  RefPtr<AsyncPanZoomController> ancestor;
+
+  // If either aApzc1 or aApzc2 is null, min(depth1, depth2) will be 0 and this
+  // function will return null.
+
+  // Calculate depth of the APZCs in the tree
+  int depth1 = 0, depth2 = 0;
+  for (AsyncPanZoomController* parent = aApzc1; parent;
+       parent = parent->GetParent()) {
+    depth1++;
+  }
+  for (AsyncPanZoomController* parent = aApzc2; parent;
+       parent = parent->GetParent()) {
+    depth2++;
+  }
+
+  // At most one of the following two loops will be executed; the deeper APZC
+  // pointer will get walked up to the depth of the shallower one.
+  int minDepth = depth1 < depth2 ? depth1 : depth2;
+  while (depth1 > minDepth) {
+    depth1--;
+    aApzc1 = aApzc1->GetParent();
+  }
+  while (depth2 > minDepth) {
+    depth2--;
+    aApzc2 = aApzc2->GetParent();
+  }
+
+  // Walk up the ancestor chains of both APZCs, always staying at the same depth
+  // for either APZC, and return the the first common ancestor encountered.
+  while (true) {
+    if (aApzc1 == aApzc2) {
+      ancestor = aApzc1;
+      break;
+    }
+    if (depth1 <= 0) {
+      break;
+    }
+    aApzc1 = aApzc1->GetParent();
+    aApzc2 = aApzc2->GetParent();
+  }
+  return ancestor.forget();
+}
+
+bool APZCTreeManager::IsFixedToRootContent(
+    const HitTestingTreeNode* aNode) const {
+  MutexAutoLock lock(mMapLock);
+  return IsFixedToRootContent(FixedPositionInfo(aNode), lock);
+}
+
+bool APZCTreeManager::IsFixedToRootContent(
+    const FixedPositionInfo& aFixedInfo,
+    const MutexAutoLock& aProofOfMapLock) const {
+  ScrollableLayerGuid::ViewID fixedTarget = aFixedInfo.mFixedPosTarget;
+  if (fixedTarget == ScrollableLayerGuid::NULL_SCROLL_ID) {
+    return false;
+  }
+  auto it =
+      mApzcMap.find(ScrollableLayerGuid(aFixedInfo.mLayersId, 0, fixedTarget));
+  if (it == mApzcMap.end()) {
+    return false;
+  }
+  RefPtr<AsyncPanZoomController> targetApzc = it->second.apzc;
+  return targetApzc && targetApzc->IsRootContent();
+}
+
+SideBits APZCTreeManager::SidesStuckToRootContent(
+    const HitTestingTreeNode* aNode, AsyncTransformConsumer aMode) const {
+  MutexAutoLock lock(mMapLock);
+  return SidesStuckToRootContent(StickyPositionInfo(aNode), aMode, lock);
+}
+
+SideBits APZCTreeManager::SidesStuckToRootContent(
+    const StickyPositionInfo& aStickyInfo, AsyncTransformConsumer aMode,
+    const MutexAutoLock& aProofOfMapLock) const {
+  SideBits result = SideBits::eNone;
+
+  ScrollableLayerGuid::ViewID stickyTarget = aStickyInfo.mStickyPosTarget;
+  if (stickyTarget == ScrollableLayerGuid::NULL_SCROLL_ID) {
+    return result;
+  }
+
+  // We support the dynamic toolbar at top and bottom.
+  if ((aStickyInfo.mFixedPosSides & SideBits::eTopBottom) == SideBits::eNone) {
+    return result;
+  }
+
+  auto it = mApzcMap.find(
+      ScrollableLayerGuid(aStickyInfo.mLayersId, 0, stickyTarget));
+  if (it == mApzcMap.end()) {
+    return result;
+  }
+  RefPtr<AsyncPanZoomController> stickyTargetApzc = it->second.apzc;
+  if (!stickyTargetApzc || !stickyTargetApzc->IsRootContent()) {
+    return result;
+  }
+
+  ParentLayerPoint translation =
+      stickyTargetApzc
+          ->GetCurrentAsyncTransform(
+              aMode, AsyncTransformComponents{AsyncTransformComponent::eLayout})
+          .mTranslation;
+
+  if (apz::IsStuckAtTop(translation.y, aStickyInfo.mStickyScrollRangeInner,
+                        aStickyInfo.mStickyScrollRangeOuter)) {
+    result |= SideBits::eTop;
+  }
+  if (apz::IsStuckAtBottom(translation.y, aStickyInfo.mStickyScrollRangeInner,
+                           aStickyInfo.mStickyScrollRangeOuter)) {
+    result |= SideBits::eBottom;
+  }
+  return result;
+}
+
+LayerToParentLayerMatrix4x4 APZCTreeManager::ComputeTransformForScrollThumbNode(
+    const HitTestingTreeNode* aNode) const {
+  mTreeLock.AssertCurrentThreadIn();
+  // The async transform applied here is for hit-testing purposes, and is
+  // intended to match the one we tell WebRender to apply in
+  // SampleForWebRender for rendering purposes.
+  MOZ_ASSERT(aNode->IsScrollThumbNode());
+  // If the scrollable element scrolled by the thumb is layerized, compute and
+  // apply the transformation that will be applied to the thumb in
+  // AsyncCompositionManager.
+  ScrollableLayerGuid guid{aNode->GetLayersId(), 0, aNode->GetScrollTargetId()};
+  if (RefPtr<HitTestingTreeNode> scrollTargetNode =
+          GetTargetNode(guid, &ScrollableLayerGuid::EqualsIgnoringPresShell)) {
+    AsyncPanZoomController* scrollTargetApzc = scrollTargetNode->GetApzc();
+    MOZ_ASSERT(scrollTargetApzc);
+    return scrollTargetApzc->CallWithLastContentPaintMetrics(
+        [&](const FrameMetrics& aMetrics) {
+          return ComputeTransformForScrollThumb(
+              aNode->GetTransform() * AsyncTransformMatrix(),
+              scrollTargetNode->GetTransform().ToUnknownMatrix(),
+              scrollTargetApzc, aMetrics, aNode->GetScrollbarData(),
+              scrollTargetNode->IsAncestorOf(aNode));
+        });
+  }
+  // Otherwise, the node does not have an async transform.
+  return aNode->GetTransform() * AsyncTransformMatrix();
+}
+
+already_AddRefed<wr::WebRenderAPI> APZCTreeManager::GetWebRenderAPI() const {
+  RefPtr<wr::WebRenderAPI> api;
+  CompositorBridgeParent::CallWithIndirectShadowTree(
+      mRootLayersId, [&](LayerTreeState& aState) -> void {
+        if (aState.mWrBridge) {
+          api = aState.mWrBridge->GetWebRenderAPI();
+        }
+      });
+  return api.forget();
+}
+
+/*static*/
+already_AddRefed<GeckoContentController> APZCTreeManager::GetContentController(
+    LayersId aLayersId) {
+  RefPtr<GeckoContentController> controller;
+  CompositorBridgeParent::CallWithIndirectShadowTree(
+      aLayersId,
+      [&](LayerTreeState& aState) -> void { controller = aState.mController; });
+  return controller.forget();
+}
+
+ScreenMargin APZCTreeManager::GetCompositorFixedLayerMargins(
+    const MutexAutoLock& aProofOfMapLock) const {
+  ScreenMargin result = mCompositorFixedLayerMargins;
+  if (StaticPrefs::apz_fixed_margin_override_enabled()) {
+    result.top = StaticPrefs::apz_fixed_margin_override_top();
+    result.bottom = StaticPrefs::apz_fixed_margin_override_bottom();
+  }
+  return result;
+}
+
+bool APZCTreeManager::GetAPZTestData(LayersId aLayersId,
+                                     APZTestData* aOutData) {
+  AssertOnUpdaterThread();
+
+  {  // copy the relevant test data into aOutData while holding the
+     // mTestDataLock
+    MutexAutoLock lock(mTestDataLock);
+    auto it = mTestData.find(aLayersId);
+    if (it == mTestData.end()) {
+      return false;
+    }
+    *aOutData = *(it->second);
+  }
+
+  {  // add some additional "current state" into the returned APZTestData
+    MutexAutoLock mapLock(mMapLock);
+
+    ClippedCompositionBoundsMap clippedCompBounds;
+    for (const auto& mapping : mApzcMap) {
+      if (mapping.first.mLayersId != aLayersId) {
+        continue;
+      }
+
+      ParentLayerRect clippedBounds = ComputeClippedCompositionBounds(
+          mapLock, clippedCompBounds, mapping.first);
+      AsyncPanZoomController* apzc = mapping.second.apzc;
+      std::string viewId = std::to_string(mapping.first.mScrollId);
+      std::string apzcState;
+      if (apzc->GetCheckerboardMagnitude(clippedBounds)) {
+        apzcState += "checkerboarding,";
+      }
+      if (apzc->IsOverscrolled()) {
+        apzcState += "overscrolled,";
+      }
+      aOutData->RecordAdditionalData(viewId, apzcState);
+    }
+  }
+  return true;
+}
+
+void APZCTreeManager::SendSubtreeTransformsToChromeMainThread(
+    const AsyncPanZoomController* aAncestor) {
+  RefPtr<GeckoContentController> controller =
+      GetContentController(mRootLayersId);
+  if (!controller) {
+    return;
+  }
+  nsTArray<MatrixMessage> messages;
+  bool underAncestor = (aAncestor == nullptr);
+  bool shouldNotify = false;
+  {
+    RecursiveMutexAutoLock lock(mTreeLock);
+    if (!mRootNode) {
+      // Event dispatched during shutdown, after ClearTree().
+      // Note, mRootNode needs to be checked with mTreeLock held.
+      return;
+    }
+    // This formulation duplicates matrix multiplications closer
+    // to the root of the tree. For now, aiming for separation
+    // of concerns rather than minimum number of multiplications.
+    ForEachNode<ReverseIterator>(
+        mRootNode.get(),
+        [&](HitTestingTreeNode* aNode) {
+          mTreeLock.AssertCurrentThreadIn();
+          bool atAncestor = (aAncestor && aNode->GetApzc() == aAncestor);
+          MOZ_ASSERT(!(underAncestor && atAncestor));
+          underAncestor |= atAncestor;
+          if (!underAncestor) {
+            return;
+          }
+          LayersId layersId = aNode->GetLayersId();
+          HitTestingTreeNode* parent = aNode->GetParent();
+          if (!parent) {
+            messages.AppendElement(MatrixMessage(Some(LayerToScreenMatrix4x4()),
+                                                 ScreenRect(), layersId));
+          } else if (layersId != parent->GetLayersId()) {
+            if (mDetachedLayersIds.find(layersId) != mDetachedLayersIds.end()) {
+              messages.AppendElement(
+                  MatrixMessage(Nothing(), ScreenRect(), layersId));
+            } else {
+              // It's important to pass aRemoteLayersId=layersId here (not
+              // parent->GetLayersId()). layersId is the LayersId of the
+              // remote content for which this transform is being computed.
+              messages.AppendElement(MatrixMessage(
+                  Some(parent->GetTransformToGecko(layersId)),
+                  parent->GetRemoteDocumentScreenRect(layersId), layersId));
+            }
+          }
+        },
+        [&](HitTestingTreeNode* aNode) {
+          bool atAncestor = (aAncestor && aNode->GetApzc() == aAncestor);
+          if (atAncestor) {
+            MOZ_ASSERT(underAncestor);
+            underAncestor = false;
+          }
+        });
+    if (messages != mLastMessages) {
+      mLastMessages = messages;
+      shouldNotify = true;
+    }
+  }
+  if (shouldNotify) {
+    controller->NotifyLayerTransforms(std::move(messages));
+  }
+}
+
+void APZCTreeManager::SetFixedLayerMargins(ScreenIntCoord aTop,
+                                           ScreenIntCoord aBottom) {
+  MutexAutoLock lock(mMapLock);
+  mCompositorFixedLayerMargins.top = ScreenCoord(aTop);
+  mCompositorFixedLayerMargins.bottom = ScreenCoord(aBottom);
+}
+
+/*static*/
+LayerToParentLayerMatrix4x4 APZCTreeManager::ComputeTransformForScrollThumb(
+    const LayerToParentLayerMatrix4x4& aCurrentTransform,
+    const Matrix4x4& aScrollableContentTransform, AsyncPanZoomController* aApzc,
+    const FrameMetrics& aMetrics, const ScrollbarData& aScrollbarData,
+    bool aScrollbarIsDescendant) {
+  return apz::ComputeTransformForScrollThumb(
+      aCurrentTransform, aScrollableContentTransform, aApzc, aMetrics,
+      aScrollbarData, aScrollbarIsDescendant);
+}
+
+APZSampler* APZCTreeManager::GetSampler() const {
+  // We should always have a sampler here, since in practice the sampler
+  // is destroyed at the same time that this APZCTreeMAnager instance is.
+  MOZ_ASSERT(mSampler);
+  return mSampler;
+}
+
+void APZCTreeManager::AssertOnSamplerThread() {
+  GetSampler()->AssertOnSamplerThread();
+}
+
+APZUpdater* APZCTreeManager::GetUpdater() const {
+  // We should always have an updater here, since in practice the updater
+  // is destroyed at the same time that this APZCTreeManager instance is.
+  MOZ_ASSERT(mUpdater);
+  return mUpdater;
+}
+
+void APZCTreeManager::AssertOnUpdaterThread() {
+  GetUpdater()->AssertOnUpdaterThread();
+}
+
+MOZ_PUSH_IGNORE_THREAD_SAFETY
+void APZCTreeManager::LockTree() {
+  AssertOnUpdaterThread();
+  mTreeLock.Lock();
+}
+
+void APZCTreeManager::UnlockTree() {
+  AssertOnUpdaterThread();
+  mTreeLock.Unlock();
+}
+MOZ_POP_THREAD_SAFETY
+
+void APZCTreeManager::SetDPI(float aDpiValue) {
+  if (!APZThreadUtils::IsControllerThread()) {
+    APZThreadUtils::RunOnControllerThread(
+        NewRunnableMethod<float>("layers::APZCTreeManager::SetDPI", this,
+                                 &APZCTreeManager::SetDPI, aDpiValue));
+    return;
+  }
+
+  APZThreadUtils::AssertOnControllerThread();
+  mDPI = aDpiValue;
+}
+
+float APZCTreeManager::GetDPI() const {
+  APZThreadUtils::AssertOnControllerThread();
+  return mDPI;
+}
+
+APZCTreeManager::FixedPositionInfo::FixedPositionInfo(
+    const HitTestingTreeNode* aNode) {
+  mFixedPositionAnimationId = aNode->GetFixedPositionAnimationId();
+  mFixedPosSides = aNode->GetFixedPosSides();
+  mFixedPosTarget = aNode->GetFixedPosTarget();
+  mLayersId = aNode->GetLayersId();
+}
+
+APZCTreeManager::StickyPositionInfo::StickyPositionInfo(
+    const HitTestingTreeNode* aNode) {
+  mStickyPositionAnimationId = aNode->GetStickyPositionAnimationId();
+  mFixedPosSides = aNode->GetFixedPosSides();
+  mStickyPosTarget = aNode->GetStickyPosTarget();
+  mLayersId = aNode->GetLayersId();
+  mStickyScrollRangeInner = aNode->GetStickyScrollRangeInner();
+  mStickyScrollRangeOuter = aNode->GetStickyScrollRangeOuter();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/APZCTreeManager.h b/gfx/layers/apz/src/APZCTreeManager.h
new file mode 100644
index 0000000000..ff9bba51ca
--- /dev/null
+++ b/gfx/layers/apz/src/APZCTreeManager.h
@@ -0,0 +1,1110 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_APZCTreeManager_h
+#define mozilla_layers_APZCTreeManager_h
+
+#include <unordered_map>  // for std::unordered_map
+
+#include "FocusState.h"          // for FocusState
+#include "HitTestingTreeNode.h"  // for HitTestingTreeNodeAutoLock
+#include "IAPZHitTester.h"       // for IAPZHitTester::HitTestResult
+#include "gfxPoint.h"            // for gfxPoint
+#include "mozilla/Assertions.h"  // for MOZ_ASSERT_HELPER2
+#include "mozilla/DataMutex.h"   // for DataMutex
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/gfx/Logging.h"              // for gfx::TreeLog
+#include "mozilla/gfx/Matrix.h"               // for Matrix4x4
+#include "mozilla/layers/APZInputBridge.h"    // for APZInputBridge
+#include "mozilla/layers/APZTestData.h"       // for APZTestData
+#include "mozilla/layers/APZUtils.h"          // for GeckoViewMetrics
+#include "mozilla/layers/IAPZCTreeManager.h"  // for IAPZCTreeManager
+#include "mozilla/layers/ScrollbarData.h"
+#include "mozilla/layers/LayersTypes.h"
+#include "mozilla/layers/KeyboardMap.h"      // for KeyboardMap
+#include "mozilla/layers/TouchCounter.h"     // for TouchCounter
+#include "mozilla/layers/ZoomConstraints.h"  // for ZoomConstraints
+#include "mozilla/webrender/webrender_ffi.h"
+#include "mozilla/RecursiveMutex.h"  // for RecursiveMutex
+#include "mozilla/RefPtr.h"          // for RefPtr
+#include "mozilla/TimeStamp.h"       // for mozilla::TimeStamp
+#include "mozilla/UniquePtr.h"       // for UniquePtr
+#include "nsCOMPtr.h"                // for already_AddRefed
+#include "nsTArray.h"
+#include "OvershootDetector.h"
+
+namespace mozilla {
+class MultiTouchInput;
+
+namespace wr {
+class TransactionWrapper;
+class WebRenderAPI;
+}  // namespace wr
+
+namespace layers {
+
+class Layer;
+class AsyncPanZoomController;
+class APZCTreeManagerParent;
+class APZSampler;
+class APZUpdater;
+class CompositorBridgeParent;
+class OverscrollHandoffChain;
+struct OverscrollHandoffState;
+class FocusTarget;
+struct FlingHandoffState;
+class InputQueue;
+struct InputBlockCallbackInfo;
+class GeckoContentController;
+class HitTestingTreeNode;
+class SampleTime;
+class WebRenderScrollDataWrapper;
+struct AncestorTransform;
+struct ScrollThumbData;
+struct ZoomTarget;
+
+/**
+ * ****************** NOTE ON LOCK ORDERING IN APZ **************************
+ *
+ * To avoid deadlock, APZ imposes and respects a global ordering on threads
+ * and locks relevant to APZ.
+ *
+ * Please see the "Threading / Locking Overview" section of
+ * gfx/docs/AsyncPanZoom.rst (hosted in rendered form at
+ * https://firefox-source-docs.mozilla.org/gfx/gfx/AsyncPanZoom.html#threading-locking-overview)
+ * for what the ordering is, and what are the rules for respecting it.
+ * **************************************************************************
+ */
+
+/**
+ * This class manages the tree of AsyncPanZoomController instances. There is one
+ * instance of this class owned by each CompositorBridgeParent, and it contains
+ * as many AsyncPanZoomController instances as there are scrollable container
+ * layers. This class generally lives on the updater thread, although some
+ * functions may be called from other threads as noted; thread safety is ensured
+ * internally.
+ *
+ * The bulk of the work of this class happens as part of the
+ * UpdateHitTestingTree function, which is when a layer tree update is received
+ * by the compositor. This function walks through the layer tree and creates a
+ * tree of HitTestingTreeNode instances to match the layer tree and for use in
+ * hit-testing on the controller thread. APZC instances may be preserved across
+ * calls to this function if the corresponding layers are still present in the
+ * layer tree.
+ *
+ * The other functions on this class are used by various pieces of client code
+ * to notify the APZC instances of events relevant to them. This includes, for
+ * example, user input events that drive panning and zooming, changes to the
+ * scroll viewport area, and changes to pan/zoom constraints.
+ *
+ * Note that the ClearTree function MUST be called when this class is no longer
+ * needed; see the method documentation for details.
+ *
+ * Behaviour of APZ is controlled by a number of preferences shown
+ * \ref APZCPrefs "here".
+ */
+class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
+  typedef mozilla::layers::AllowedTouchBehavior AllowedTouchBehavior;
+  typedef mozilla::layers::AsyncDragMetrics AsyncDragMetrics;
+  using HitTestResult = IAPZHitTester::HitTestResult;
+
+  /*
+   * A result from APZCTreeManager::FindHandoffParent.
+   */
+  struct TargetApzcForNodeResult {
+    // The APZC to handoff overscroll to.
+    AsyncPanZoomController* mApzc;
+    // Targeting a document's root APZC from content fixed to the document.
+    bool mIsFixed;
+  };
+
+  // Helper struct to hold some state while we build the hit-testing tree. The
+  // sole purpose of this struct is to shorten the argument list to
+  // UpdateHitTestingTree. All the state that we don't need to
+  // push on the stack during recursion and pop on unwind is stored here.
+  struct TreeBuildingState;
+
+ public:
+  static mozilla::LazyLogModule sLog;
+
+  static already_AddRefed<APZCTreeManager> Create(
+      LayersId aRootLayersId, UniquePtr<IAPZHitTester> aHitTester = nullptr);
+  void SetSampler(APZSampler* aSampler);
+  void SetUpdater(APZUpdater* aUpdater);
+
+  /**
+   * Notifies this APZCTreeManager that the associated compositor is now
+   * responsible for managing another layers id, which got moved over from
+   * some other compositor. That other compositor's APZCTreeManager is also
+   * provided. This allows APZCTreeManager to transfer any necessary state
+   * from the old APZCTreeManager related to that layers id.
+   * This function must be called on the updater thread.
+   */
+  void NotifyLayerTreeAdopted(LayersId aLayersId,
+                              const RefPtr<APZCTreeManager>& aOldTreeManager);
+
+  /**
+   * Notifies this APZCTreeManager that a layer tree being managed by the
+   * associated compositor has been removed/destroyed. Note that this does
+   * NOT get called during shutdown situations, when the root layer tree is
+   * also getting destroyed.
+   * This function must be called on the updater thread.
+   */
+  void NotifyLayerTreeRemoved(LayersId aLayersId);
+
+  /**
+   * Rebuild the focus state based on the focus target from the layer tree
+   * update that just occurred. This must be called on the updater thread.
+   *
+   * @param aRootLayerTreeId The layer tree ID of the root layer corresponding
+   *                         to this APZCTreeManager
+   * @param aOriginatingLayersId The layer tree ID of the layer corresponding to
+   *                             this layer tree update.
+   */
+  void UpdateFocusState(LayersId aRootLayerTreeId,
+                        LayersId aOriginatingLayersId,
+                        const FocusTarget& aFocusTarget);
+
+  /**
+   * Rebuild the hit-testing tree based on an incoming WebRender transaction.
+   * Preserve nodes and APZC instances where possible, but retire those whose
+   * layers are no longer in the layer tree.
+   * (Note: "layer tree" here refers to the tree of WebRenderLayerScrollData
+   *  nodes sent as part of a WebRender transaction.)
+   *
+   * This must be called on the updater thread.
+   *
+   * @param aRoot The root of the (full) layer tree
+   * @param aOriginatingLayersId The layers id of the subtree that triggered
+   *                             this repaint, and to which aIsFirstPaint
+   *                             applies.
+   * @param aIsFirstPaint True if the transaction that this is called in
+   *                      response to included a first-paint. If this is true,
+   *                      the part of the tree that is affected by the
+   *                      first-paint flag is indicated by the
+   *                      aOriginatingLayersId parameter.
+   * @param aPaintSequenceNumber The sequence number of the paint that triggered
+   *                             this layer update. Note that every child
+   *                             process' layer subtree has its own sequence
+   *                             numbers.
+   */
+  void UpdateHitTestingTree(const WebRenderScrollDataWrapper& aRoot,
+                            bool aIsFirstPaint, LayersId aOriginatingLayersId,
+                            uint32_t aPaintSequenceNumber);
+
+  /**
+   * Called when webrender is enabled, from the sampler thread. This function
+   * populates the provided transaction with any async scroll offsets needed.
+   * It also advances APZ animations to the specified sample time, and requests
+   * another composite if there are still active animations.
+   * In effect it is the webrender equivalent of (part of) the code in
+   * AsyncCompositionManager.
+   */
+  void SampleForWebRender(const Maybe<VsyncId>& aVsyncId,
+                          wr::TransactionWrapper& aTxn,
+                          const SampleTime& aSampleTime);
+
+  /**
+   * Refer to the documentation of APZInputBridge::ReceiveInputEvent() and
+   * APZEventResult.
+   */
+  APZEventResult ReceiveInputEvent(
+      InputData& aEvent,
+      InputBlockCallback&& aCallback = InputBlockCallback()) override;
+
+  /**
+   * Set the keyboard shortcuts to use for translating keyboard events.
+   */
+  void SetKeyboardMap(const KeyboardMap& aKeyboardMap) override;
+
+  /**
+   * Kicks an animation to zoom to a rect. This may be either a zoom out or zoom
+   * in. The actual animation is done on the sampler thread after being set
+   * up. |aRect| must be given in CSS pixels, relative to the document.
+   * |aFlags| is a combination of the ZoomToRectBehavior enum values.
+   */
+  void ZoomToRect(const ScrollableLayerGuid& aGuid,
+                  const ZoomTarget& aZoomTarget,
+                  const uint32_t aFlags = DEFAULT_BEHAVIOR) override;
+
+  /**
+   * If we have touch listeners, this should always be called when we know
+   * definitively whether or not content has preventDefaulted any touch events
+   * that have come in. If |aPreventDefault| is true, any touch events in the
+   * queue will be discarded. This function must be called on the controller
+   * thread.
+   */
+  void ContentReceivedInputBlock(uint64_t aInputBlockId,
+                                 bool aPreventDefault) override;
+
+  /**
+   * When the event regions code is enabled, this function should be invoked to
+   * to confirm the target of the input block. This is only needed in cases
+   * where the initial input event of the block hit a dispatch-to-content region
+   * but is safe to call for all input blocks.
+   * The different elements in the array of targets correspond to the targets
+   * for the different touch points. In the case where the touch point has no
+   * target, or the target is not a scrollable frame, the target's |mScrollId|
+   * should be set to ScrollableLayerGuid::NULL_SCROLL_ID.
+   * Note: For mouse events that start a scrollbar drag, both SetTargetAPZC()
+   *       and StartScrollbarDrag() will be called, and the calls may happen
+   *       in either order. That's fine - whichever arrives first will confirm
+   *       the block, and StartScrollbarDrag() will fill in the drag metrics.
+   *       If the block is confirmed before we have drag metrics, some events
+   *       in the drag block may be handled as no-ops until the drag metrics
+   *       arrive.
+   */
+  void SetTargetAPZC(uint64_t aInputBlockId,
+                     const nsTArray<ScrollableLayerGuid>& aTargets) override;
+
+  /**
+   * Updates any zoom constraints contained in the <meta name="viewport"> tag.
+   * If the |aConstraints| is Nothing() then previously-provided constraints for
+   * the given |aGuid| are cleared.
+   */
+  void UpdateZoomConstraints(
+      const ScrollableLayerGuid& aGuid,
+      const Maybe<ZoomConstraints>& aConstraints) override;
+
+  /**
+   * Calls Destroy() on all APZC instances attached to the tree, and resets the
+   * tree back to empty. This function must be called exactly once during the
+   * lifetime of this APZCTreeManager, when this APZCTreeManager is no longer
+   * needed. Failing to call this function may prevent objects from being freed
+   * properly.
+   * This must be called on the updater thread.
+   */
+  void ClearTree();
+
+  /**
+   * Sets the dpi value used by all AsyncPanZoomControllers attached to this
+   * tree manager.
+   * DPI defaults to 160 if not set using SetDPI() at any point.
+   */
+  void SetDPI(float aDpiValue) override;
+
+  /**
+   * Returns the current dpi value in use.
+   */
+  float GetDPI() const;
+
+  /**
+   * Find the hit testing node for the scrollbar thumb that matches these
+   * drag metrics. Initializes aOutThumbNode with the node, if there is one.
+   */
+  void FindScrollThumbNode(const AsyncDragMetrics& aDragMetrics,
+                           LayersId aLayersId,
+                           HitTestingTreeNodeAutoLock& aOutThumbNode);
+
+  /**
+   * Sets allowed touch behavior values for current touch-session for specific
+   * input block (determined by aInputBlock).
+   * Should be invoked by the widget. Each value of the aValues arrays
+   * corresponds to the different touch point that is currently active.
+   * Must be called after receiving the TOUCH_START event that starts the
+   * touch-session.
+   */
+  void SetAllowedTouchBehavior(
+      uint64_t aInputBlockId,
+      const nsTArray<TouchBehaviorFlags>& aValues) override;
+
+  void SetBrowserGestureResponse(uint64_t aInputBlockId,
+                                 BrowserGestureResponse aResponse) override;
+
+  /**
+   * This is a callback for AsyncPanZoomController to call when it wants to
+   * scroll in response to a touch-move event, or when it needs to hand off
+   * overscroll to the next APZC. Note that because of scroll grabbing, the
+   * first APZC to scroll may not be the one that is receiving the touch events.
+   *
+   * |aPrev| is the APZC that received the touch events triggering the scroll
+   *   (in the case of an initial scroll), or the last APZC to scroll (in the
+   *   case of overscroll)
+   * |aStartPoint| and |aEndPoint| are in |aPrev|'s transformed screen
+   *   coordinates (i.e. the same coordinates in which touch points are given to
+   *   APZCs). The amount of (over)scroll is represented by two points rather
+   *   than a displacement because with certain 3D transforms, the same
+   *   displacement between different points in transformed coordinates can
+   *   represent different displacements in untransformed coordinates.
+   * |aOverscrollHandoffChain| is the overscroll handoff chain used for
+   *   determining the order in which scroll should be handed off between
+   *   APZCs
+   * |aOverscrollHandoffChainIndex| is the next position in the overscroll
+   *   handoff chain that should be scrolled.
+   *
+   * aStartPoint and aEndPoint will be modified depending on how much of the
+   * scroll each APZC consumes. This is to allow the sending APZC to go into
+   * an overscrolled state if no APZC further up in the handoff chain accepted
+   * the entire scroll.
+   *
+   * The function will return true if the entire scroll was consumed, and
+   * false otherwise. As this function also modifies aStartPoint and aEndPoint,
+   * when scroll is consumed, it should always the case that this function
+   * returns true if and only if IsZero(aStartPoint - aEndPoint), using the
+   * modified aStartPoint and aEndPoint after the function returns.
+   *
+   * The way this method works is best illustrated with an example.
+   * Consider three nested APZCs, A, B, and C, with C being the innermost one.
+   * Say B is scroll-grabbing.
+   * The touch events go to C because it's the innermost one (so e.g. taps
+   * should go through C), but the overscroll handoff chain is B -> C -> A
+   * because B is scroll-grabbing.
+   * For convenience I'll refer to the three APZC objects as A, B, and C, and
+   * to the tree manager object as TM.
+   * Here's what happens when C receives a touch-move event:
+   *   - C.TrackTouch() calls TM.DispatchScroll() with index = 0.
+   *   - TM.DispatchScroll() calls B.AttemptScroll() (since B is at index 0 in
+   *     the chain).
+   *   - B.AttemptScroll() scrolls B. If there is overscroll, it calls
+   *     TM.DispatchScroll() with index = 1.
+   *   - TM.DispatchScroll() calls C.AttemptScroll() (since C is at index 1 in
+   *     the chain)
+   *   - C.AttemptScroll() scrolls C. If there is overscroll, it calls
+   *     TM.DispatchScroll() with index = 2.
+   *   - TM.DispatchScroll() calls A.AttemptScroll() (since A is at index 2 in
+   *     the chain)
+   *   - A.AttemptScroll() scrolls A. If there is overscroll, it calls
+   *     TM.DispatchScroll() with index = 3.
+   *   - TM.DispatchScroll() discards the rest of the scroll as there are no
+   *     more elements in the chain.
+   *
+   * Note: this should be used for panning only. For handing off overscroll for
+   *       a fling, use DispatchFling().
+   */
+  bool DispatchScroll(AsyncPanZoomController* aPrev,
+                      ParentLayerPoint& aStartPoint,
+                      ParentLayerPoint& aEndPoint,
+                      OverscrollHandoffState& aOverscrollHandoffState);
+
+  /**
+   * This is a callback for AsyncPanZoomController to call when it wants to
+   * start a fling in response to a touch-end event, or when it needs to hand
+   * off a fling to the next APZC. Note that because of scroll grabbing, the
+   * first APZC to fling may not be the one that is receiving the touch events.
+   *
+   * @param aApzc the APZC that wants to start or hand off the fling
+   * @param aHandoffState a collection of state about the operation,
+   *                      which contains the following:
+   *
+   *        mVelocity the current velocity of the fling, in |aApzc|'s screen
+   *                  pixels per millisecond
+   *        mChain the chain of APZCs along which the fling
+   *                   should be handed off
+   *        mIsHandoff is true if |aApzc| is handing off an existing fling (in
+   *                   this case the fling is given to the next APZC in the
+   *                   handoff chain after |aApzc|), and false is |aApzc| wants
+   *                   start a fling (in this case the fling is given to the
+   *                   first APZC in the chain)
+   *
+   * The return value is the "residual velocity", the portion of
+   * |aHandoffState.mVelocity| that was not consumed by APZCs in the
+   * handoff chain doing flings.
+   * The caller can use this value to determine whether it should consume
+   * the excess velocity by going into overscroll.
+   */
+  ParentLayerPoint DispatchFling(AsyncPanZoomController* aApzc,
+                                 const FlingHandoffState& aHandoffState);
+
+  void StartScrollbarDrag(const ScrollableLayerGuid& aGuid,
+                          const AsyncDragMetrics& aDragMetrics) override;
+
+  bool StartAutoscroll(const ScrollableLayerGuid& aGuid,
+                       const ScreenPoint& aAnchorLocation) override;
+
+  void StopAutoscroll(const ScrollableLayerGuid& aGuid) override;
+
+  /*
+   * Build the chain of APZCs that will handle overscroll for a pan starting at
+   * |aInitialTarget|.
+   */
+  RefPtr<const OverscrollHandoffChain> BuildOverscrollHandoffChain(
+      const RefPtr<AsyncPanZoomController>& aInitialTarget);
+
+  /**
+   * Function used to disable LongTap gestures.
+   *
+   * On slow running tests, drags and touch events can be misinterpreted
+   * as a long tap. This allows tests to disable long tap gesture detection.
+   */
+  void SetLongTapEnabled(bool aTapGestureEnabled) override;
+
+  APZInputBridge* InputBridge() override { return this; }
+
+  /**
+   * Add a callback to be invoked when |aInputBlockId| is ready for handling.
+   *
+   * Should only be used for input blocks that are not yet ready for handling
+   * at the time this is called. If the input block was already handled,
+   * the callback will never be called.
+   *
+   * Only one callback can be registered for an input block at a time.
+   * Subsequent attempts to register a callback for an input block will be
+   * ignored until the existing callback is triggered.
+   */
+  void AddInputBlockCallback(uint64_t aInputBlockId,
+                             InputBlockCallbackInfo&& aCallbackInfo);
+
+  // Methods to help process WidgetInputEvents (or manage conversion to/from
+  // InputData)
+
+  void ProcessUnhandledEvent(LayoutDeviceIntPoint* aRefPoint,
+                             ScrollableLayerGuid* aOutTargetGuid,
+                             uint64_t* aOutFocusSequenceNumber,
+                             LayersId* aOutLayersId) override;
+
+  void UpdateWheelTransaction(
+      LayoutDeviceIntPoint aRefPoint, EventMessage aEventMessage,
+      const Maybe<ScrollableLayerGuid>& aTargetGuid) override;
+
+  bool GetAPZTestData(LayersId aLayersId, APZTestData* aOutData);
+
+  /**
+   * Iterates over the hit testing tree, collects LayersIds and associated
+   * transforms from layer coordinate space to root coordinate space, and
+   * sends these over to the main thread of the chrome process. If the provided
+   * |aAncestor| argument is non-null, then only the transforms for layer
+   * subtrees scrolled by the aAncestor (i.e. descendants of aAncestor) will be
+   * sent.
+   */
+  void SendSubtreeTransformsToChromeMainThread(
+      const AsyncPanZoomController* aAncestor);
+
+  /**
+   * Set fixed layer margins for dynamic toolbar.
+   */
+  void SetFixedLayerMargins(ScreenIntCoord aTop, ScreenIntCoord aBottom);
+
+  /**
+   * Refer to apz::ComputeTransformForScrollThumb() for a description
+   * of the parameters.
+   */
+  static LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumb(
+      const LayerToParentLayerMatrix4x4& aCurrentTransform,
+      const gfx::Matrix4x4& aScrollableContentTransform,
+      AsyncPanZoomController* aApzc, const FrameMetrics& aMetrics,
+      const ScrollbarData& aScrollbarData, bool aScrollbarIsDescendant);
+
+  /**
+   * Dispatch a flush complete notification from the repaint thread of the
+   * content controller for the given layers id.
+   */
+  static void FlushApzRepaints(LayersId aLayersId);
+
+  /**
+   * Mark |aLayersId| as having been moved from the compositor that owns this
+   * tree manager to a compositor that doesn't use APZ.
+   * See |mDetachedLayersIds| for more details.
+   */
+  void MarkAsDetached(LayersId aLayersId);
+
+  // Assert that the current thread is the sampler thread for this APZCTM.
+  void AssertOnSamplerThread();
+  // Assert that the current thread is the updater thread for this APZCTM.
+  void AssertOnUpdaterThread();
+
+  // Returns a pointer to the WebRenderAPI this APZCTreeManager is for.
+  // This might be null (for example, if WebRender is not enabled).
+  already_AddRefed<wr::WebRenderAPI> GetWebRenderAPI() const;
+
+ protected:
+  APZCTreeManager(LayersId aRootLayersId, UniquePtr<IAPZHitTester> aHitTester);
+
+  void Init();
+
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~APZCTreeManager();
+
+  APZSampler* GetSampler() const;
+  APZUpdater* GetUpdater() const;
+
+  bool AdvanceAnimationsInternal(const MutexAutoLock& aProofOfMapLock,
+                                 const SampleTime& aSampleTime);
+
+  // We need to allow APZUpdater to lock and unlock this tree during a WR
+  // scene swap. We do this using private helpers to avoid exposing these
+  // functions to the world.
+ private:
+  friend class APZUpdater;
+  void LockTree() MOZ_CAPABILITY_ACQUIRE(mTreeLock);
+  void UnlockTree() MOZ_CAPABILITY_RELEASE(mTreeLock);
+
+  // Protected hooks for gtests subclass
+  virtual already_AddRefed<AsyncPanZoomController> NewAPZCInstance(
+      LayersId aLayersId, GeckoContentController* aController);
+
+ public:
+  // Public hook for gtests subclass
+  virtual SampleTime GetFrameTime();
+
+  // Also used for controlling time during tests
+  void SetTestSampleTime(const Maybe<TimeStamp>& aTime);
+
+ private:
+  mutable DataMutex<Maybe<TimeStamp>> mTestSampleTime;
+  CopyableTArray<MatrixMessage> mLastMessages;
+
+ public:
+  /* Some helper functions to find an APZC given some identifying input. These
+     functions lock the tree of APZCs while they find the right one, and then
+     return an addref'd pointer to it. This allows caller code to just use the
+     target APZC without worrying about it going away. These are public for
+     testing code and generally should not be used by other production code.
+  */
+  RefPtr<HitTestingTreeNode> GetRootNode() const;
+  HitTestResult GetTargetAPZC(const ScreenPoint& aPoint);
+  already_AddRefed<AsyncPanZoomController> GetTargetAPZC(
+      const LayersId& aLayersId,
+      const ScrollableLayerGuid::ViewID& aScrollId) const;
+  already_AddRefed<AsyncPanZoomController> GetTargetAPZC(
+      const LayersId& aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+      const MutexAutoLock& aProofOfMapLock) const;
+  ScreenToParentLayerMatrix4x4 GetScreenToApzcTransform(
+      const AsyncPanZoomController* aApzc) const;
+  ParentLayerToScreenMatrix4x4 GetApzcToGeckoTransformForHit(
+      HitTestResult& aHitResult) const;
+  ParentLayerToScreenMatrix4x4 GetApzcToGeckoTransform(
+      const AsyncPanZoomController* aApzc,
+      const AsyncTransformComponents& aComponents) const;
+
+  /*
+   * A common utility function used for GetApzcToGeckoTransform and
+   * GetOopifApzcToRootContentApzcTransform.
+   *
+   * NOTE: The matrix returned by this function can NOT be used to convert
+   * metrics in |aStartApzc| to |aStopApzc|. If you want the conversion matrix,
+   * you will have to use either GetApzcToGeckoTransform or
+   * GetOopifApzcToRootContentApzcTransform.
+   */
+  ParentLayerToParentLayerMatrix4x4 GetApzcToApzcTransform(
+      const AsyncPanZoomController* aStartApzc,
+      const AsyncPanZoomController* aStopApzc,
+      const AsyncTransformComponents& aComponents) const;
+
+  /*
+   * Returns the matrix which transforms coordinates relative to the layout
+   * viewport of |aApzc|, to be relative to the document origin of the root
+   * content APZC of |aApzc|.
+   * |aApzc| must be the root APZC of an out-of-process iframe.
+   */
+  CSSToCSSMatrix4x4 GetOopifToRootContentTransform(
+      AsyncPanZoomController* aApzc) const;
+
+  /**
+   * Convert the given |aRect| in the document coordinates of |aApzc| to the top
+   * level document coordinates.
+   * |aApzc| must be an in-process root APZC.
+   */
+  CSSRect ConvertRectInApzcToRoot(AsyncPanZoomController* aApzc,
+                                  const CSSRect& aRect) const;
+
+  ScreenPoint GetCurrentMousePosition() const;
+  void SetCurrentMousePosition(const ScreenPoint& aNewPos);
+
+  /**
+   * Convert a screen point of an event targeting |aApzc| to Gecko
+   * coordinates.
+   */
+  Maybe<ScreenIntPoint> ConvertToGecko(const ScreenIntPoint& aPoint,
+                                       AsyncPanZoomController* aApzc);
+
+  /**
+   * Find the zoomable APZC in the same layer subtree (i.e. with the same
+   * layers id) as the given APZC.
+   */
+  already_AddRefed<AsyncPanZoomController> FindZoomableApzc(
+      AsyncPanZoomController* aStart) const;
+
+  AsyncPanZoomController* FindRootApzcFor(LayersId aLayersId) const;
+
+  ScreenMargin GetCompositorFixedLayerMargins() const;
+
+  void AdjustEventPointForDynamicToolbar(ScreenIntPoint& aEventPoint,
+                                         const HitTestResult& aHit);
+
+  APZScrollGeneration NewAPZScrollGeneration() {
+    // In the production code this function gets only called from the sampler
+    // thread but in tests using nsIDOMWindowUtils.setAsyncScrollOffset this
+    // function gets called from the controller thread so we need to lock the
+    // mutex for this counter.
+    MutexAutoLock lock(mScrollGenerationLock);
+    return mScrollGenerationCounter.NewAPZGeneration();
+  }
+
+  template <typename Callback>
+  void CallWithMapLock(Callback& aCallback) {
+    MutexAutoLock lock(mMapLock);
+    aCallback(lock);
+  }
+
+ private:
+  using GuidComparator = ScrollableLayerGuid::Comparator;
+  using ScrollNode = WebRenderScrollDataWrapper;
+
+  /* Helpers */
+
+  void AttachNodeToTree(HitTestingTreeNode* aNode, HitTestingTreeNode* aParent,
+                        HitTestingTreeNode* aNextSibling)
+      MOZ_REQUIRES(mTreeLock);
+  already_AddRefed<AsyncPanZoomController> GetTargetAPZC(
+      const ScrollableLayerGuid& aGuid);
+  already_AddRefed<HitTestingTreeNode> GetTargetNode(
+      const ScrollableLayerGuid& aGuid, GuidComparator aComparator) const;
+  HitTestingTreeNode* FindTargetNode(HitTestingTreeNode* aNode,
+                                     const ScrollableLayerGuid& aGuid,
+                                     GuidComparator aComparator);
+  TargetApzcForNodeResult GetTargetApzcForNode(const HitTestingTreeNode* aNode);
+  TargetApzcForNodeResult FindHandoffParent(
+      const AsyncPanZoomController* aApzc);
+  /**
+   * Find the root __content__ APZC for |aLayersId|.
+   * If |aLayersId| is NOT for the LayersId for the root content, this function
+   * returns nullptr.
+   *
+   * NOTE: Only the top-level content document will have a root content APZC.
+   */
+  HitTestingTreeNode* FindRootNodeForLayersId(LayersId aLayersId) const;
+  AsyncPanZoomController* FindRootContentApzcForLayersId(
+      LayersId aLayersId) const;
+  already_AddRefed<AsyncPanZoomController> GetZoomableTarget(
+      AsyncPanZoomController* aApzc1, AsyncPanZoomController* aApzc2) const;
+  already_AddRefed<AsyncPanZoomController> CommonAncestor(
+      AsyncPanZoomController* aApzc1, AsyncPanZoomController* aApzc2) const;
+
+  struct FixedPositionInfo;
+  struct StickyPositionInfo;
+
+  // Returns true if |aNode| is a fixed layer that is fixed to the root content
+  // APZC.
+  // The map lock is required within these functions; if the map lock is already
+  // being held by the caller, the second overload should be used. If the map
+  // lock is not being held at the call site, the first overload should be used.
+  bool IsFixedToRootContent(const HitTestingTreeNode* aNode) const;
+  bool IsFixedToRootContent(const FixedPositionInfo& aFixedInfo,
+                            const MutexAutoLock& aProofOfMapLock) const;
+
+  // Returns the vertical sides of |aNode| that are stuck to the root content.
+  // The map lock is required within these functions; if the map lock is already
+  // being held by the caller, the second overload should be used. If the map
+  // lock is not being held at the call site, the first overload should be used.
+  SideBits SidesStuckToRootContent(const HitTestingTreeNode* aNode,
+                                   AsyncTransformConsumer aMode) const;
+  SideBits SidesStuckToRootContent(const StickyPositionInfo& aStickyInfo,
+                                   AsyncTransformConsumer aMode,
+                                   const MutexAutoLock& aProofOfMapLock) const;
+
+  /**
+   * Perform hit testing for a touch-start event.
+   *
+   * @param aEvent The touch-start event.
+   *
+   * The remaining parameters are out-parameter used to communicate additional
+   * return values:
+   *
+   * @param aOutTouchBehaviors
+   *     The touch behaviours that should be allowed for this touch block.
+
+   * @return The results of the hit test, including the APZC that was hit.
+   */
+  HitTestResult GetTouchInputBlockAPZC(
+      const MultiTouchInput& aEvent,
+      nsTArray<TouchBehaviorFlags>* aOutTouchBehaviors);
+
+  /**
+   * A helper structure for use by ReceiveInputEvent() and its helpers.
+   */
+  struct InputHandlingState {
+    // A reference to the event being handled.
+    InputData& mEvent;
+
+    // The value that will be returned by ReceiveInputEvent().
+    APZEventResult mResult;
+
+    // If we performed a hit-test while handling this input event, or
+    // reused the result of a previous hit-test in the input block,
+    // this is populated with the result of the hit test.
+    HitTestResult mHit;
+
+    // Called at the end of ReceiveInputEvent() to perform any final
+    // computations, and then return mResult.
+    // If the event will have a delayed result then this takes care of adding
+    // the specified callback to the APZCTreeManager.
+    APZEventResult Finish(APZCTreeManager& aTreeManager,
+                          InputBlockCallback&& aCallback);
+  };
+
+  void ProcessTouchInput(InputHandlingState& aState, MultiTouchInput& aInput);
+  /**
+   * Given a mouse-down event that hit a scroll thumb node, set up APZ
+   * dragging of the scroll thumb.
+   *
+   * Must be called after the mouse event has been sent to InputQueue.
+   *
+   * @param aMouseInput The mouse-down event.
+   * @param aScrollThumbNode Tthe scroll thumb node that was hit.
+   * @param aApzc
+   *     The APZC for the scroll frame scrolled by the scroll thumb, if that
+   *     scroll frame is layerized. (A thumb can be layerized without its
+   *     target scroll frame being layerized.) Otherwise, an enclosing APZC.
+   */
+  void SetupScrollbarDrag(MouseInput& aMouseInput,
+                          const HitTestingTreeNodeAutoLock& aScrollThumbNode,
+                          AsyncPanZoomController* aApzc);
+  /**
+   * Process a touch event that's part of a scrollbar touch-drag gesture.
+   *
+   * @param aInput The touch event.
+   * @param aScrollThumbNode
+   *     If this is the touch-start event, the node representing the scroll
+   *     thumb we are starting to drag. Otherwise nullptr.
+   * @param aHitInfo
+   *     The hit-test flags for the touch input.
+   * @return See ReceiveInputEvent() for what the return value means.
+   */
+  APZEventResult ProcessTouchInputForScrollbarDrag(
+      MultiTouchInput& aInput,
+      const HitTestingTreeNodeAutoLock& aScrollThumbNode,
+      const gfx::CompositorHitTestInfo& aHitInfo);
+  void FlushRepaintsToClearScreenToGeckoTransform();
+
+  void SynthesizePinchGestureFromMouseWheel(
+      const ScrollWheelInput& aWheelInput,
+      const RefPtr<AsyncPanZoomController>& aTarget);
+
+  already_AddRefed<HitTestingTreeNode> RecycleOrCreateNode(
+      const RecursiveMutexAutoLock& aProofOfTreeLock, TreeBuildingState& aState,
+      AsyncPanZoomController* aApzc, LayersId aLayersId);
+  HitTestingTreeNode* PrepareNodeForLayer(
+      const RecursiveMutexAutoLock& aProofOfTreeLock, const ScrollNode& aLayer,
+      const FrameMetrics& aMetrics, LayersId aLayersId,
+      const Maybe<ZoomConstraints>& aZoomConstraints,
+      const AncestorTransform& aAncestorTransform, HitTestingTreeNode* aParent,
+      HitTestingTreeNode* aNextSibling, TreeBuildingState& aState);
+
+  void PrintLayerInfo(const ScrollNode& aLayer);
+
+  void NotifyScrollbarDragInitiated(uint64_t aDragBlockId,
+                                    const ScrollableLayerGuid& aGuid,
+                                    ScrollDirection aDirection) const;
+  void NotifyScrollbarDragRejected(const ScrollableLayerGuid& aGuid) const;
+  void NotifyAutoscrollRejected(const ScrollableLayerGuid& aGuid) const;
+
+  // Returns the transform that converts from |aNode|'s coordinates
+  // to the coordinates of |aNode|'s parent in the hit-testing tree. Requires
+  // the caller to hold mTreeLock.
+  LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumbNode(
+      const HitTestingTreeNode* aNode) const MOZ_REQUIRES(mTreeLock);
+
+  // Look up the GeckoContentController for the given layers id.
+  static already_AddRefed<GeckoContentController> GetContentController(
+      LayersId aLayersId);
+
+  using ClippedCompositionBoundsMap =
+      std::unordered_map<ScrollableLayerGuid, ParentLayerRect,
+                         ScrollableLayerGuid::HashIgnoringPresShellFn,
+                         ScrollableLayerGuid::EqualIgnoringPresShellFn>;
+  // This is a recursive function that populates `aDestMap` with the clipped
+  // composition bounds for the APZC corresponding to `aGuid` and returns those
+  // bounds as a convenience. It recurses to also populate `aDestMap` with that
+  // APZC's ancestors. In order to do this it needs to access mApzcMap
+  // and therefore requires the caller to hold the map lock.
+  ParentLayerRect ComputeClippedCompositionBounds(
+      const MutexAutoLock& aProofOfMapLock,
+      ClippedCompositionBoundsMap& aDestMap, ScrollableLayerGuid aGuid);
+
+  ScreenMargin GetCompositorFixedLayerMargins(
+      const MutexAutoLock& aProofOfMapLock) const;
+
+ protected:
+  /* The input queue where input events are held until we know enough to
+   * figure out where they're going. Protected so gtests can access it.
+   */
+  RefPtr<InputQueue> mInputQueue;
+
+  /** A lock that protects mApzcMap, mScrollThumbInfo, mRootScrollbarInfo,
+   * mFixedPositionInfo, and mStickyPositionInfo.
+   */
+  mutable mozilla::Mutex mMapLock;
+
+ private:
+  /* Layers id for the root CompositorBridgeParent that owns this
+   * APZCTreeManager. */
+  LayersId mRootLayersId;
+
+  /* Pointer to the APZSampler instance that is bound to this APZCTreeManager.
+   * The sampler has a RefPtr to this class, and this non-owning raw pointer
+   * back to the APZSampler is nulled out in the sampler's destructor, so this
+   * pointer should always be valid.
+   */
+  APZSampler* MOZ_NON_OWNING_REF mSampler;
+  /* Pointer to the APZUpdater instance that is bound to this APZCTreeManager.
+   * The updater has a RefPtr to this class, and this non-owning raw pointer
+   * back to the APZUpdater is nulled out in the updater's destructor, so this
+   * pointer should always be valid.
+   */
+  APZUpdater* MOZ_NON_OWNING_REF mUpdater;
+
+  /* Whenever walking or mutating the tree rooted at mRootNode, mTreeLock must
+   * be held. This lock does not need to be held while manipulating a single
+   * APZC instance in isolation (that is, if its tree pointers are not being
+   * accessed or mutated). The lock also needs to be held when accessing the
+   * mRootNode instance variable, as that is considered part of the APZC tree
+   * management state.
+   * IMPORTANT: See the note about lock ordering at the top of this file. */
+  mutable mozilla::RecursiveMutex mTreeLock;
+  RefPtr<HitTestingTreeNode> mRootNode MOZ_GUARDED_BY(mTreeLock);
+
+  /*
+   * A set of LayersIds for which APZCTM should only send empty
+   * MatrixMessages via NotifyLayerTransform().
+   * This is used in cases where a tab has been transferred to a non-APZ
+   * compositor (and thus will not receive MatrixMessages reflecting its new
+   * transforms) and we need to make sure it doesn't get stuck with transforms
+   * from its old tree manager (us).
+   * Acquire mTreeLock before accessing this.
+   */
+  std::unordered_set<LayersId, LayersId::HashFn> mDetachedLayersIds
+      MOZ_GUARDED_BY(mTreeLock);
+
+  /**
+   * Helper structure to store a bunch of things in mApzcMap so that they can
+   * be used from the sampler thread.
+   */
+  struct ApzcMapData {
+    // A pointer to the APZC itself
+    RefPtr<AsyncPanZoomController> apzc;
+    // The parent APZC's guid, or Nothing() if there is no parent
+    Maybe<ScrollableLayerGuid> parent;
+  };
+
+  /**
+   * A map for quick access to get some APZC data by guid, without having to
+   * acquire the tree lock. mMapLock must be acquired while accessing or
+   * modifying mApzcMap.
+   */
+  std::unordered_map<ScrollableLayerGuid, ApzcMapData,
+                     ScrollableLayerGuid::HashIgnoringPresShellFn,
+                     ScrollableLayerGuid::EqualIgnoringPresShellFn>
+      mApzcMap;
+  /**
+   * A helper structure to store all the information needed to compute the
+   * async transform for a scrollthumb on the sampler thread.
+   */
+  struct ScrollThumbInfo {
+    uint64_t mThumbAnimationId;
+    CSSTransformMatrix mThumbTransform;
+    ScrollbarData mThumbData;
+    ScrollableLayerGuid mTargetGuid;
+    CSSTransformMatrix mTargetTransform;
+    bool mTargetIsAncestor;
+
+    ScrollThumbInfo(const uint64_t& aThumbAnimationId,
+                    const CSSTransformMatrix& aThumbTransform,
+                    const ScrollbarData& aThumbData,
+                    const ScrollableLayerGuid& aTargetGuid,
+                    const CSSTransformMatrix& aTargetTransform,
+                    bool aTargetIsAncestor)
+        : mThumbAnimationId(aThumbAnimationId),
+          mThumbTransform(aThumbTransform),
+          mThumbData(aThumbData),
+          mTargetGuid(aTargetGuid),
+          mTargetTransform(aTargetTransform),
+          mTargetIsAncestor(aTargetIsAncestor) {
+      MOZ_ASSERT(mTargetGuid.mScrollId == mThumbData.mTargetViewId);
+    }
+  };
+  /**
+   * If this APZCTreeManager is being used with WebRender, this vector gets
+   * populated during a layers update. It holds a package of information needed
+   * to compute and set the async transforms on scroll thumbs. This information
+   * is extracted from the HitTestingTreeNodes for the WebRender case because
+   * accessing the HitTestingTreeNodes requires holding the tree lock which
+   * we cannot do on the WR sampler thread. mScrollThumbInfo, however, can
+   * be accessed while just holding the mMapLock which is safe to do on the
+   * sampler thread.
+   * mMapLock must be acquired while accessing or modifying mScrollThumbInfo.
+   */
+  std::vector<ScrollThumbInfo> mScrollThumbInfo;
+
+  /**
+   * A helper structure to store all the information needed to compute the
+   * async transform for a scrollthumb on the sampler thread.
+   */
+  struct RootScrollbarInfo {
+    uint64_t mScrollbarAnimationId;
+    ScrollDirection mScrollDirection;
+
+    RootScrollbarInfo(const uint64_t& aScrollbarAnimationId,
+                      const ScrollDirection aScrollDirection)
+        : mScrollbarAnimationId(aScrollbarAnimationId),
+          mScrollDirection(aScrollDirection) {}
+  };
+  /**
+   * If this APZCTreeManager is being used with WebRender, this vector gets
+   * populated during a layers update. It holds a package of information needed
+   * to compute and set the async transforms on root scrollbars. This
+   * information is extracted from the HitTestingTreeNodes for the WebRender
+   * case because accessing the HitTestingTreeNodes requires holding the tree
+   * lock which we cannot do on the WR sampler thread. mRootScrollbarInfo,
+   * however, can be accessed while just holding the mMapLock which is safe to
+   * do on the sampler thread.
+   * mMapLock must be acquired while accessing or modifying mRootScrollbarInfo.
+   */
+  std::vector<RootScrollbarInfo> mRootScrollbarInfo;
+
+  /**
+   * A helper structure to store all the information needed to compute the
+   * async transform for a fixed position element on the sampler thread.
+   */
+  struct FixedPositionInfo {
+    Maybe<uint64_t> mFixedPositionAnimationId;
+    SideBits mFixedPosSides;
+    ScrollableLayerGuid::ViewID mFixedPosTarget;
+    LayersId mLayersId;
+
+    explicit FixedPositionInfo(const HitTestingTreeNode* aNode);
+  };
+  /**
+   * If this APZCTreeManager is being used with WebRender, this vector gets
+   * populated during a layers update. It holds a package of information needed
+   * to compute and set the async transforms on fixed position content. This
+   * information is extracted from the HitTestingTreeNodes for the WebRender
+   * case because accessing the HitTestingTreeNodes requires holding the tree
+   * lock which we cannot do on the WR sampler thread. mFixedPositionInfo,
+   * however, can be accessed while just holding the mMapLock which is safe to
+   * do on the sampler thread. mMapLock must be acquired while accessing or
+   * modifying mFixedPositionInfo.
+   */
+  std::vector<FixedPositionInfo> mFixedPositionInfo;
+
+  /**
+   * A helper structure to store all the information needed to compute the
+   * async transform for a sticky position element on the sampler thread.
+   */
+  struct StickyPositionInfo {
+    Maybe<uint64_t> mStickyPositionAnimationId;
+    SideBits mFixedPosSides;
+    ScrollableLayerGuid::ViewID mStickyPosTarget;
+    LayersId mLayersId;
+    LayerRectAbsolute mStickyScrollRangeInner;
+    LayerRectAbsolute mStickyScrollRangeOuter;
+
+    explicit StickyPositionInfo(const HitTestingTreeNode* aNode);
+  };
+  /**
+   * If this APZCTreeManager is being used with WebRender, this vector gets
+   * populated during a layers update. It holds a package of information needed
+   * to compute and set the async transforms on sticky position content. This
+   * information is extracted from the HitTestingTreeNodes for the WebRender
+   * case because accessing the HitTestingTreeNodes requires holding the tree
+   * lock which we cannot do on the WR sampler thread. mStickyPositionInfo,
+   * however, can be accessed while just holding the mMapLock which is safe to
+   * do on the sampler thread. mMapLock must be acquired while accessing or
+   * modifying mStickyPositionInfo.
+   */
+  std::vector<StickyPositionInfo> mStickyPositionInfo;
+
+  /* Holds the zoom constraints for scrollable layers, as determined by the
+   * the main-thread gecko code. This can only be accessed on the updater
+   * thread. */
+  std::unordered_map<ScrollableLayerGuid, ZoomConstraints,
+                     ScrollableLayerGuid::HashIgnoringPresShellFn,
+                     ScrollableLayerGuid::EqualIgnoringPresShellFn>
+      mZoomConstraints;
+  /* A list of keyboard shortcuts to use for translating keyboard inputs into
+   * keyboard actions. This is gathered on the main thread from XBL bindings.
+   * This must only be accessed on the controller thread.
+   */
+  KeyboardMap mKeyboardMap;
+  /* This tracks the focus targets of chrome and content and whether we have
+   * a current focus target or whether we are waiting for a new confirmation.
+   */
+  FocusState mFocusState;
+  /* This tracks the hit test result info for the current touch input block.
+   * In particular, it tracks the target APZC, the hit test flags, and the
+   * fixed pos sides. This is populated at the start of a touch block based
+   * on the hit-test result, and used for subsequent touch events in the block.
+   * This allows touch points to move outside the thing they started on, but
+   * still have the touch events delivered to the same initial APZC. This will
+   * only ever be touched on the input delivery thread, and so does not require
+   * locking.
+   */
+  HitTestResult mTouchBlockHitResult;
+  /* Sometimes we want to ignore all touches except one. In such cases, this
+   * is set to the identifier of the touch we are not ignoring; in other cases,
+   * this is set to -1.
+   */
+  int32_t mRetainedTouchIdentifier;
+  /* This tracks whether the current input block represents a touch-drag of
+   * a scrollbar. In this state, touch events are forwarded to content as touch
+   * events, but converted to mouse events before going into InputQueue and
+   * being handled by an APZC (to reuse the APZ code for scrollbar dragging
+   * with a mouse).
+   */
+  bool mInScrollbarTouchDrag;
+  /* Tracks the number of touch points we are tracking that are currently on
+   * the screen. */
+  TouchCounter mTouchCounter;
+  /* If a tap gesture event sent directly by widget code (rather than gesture
+   * detected from touch events by APZ) is being processed, this stores the
+   * result of hit testing for that tap gesture event.
+   */
+  HitTestResult mTapGestureHitResult;
+  /* Stores the current mouse position in screen coordinates.
+   */
+  mutable DataMutex<ScreenPoint> mCurrentMousePosition;
+  /* Extra margins that should be applied to content that fixed wrt. the
+   * RCD-RSF, to account for the dynamic toolbar.
+   * Acquire mMapLock before accessing this.
+   */
+  ScreenMargin mCompositorFixedLayerMargins;
+  /* Similar to above |mCompositorFixedLayerMargins|. But this value is the
+   * margins on the main-thread at the last time position:fixed elements were
+   * updated during the dynamic toolbar transitions.
+   * Acquire mMapLock before accessing this.
+   */
+  ScreenMargin mGeckoFixedLayerMargins;
+  /* For logging the APZC tree for debugging (enabled by the apz.printtree
+   * pref). The purpose of using LOG_CRITICAL is so that you don't also need to
+   * change the gfx.logging.level pref to see the output. */
+  gfx::TreeLog<gfx::LOG_CRITICAL> mApzcTreeLog;
+
+  class CheckerboardFlushObserver;
+  friend class CheckerboardFlushObserver;
+  RefPtr<CheckerboardFlushObserver> mFlushObserver;
+
+  // Map from layers id to APZTestData. Accesses and mutations must be
+  // protected by the mTestDataLock.
+  std::unordered_map<LayersId, UniquePtr<APZTestData>, LayersId::HashFn>
+      mTestData;
+  mutable mozilla::Mutex mTestDataLock;
+
+  // A state machine that tries to record how much users are overshooting
+  // their desired scroll destination while using the scrollwheel.
+  OvershootDetector mOvershootDetector;
+
+  // This must only be touched on the controller thread.
+  float mDPI;
+
+  friend class IAPZHitTester;
+  UniquePtr<IAPZHitTester> mHitTester;
+
+  // NOTE: This ScrollGenerationCounter needs to be per APZCTreeManager since
+  // the generation is bumped up on the sampler theread which is per
+  // APZCTreeManager.
+  ScrollGenerationCounter mScrollGenerationCounter;
+  mozilla::Mutex mScrollGenerationLock;
+
+#if defined(MOZ_WIDGET_ANDROID)
+ private:
+  // Last Frame metrics sent to java through UIController.
+  GeckoViewMetrics mLastRootMetrics;
+#endif  // defined(MOZ_WIDGET_ANDROID)
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_PanZoomController_h
diff --git a/gfx/layers/apz/src/APZInputBridge.cpp b/gfx/layers/apz/src/APZInputBridge.cpp
new file mode 100644
index 0000000000..6a313cf27e
--- /dev/null
+++ b/gfx/layers/apz/src/APZInputBridge.cpp
@@ -0,0 +1,458 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/APZInputBridge.h"
+
+#include "AsyncPanZoomController.h"
+#include "InputData.h"               // for MouseInput, etc
+#include "InputBlockState.h"         // for InputBlockState
+#include "OverscrollHandoffState.h"  // for OverscrollHandoffState
+#include "mozilla/EventForwards.h"
+#include "mozilla/dom/WheelEventBinding.h"  // for WheelEvent constants
+#include "mozilla/EventStateManager.h"      // for EventStateManager
+#include "mozilla/layers/APZThreadUtils.h"  // for AssertOnControllerThread, etc
+#include "mozilla/MouseEvents.h"            // for WidgetMouseEvent
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_general.h"
+#include "mozilla/StaticPrefs_test.h"
+#include "mozilla/TextEvents.h"           // for WidgetKeyboardEvent
+#include "mozilla/TouchEvents.h"          // for WidgetTouchEvent
+#include "mozilla/WheelHandlingHelper.h"  // for WheelDeltaHorizontalizer,
+                                          //     WheelDeltaAdjustmentStrategy
+
+namespace mozilla {
+namespace layers {
+
+APZEventResult::APZEventResult()
+    : mStatus(nsEventStatus_eIgnore),
+      mInputBlockId(InputBlockState::NO_BLOCK_ID) {}
+
+APZEventResult::APZEventResult(
+    const RefPtr<AsyncPanZoomController>& aInitialTarget,
+    TargetConfirmationFlags aFlags)
+    : APZEventResult() {
+  mHandledResult = [&]() -> Maybe<APZHandledResult> {
+    if (!aInitialTarget->IsRootContent()) {
+      // If the initial target is not the root, this will definitely not be
+      // handled by the root. (The confirmed target is either the initial
+      // target, or a descendant.)
+      return Some(
+          APZHandledResult{APZHandledPlace::HandledByContent, aInitialTarget});
+    }
+
+    if (!aFlags.mDispatchToContent) {
+      // If the initial target is the root and we don't need to dispatch to
+      // content, the event will definitely be handled by the root.
+      return Some(
+          APZHandledResult{APZHandledPlace::HandledByRoot, aInitialTarget});
+    }
+
+    // Otherwise, we're not sure.
+    return Nothing();
+  }();
+  aInitialTarget->GetGuid(&mTargetGuid);
+}
+
+void APZEventResult::SetStatusAsConsumeDoDefault(
+    const InputBlockState& aBlock) {
+  SetStatusAsConsumeDoDefault(aBlock.GetTargetApzc());
+}
+
+void APZEventResult::SetStatusAsConsumeDoDefault(
+    const RefPtr<AsyncPanZoomController>& aTarget) {
+  mStatus = nsEventStatus_eConsumeDoDefault;
+  mHandledResult =
+      Some(aTarget && aTarget->IsRootContent()
+               ? APZHandledResult{APZHandledPlace::HandledByRoot, aTarget}
+               : APZHandledResult{APZHandledPlace::HandledByContent, aTarget});
+}
+
+void APZEventResult::SetStatusForTouchEvent(
+    const InputBlockState& aBlock, TargetConfirmationFlags aFlags,
+    PointerEventsConsumableFlags aConsumableFlags,
+    const AsyncPanZoomController* aTarget) {
+  // Note, we need to continue setting mStatus to eIgnore in the {mHasRoom=true,
+  // mAllowedByTouchAction=false} case because this is the behaviour expected by
+  // APZEventState::ProcessTouchEvent() when it determines when to send a
+  // `pointercancel` event. TODO: Use something more descriptive than
+  // nsEventStatus for this purpose.
+  mStatus = aConsumableFlags.IsConsumable() ? nsEventStatus_eConsumeDoDefault
+                                            : nsEventStatus_eIgnore;
+
+  UpdateHandledResult(aBlock, aConsumableFlags, aTarget,
+                      aFlags.mDispatchToContent);
+}
+
+void APZEventResult::UpdateHandledResult(
+    const InputBlockState& aBlock,
+    PointerEventsConsumableFlags aConsumableFlags,
+    const AsyncPanZoomController* aTarget, bool aDispatchToContent) {
+  // If the touch event's effect is disallowed by touch-action, treat it as if
+  // a touch event listener had preventDefault()-ed it (i.e. return
+  // HandledByContent, except we can do it eagerly rather than having to wait
+  // for the listener to run).
+  if (!aConsumableFlags.mAllowedByTouchAction) {
+    mHandledResult =
+        Some(APZHandledResult{APZHandledPlace::HandledByContent, aTarget});
+    mHandledResult->mOverscrollDirections = ScrollDirections();
+    return;
+  }
+
+  if (mHandledResult && !aDispatchToContent && !aConsumableFlags.mHasRoom) {
+    // Set result to Unhandled if we have no room to scroll, unless it
+    // was HandledByContent because we're over a dispatch-to-content region,
+    // in which case it should remain HandledByContent.
+    mHandledResult->mPlace = APZHandledPlace::Unhandled;
+  }
+
+  if (aTarget && !aTarget->IsRootContent()) {
+    // If the event targets a subframe but the subframe and its ancestors
+    // are all scrolled to the top, we want an upward swipe to allow
+    // triggering pull-to-refresh.
+    bool mayTriggerPullToRefresh =
+        aBlock.GetOverscrollHandoffChain()->ScrollingUpWillTriggerPullToRefresh(
+            aTarget);
+    if (mayTriggerPullToRefresh) {
+      // Similar to what is done for the dynamic toolbar, we need to ensure
+      // that if the input has the dispatch to content flag, we need to change
+      // the handled result to Nothing(), so that GeckoView can wait for the
+      // result.
+      mHandledResult = (aDispatchToContent)
+                           ? Nothing()
+                           : Some(APZHandledResult{APZHandledPlace::Unhandled,
+                                                   aTarget, true});
+    }
+
+    auto [mayMoveDynamicToolbar, rootApzc] =
+        aBlock.GetOverscrollHandoffChain()->ScrollingDownWillMoveDynamicToolbar(
+            aTarget);
+    if (mayMoveDynamicToolbar) {
+      MOZ_ASSERT(rootApzc && rootApzc->IsRootContent());
+      // The event is actually consumed by a non-root APZC but scroll
+      // positions in all relevant APZCs are at the bottom edge, so if there's
+      // still contents covered by the dynamic toolbar we need to move the
+      // dynamic toolbar to make the covered contents visible, thus we need
+      // to tell it to GeckoView so we handle it as if it's consumed in the
+      // root APZC.
+      // IMPORTANT NOTE: If the incoming TargetConfirmationFlags has
+      // mDispatchToContent, we need to change it to Nothing() so that
+      // GeckoView can properly wait for results from the content on the
+      // main-thread.
+      mHandledResult =
+          aDispatchToContent
+              ? Nothing()
+              : Some(APZHandledResult{aConsumableFlags.IsConsumable()
+                                          ? APZHandledPlace::HandledByRoot
+                                          : APZHandledPlace::Unhandled,
+                                      rootApzc});
+    }
+  }
+}
+
+void APZEventResult::SetStatusForFastFling(
+    const TouchBlockState& aBlock, TargetConfirmationFlags aFlags,
+    PointerEventsConsumableFlags aConsumableFlags,
+    const AsyncPanZoomController* aTarget) {
+  MOZ_ASSERT(aBlock.IsDuringFastFling());
+
+  // Set eConsumeNoDefault for fast fling since we don't want to send the event
+  // to content at all.
+  mStatus = nsEventStatus_eConsumeNoDefault;
+
+  // In the case of fast fling, the event will never be sent to content, so we
+  // want a result where `aDispatchToContent` is false whatever the original
+  // `aFlags.mDispatchToContent` is.
+  UpdateHandledResult(aBlock, aConsumableFlags, aTarget, false /*
+  aDispatchToContent */);
+}
+
+static bool WillHandleMouseEvent(const WidgetMouseEventBase& aEvent) {
+  return aEvent.mMessage == eMouseMove || aEvent.mMessage == eMouseDown ||
+         aEvent.mMessage == eMouseUp || aEvent.mMessage == eDragEnd ||
+         (StaticPrefs::test_events_async_enabled() &&
+          aEvent.mMessage == eMouseHitTest);
+}
+
+/* static */
+Maybe<APZWheelAction> APZInputBridge::ActionForWheelEvent(
+    WidgetWheelEvent* aEvent) {
+  if (!(aEvent->mDeltaMode == dom::WheelEvent_Binding::DOM_DELTA_LINE ||
+        aEvent->mDeltaMode == dom::WheelEvent_Binding::DOM_DELTA_PIXEL ||
+        aEvent->mDeltaMode == dom::WheelEvent_Binding::DOM_DELTA_PAGE)) {
+    return Nothing();
+  }
+  return EventStateManager::APZWheelActionFor(aEvent);
+}
+
+APZEventResult APZInputBridge::ReceiveInputEvent(
+    WidgetInputEvent& aEvent, InputBlockCallback&& aCallback) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  APZEventResult result;
+
+  switch (aEvent.mClass) {
+    case eMouseEventClass:
+    case eDragEventClass: {
+      WidgetMouseEvent& mouseEvent = *aEvent.AsMouseEvent();
+      if (WillHandleMouseEvent(mouseEvent)) {
+        MouseInput input(mouseEvent);
+        input.mOrigin =
+            ScreenPoint(mouseEvent.mRefPoint.x, mouseEvent.mRefPoint.y);
+
+        result = ReceiveInputEvent(input, std::move(aCallback));
+
+        mouseEvent.mRefPoint = TruncatedToInt(ViewAs<LayoutDevicePixel>(
+            input.mOrigin,
+            PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent));
+        mouseEvent.mFlags.mHandledByAPZ = input.mHandledByAPZ;
+        mouseEvent.mFocusSequenceNumber = input.mFocusSequenceNumber;
+#ifdef XP_MACOSX
+        // It's not assumed that the click event has already been prevented,
+        // except mousedown event with ctrl key is pressed where we prevent
+        // click event from widget on Mac platform.
+        MOZ_ASSERT_IF(!mouseEvent.IsControl() ||
+                          mouseEvent.mMessage != eMouseDown ||
+                          mouseEvent.mButton != MouseButton::ePrimary,
+                      !mouseEvent.mClickEventPrevented);
+#else
+        MOZ_ASSERT(
+            !mouseEvent.mClickEventPrevented,
+            "It's not assumed that the click event has already been prevented");
+#endif
+        mouseEvent.mClickEventPrevented |= input.mPreventClickEvent;
+        MOZ_ASSERT_IF(mouseEvent.mClickEventPrevented,
+                      mouseEvent.mMessage == eMouseDown ||
+                          mouseEvent.mMessage == eMouseUp);
+        aEvent.mLayersId = input.mLayersId;
+
+        if (mouseEvent.IsReal()) {
+          UpdateWheelTransaction(mouseEvent.mRefPoint, mouseEvent.mMessage,
+                                 Some(result.mTargetGuid));
+        }
+
+        return result;
+      }
+
+      if (mouseEvent.IsReal()) {
+        UpdateWheelTransaction(mouseEvent.mRefPoint, mouseEvent.mMessage,
+                               Nothing());
+      }
+
+      ProcessUnhandledEvent(&mouseEvent.mRefPoint, &result.mTargetGuid,
+                            &aEvent.mFocusSequenceNumber, &aEvent.mLayersId);
+      return result;
+    }
+    case eTouchEventClass: {
+      WidgetTouchEvent& touchEvent = *aEvent.AsTouchEvent();
+      MultiTouchInput touchInput(touchEvent);
+      result = ReceiveInputEvent(touchInput, std::move(aCallback));
+      // touchInput was modified in-place to possibly remove some
+      // touch points (if we are overscrolled), and the coordinates were
+      // modified using the APZ untransform. We need to copy these changes
+      // back into the WidgetInputEvent.
+      touchEvent.mTouches.Clear();
+      touchEvent.mTouches.SetCapacity(touchInput.mTouches.Length());
+      for (size_t i = 0; i < touchInput.mTouches.Length(); i++) {
+        *touchEvent.mTouches.AppendElement() =
+            touchInput.mTouches[i].ToNewDOMTouch();
+      }
+      touchEvent.mFlags.mHandledByAPZ = touchInput.mHandledByAPZ;
+      touchEvent.mFocusSequenceNumber = touchInput.mFocusSequenceNumber;
+      aEvent.mLayersId = touchInput.mLayersId;
+      return result;
+    }
+    case eWheelEventClass: {
+      WidgetWheelEvent& wheelEvent = *aEvent.AsWheelEvent();
+
+      if (Maybe<APZWheelAction> action = ActionForWheelEvent(&wheelEvent)) {
+        ScrollWheelInput::ScrollMode scrollMode =
+            ScrollWheelInput::SCROLLMODE_INSTANT;
+        if (StaticPrefs::general_smoothScroll() &&
+            ((wheelEvent.mDeltaMode ==
+                  dom::WheelEvent_Binding::DOM_DELTA_LINE &&
+              StaticPrefs::general_smoothScroll_mouseWheel()) ||
+             (wheelEvent.mDeltaMode ==
+                  dom::WheelEvent_Binding::DOM_DELTA_PAGE &&
+              StaticPrefs::general_smoothScroll_pages()))) {
+          scrollMode = ScrollWheelInput::SCROLLMODE_SMOOTH;
+        }
+
+        WheelDeltaAdjustmentStrategy strategy =
+            EventStateManager::GetWheelDeltaAdjustmentStrategy(wheelEvent);
+        // Adjust the delta values of the wheel event if the current default
+        // action is to horizontalize scrolling. I.e., deltaY values are set to
+        // deltaX and deltaY and deltaZ values are set to 0.
+        // If horizontalized, the delta values will be restored and its overflow
+        // deltaX will become 0 when the WheelDeltaHorizontalizer instance is
+        // being destroyed.
+        WheelDeltaHorizontalizer horizontalizer(wheelEvent);
+        if (WheelDeltaAdjustmentStrategy::eHorizontalize == strategy) {
+          horizontalizer.Horizontalize();
+        }
+
+        // If the wheel event becomes no-op event, don't handle it as scroll.
+        if (wheelEvent.mDeltaX || wheelEvent.mDeltaY) {
+          ScreenPoint origin(wheelEvent.mRefPoint.x, wheelEvent.mRefPoint.y);
+          ScrollWheelInput input(
+              wheelEvent.mTimeStamp, 0, scrollMode,
+              ScrollWheelInput::DeltaTypeForDeltaMode(wheelEvent.mDeltaMode),
+              origin, wheelEvent.mDeltaX, wheelEvent.mDeltaY,
+              wheelEvent.mAllowToOverrideSystemScrollSpeed, strategy);
+          input.mAPZAction = action.value();
+
+          // We add the user multiplier as a separate field, rather than
+          // premultiplying it, because if the input is converted back to a
+          // WidgetWheelEvent, then EventStateManager would apply the delta a
+          // second time. We could in theory work around this by asking ESM to
+          // customize the event much sooner, and then save the
+          // "mCustomizedByUserPrefs" bit on ScrollWheelInput - but for now,
+          // this seems easier.
+          EventStateManager::GetUserPrefsForWheelEvent(
+              &wheelEvent, &input.mUserDeltaMultiplierX,
+              &input.mUserDeltaMultiplierY);
+
+          result = ReceiveInputEvent(input, std::move(aCallback));
+          wheelEvent.mRefPoint = TruncatedToInt(ViewAs<LayoutDevicePixel>(
+              input.mOrigin, PixelCastJustification::
+                                 LayoutDeviceIsScreenForUntransformedEvent));
+          wheelEvent.mFlags.mHandledByAPZ = input.mHandledByAPZ;
+          wheelEvent.mFocusSequenceNumber = input.mFocusSequenceNumber;
+          aEvent.mLayersId = input.mLayersId;
+
+          return result;
+        }
+      }
+
+      UpdateWheelTransaction(aEvent.mRefPoint, aEvent.mMessage, Nothing());
+      ProcessUnhandledEvent(&aEvent.mRefPoint, &result.mTargetGuid,
+                            &aEvent.mFocusSequenceNumber, &aEvent.mLayersId);
+      MOZ_ASSERT(result.GetStatus() == nsEventStatus_eIgnore);
+      return result;
+    }
+    case eKeyboardEventClass: {
+      WidgetKeyboardEvent& keyboardEvent = *aEvent.AsKeyboardEvent();
+
+      KeyboardInput input(keyboardEvent);
+
+      result = ReceiveInputEvent(input, std::move(aCallback));
+
+      keyboardEvent.mFlags.mHandledByAPZ = input.mHandledByAPZ;
+      keyboardEvent.mFocusSequenceNumber = input.mFocusSequenceNumber;
+      return result;
+    }
+    default: {
+      UpdateWheelTransaction(aEvent.mRefPoint, aEvent.mMessage, Nothing());
+      ProcessUnhandledEvent(&aEvent.mRefPoint, &result.mTargetGuid,
+                            &aEvent.mFocusSequenceNumber, &aEvent.mLayersId);
+      return result;
+    }
+  }
+
+  MOZ_ASSERT_UNREACHABLE("Invalid WidgetInputEvent type.");
+  result.SetStatusAsConsumeNoDefault();
+  return result;
+}
+
+APZHandledResult::APZHandledResult(APZHandledPlace aPlace,
+                                   const AsyncPanZoomController* aTarget,
+                                   bool aPopulateDirectionsForUnhandled)
+    : mPlace(aPlace) {
+  MOZ_ASSERT(aTarget);
+  switch (aPlace) {
+    case APZHandledPlace::Unhandled:
+      if (aTarget && aPopulateDirectionsForUnhandled) {
+        mScrollableDirections = aTarget->ScrollableDirections();
+        mOverscrollDirections = aTarget->GetAllowedHandoffDirections();
+      }
+      break;
+    case APZHandledPlace::HandledByContent:
+      if (aTarget) {
+        mScrollableDirections = aTarget->ScrollableDirections();
+        mOverscrollDirections = aTarget->GetAllowedHandoffDirections();
+      }
+      break;
+    case APZHandledPlace::HandledByRoot: {
+      MOZ_ASSERT(aTarget->IsRootContent());
+      if (aTarget) {
+        mScrollableDirections = aTarget->ScrollableDirections();
+        mOverscrollDirections = aTarget->GetAllowedHandoffDirections();
+      }
+      break;
+    }
+    default:
+      MOZ_ASSERT_UNREACHABLE("Invalid APZHandledPlace");
+      break;
+  }
+}
+
+std::ostream& operator<<(std::ostream& aOut, const SideBits& aSideBits) {
+  if ((aSideBits & SideBits::eAll) == SideBits::eAll) {
+    aOut << "all";
+  } else {
+    AutoTArray<nsCString, 4> strings;
+    if (aSideBits & SideBits::eTop) {
+      strings.AppendElement("top"_ns);
+    }
+    if (aSideBits & SideBits::eRight) {
+      strings.AppendElement("right"_ns);
+    }
+    if (aSideBits & SideBits::eBottom) {
+      strings.AppendElement("bottom"_ns);
+    }
+    if (aSideBits & SideBits::eLeft) {
+      strings.AppendElement("left"_ns);
+    }
+    aOut << strings;
+  }
+  return aOut;
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const ScrollDirections& aScrollDirections) {
+  if (aScrollDirections.contains(EitherScrollDirection)) {
+    aOut << "either";
+  } else if (aScrollDirections.contains(HorizontalScrollDirection)) {
+    aOut << "horizontal";
+  } else if (aScrollDirections.contains(VerticalScrollDirection)) {
+    aOut << "vertical";
+  } else {
+    aOut << "none";
+  }
+  return aOut;
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const APZHandledPlace& aHandledPlace) {
+  switch (aHandledPlace) {
+    case APZHandledPlace::Unhandled:
+      aOut << "unhandled";
+      break;
+    case APZHandledPlace::HandledByRoot: {
+      aOut << "handled-by-root";
+      break;
+    }
+    case APZHandledPlace::HandledByContent: {
+      aOut << "handled-by-content";
+      break;
+    }
+    case APZHandledPlace::Invalid: {
+      aOut << "INVALID";
+      break;
+    }
+  }
+  return aOut;
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const APZHandledResult& aHandledResult) {
+  aOut << "handled: " << aHandledResult.mPlace << ", ";
+  aOut << "scrollable: " << aHandledResult.mScrollableDirections << ", ";
+  aOut << "overscroll: " << aHandledResult.mOverscrollDirections << std::endl;
+  return aOut;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/APZPublicUtils.cpp b/gfx/layers/apz/src/APZPublicUtils.cpp
new file mode 100644
index 0000000000..6902e0738c
--- /dev/null
+++ b/gfx/layers/apz/src/APZPublicUtils.cpp
@@ -0,0 +1,111 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/APZPublicUtils.h"
+
+#include "AsyncPanZoomController.h"
+#include "mozilla/HelperMacros.h"
+#include "mozilla/StaticPrefs_general.h"
+
+namespace mozilla {
+namespace layers {
+
+namespace apz {
+
+/*static*/ void InitializeGlobalState() {
+  MOZ_ASSERT(NS_IsMainThread());
+  AsyncPanZoomController::InitializeGlobalState();
+}
+
+/*static*/ const ScreenMargin CalculatePendingDisplayPort(
+    const FrameMetrics& aFrameMetrics, const ParentLayerPoint& aVelocity) {
+  return AsyncPanZoomController::CalculatePendingDisplayPort(
+      aFrameMetrics, aVelocity, AsyncPanZoomController::ZoomInProgress::No);
+}
+
+/*static*/ gfx::IntSize GetDisplayportAlignmentMultiplier(
+    const ScreenSize& aBaseSize) {
+  return AsyncPanZoomController::GetDisplayportAlignmentMultiplier(aBaseSize);
+}
+
+ScrollAnimationBezierPhysicsSettings ComputeBezierAnimationSettingsForOrigin(
+    ScrollOrigin aOrigin) {
+  int32_t minMS = 0;
+  int32_t maxMS = 0;
+  bool isOriginSmoothnessEnabled = false;
+
+#define READ_DURATIONS(prefbase)                                              \
+  isOriginSmoothnessEnabled = StaticPrefs::general_smoothScroll() &&          \
+                              StaticPrefs::general_smoothScroll_##prefbase(); \
+  if (isOriginSmoothnessEnabled) {                                            \
+    minMS = StaticPrefs::general_smoothScroll_##prefbase##_durationMinMS();   \
+    maxMS = StaticPrefs::general_smoothScroll_##prefbase##_durationMaxMS();   \
+  }
+
+  switch (aOrigin) {
+    case ScrollOrigin::Pixels:
+      READ_DURATIONS(pixels)
+      break;
+    case ScrollOrigin::Lines:
+      READ_DURATIONS(lines)
+      break;
+    case ScrollOrigin::Pages:
+      READ_DURATIONS(pages)
+      break;
+    case ScrollOrigin::MouseWheel:
+      READ_DURATIONS(mouseWheel)
+      break;
+    case ScrollOrigin::Scrollbars:
+      READ_DURATIONS(scrollbars)
+      break;
+    default:
+      READ_DURATIONS(other)
+      break;
+  }
+
+#undef READ_DURATIONS
+
+  if (isOriginSmoothnessEnabled) {
+    static const int32_t kSmoothScrollMaxAllowedAnimationDurationMS = 10000;
+    maxMS = clamped(maxMS, 0, kSmoothScrollMaxAllowedAnimationDurationMS);
+    minMS = clamped(minMS, 0, maxMS);
+  }
+
+  // Keep the animation duration longer than the average event intervals
+  // (to "connect" consecutive scroll animations before the scroll comes to a
+  // stop).
+  double intervalRatio =
+      ((double)StaticPrefs::general_smoothScroll_durationToIntervalRatio()) /
+      100.0;
+
+  // Duration should be at least as long as the intervals -> ratio is at least 1
+  intervalRatio = std::max(1.0, intervalRatio);
+
+  return ScrollAnimationBezierPhysicsSettings{minMS, maxMS, intervalRatio};
+}
+
+ScrollMode GetScrollModeForOrigin(ScrollOrigin origin) {
+  if (!StaticPrefs::general_smoothScroll()) return ScrollMode::Instant;
+  switch (origin) {
+    case ScrollOrigin::Lines:
+      return StaticPrefs::general_smoothScroll_lines() ? ScrollMode::Smooth
+                                                       : ScrollMode::Instant;
+    case ScrollOrigin::Pages:
+      return StaticPrefs::general_smoothScroll_pages() ? ScrollMode::Smooth
+                                                       : ScrollMode::Instant;
+    case ScrollOrigin::Other:
+      return StaticPrefs::general_smoothScroll_other() ? ScrollMode::Smooth
+                                                       : ScrollMode::Instant;
+    default:
+      MOZ_ASSERT(false, "Unknown keyboard scroll origin");
+      return StaticPrefs::general_smoothScroll() ? ScrollMode::Smooth
+                                                 : ScrollMode::Instant;
+  }
+}
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/APZSampler.cpp b/gfx/layers/apz/src/APZSampler.cpp
new file mode 100644
index 0000000000..d0e251cec4
--- /dev/null
+++ b/gfx/layers/apz/src/APZSampler.cpp
@@ -0,0 +1,216 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/APZSampler.h"
+
+#include "AsyncPanZoomController.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/layers/APZThreadUtils.h"
+#include "mozilla/layers/APZUtils.h"
+#include "mozilla/layers/CompositorThread.h"
+#include "mozilla/layers/SynchronousTask.h"
+#include "TreeTraversal.h"
+#include "mozilla/webrender/WebRenderAPI.h"
+
+namespace mozilla {
+namespace layers {
+
+StaticMutex APZSampler::sWindowIdLock;
+StaticAutoPtr<std::unordered_map<uint64_t, RefPtr<APZSampler>>>
+    APZSampler::sWindowIdMap;
+
+APZSampler::APZSampler(const RefPtr<APZCTreeManager>& aApz,
+                       bool aIsUsingWebRender)
+    : mApz(aApz),
+      mIsUsingWebRender(aIsUsingWebRender),
+      mThreadIdLock("APZSampler::mThreadIdLock"),
+      mSampleTimeLock("APZSampler::mSampleTimeLock") {
+  MOZ_ASSERT(aApz);
+  mApz->SetSampler(this);
+}
+
+APZSampler::~APZSampler() { mApz->SetSampler(nullptr); }
+
+void APZSampler::Destroy() {
+  StaticMutexAutoLock lock(sWindowIdLock);
+  if (mWindowId) {
+    MOZ_ASSERT(sWindowIdMap);
+    sWindowIdMap->erase(wr::AsUint64(*mWindowId));
+  }
+}
+
+void APZSampler::SetWebRenderWindowId(const wr::WindowId& aWindowId) {
+  StaticMutexAutoLock lock(sWindowIdLock);
+  MOZ_ASSERT(!mWindowId);
+  mWindowId = Some(aWindowId);
+  if (!sWindowIdMap) {
+    sWindowIdMap = new std::unordered_map<uint64_t, RefPtr<APZSampler>>();
+    NS_DispatchToMainThread(NS_NewRunnableFunction(
+        "APZSampler::ClearOnShutdown", [] { ClearOnShutdown(&sWindowIdMap); }));
+  }
+  (*sWindowIdMap)[wr::AsUint64(aWindowId)] = this;
+}
+
+/*static*/
+void APZSampler::SetSamplerThread(const wr::WrWindowId& aWindowId) {
+  if (RefPtr<APZSampler> sampler = GetSampler(aWindowId)) {
+    MutexAutoLock lock(sampler->mThreadIdLock);
+    sampler->mSamplerThreadId = Some(PlatformThread::CurrentId());
+  }
+}
+
+/*static*/
+void APZSampler::SampleForWebRender(const wr::WrWindowId& aWindowId,
+                                    const uint64_t* aGeneratedFrameId,
+                                    wr::Transaction* aTransaction) {
+  if (RefPtr<APZSampler> sampler = GetSampler(aWindowId)) {
+    wr::TransactionWrapper txn(aTransaction);
+    Maybe<VsyncId> vsyncId =
+        aGeneratedFrameId ? Some(VsyncId{*aGeneratedFrameId}) : Nothing();
+    sampler->SampleForWebRender(vsyncId, txn);
+  }
+}
+
+void APZSampler::SetSampleTime(const SampleTime& aSampleTime) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  MutexAutoLock lock(mSampleTimeLock);
+  // This only gets called with WR, and the time provided is going to be
+  // the time at which the current vsync interval ends. i.e. it is the timestamp
+  // for the next vsync that will occur.
+  mSampleTime = aSampleTime;
+}
+
+void APZSampler::SampleForWebRender(const Maybe<VsyncId>& aVsyncId,
+                                    wr::TransactionWrapper& aTxn) {
+  AssertOnSamplerThread();
+  SampleTime sampleTime;
+  {  // scope lock
+    MutexAutoLock lock(mSampleTimeLock);
+
+    // If mSampleTime is null we're in a startup phase where the
+    // WebRenderBridgeParent hasn't yet provided us with a sample time.
+    // If we're that early there probably aren't any APZ animations happening
+    // anyway, so using Timestamp::Now() should be fine.
+    SampleTime now = SampleTime::FromNow();
+    sampleTime = mSampleTime.IsNull() ? now : mSampleTime;
+  }
+  mApz->SampleForWebRender(aVsyncId, aTxn, sampleTime);
+}
+
+AsyncTransform APZSampler::GetCurrentAsyncTransform(
+    const LayersId& aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+    AsyncTransformComponents aComponents,
+    const MutexAutoLock& aProofOfMapLock) const {
+  MOZ_ASSERT(!CompositorThreadHolder::IsInCompositorThread());
+  AssertOnSamplerThread();
+
+  RefPtr<AsyncPanZoomController> apzc =
+      mApz->GetTargetAPZC(aLayersId, aScrollId, aProofOfMapLock);
+  if (!apzc) {
+    // It's possible that this function can get called even after the target
+    // APZC has been already destroyed because destroying the animation which
+    // triggers this function call is basically processed later than the APZC,
+    // i.e. queue mCompositorAnimationsToDelete in WebRenderBridgeParent and
+    // then remove in WebRenderBridgeParent::RemoveEpochDataPriorTo.
+    return AsyncTransform{};
+  }
+
+  return apzc->GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing,
+                                        aComponents);
+}
+
+ParentLayerRect APZSampler::GetCompositionBounds(
+    const LayersId& aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+    const MutexAutoLock& aProofOfMapLock) const {
+  // This function can get called on the compositor in case of non WebRender
+  // get called on the sampler thread in case of WebRender.
+  AssertOnSamplerThread();
+
+  RefPtr<AsyncPanZoomController> apzc =
+      mApz->GetTargetAPZC(aLayersId, aScrollId, aProofOfMapLock);
+  if (!apzc) {
+    // On WebRender it's possible that this function can get called even after
+    // the target APZC has been already destroyed because destroying the
+    // animation which triggers this function call is basically processed later
+    // than the APZC one, i.e. queue mCompositorAnimationsToDelete in
+    // WebRenderBridgeParent and then remove them in
+    // WebRenderBridgeParent::RemoveEpochDataPriorTo.
+    return ParentLayerRect();
+  }
+
+  return apzc->GetCompositionBounds();
+}
+
+Maybe<APZSampler::ScrollOffsetAndRange>
+APZSampler::GetCurrentScrollOffsetAndRange(
+    const LayersId& aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+    const MutexAutoLock& aProofOfMapLock) const {
+  // Note: This is called from OMTA Sampler thread, or Compositor thread for
+  // testing.
+
+  RefPtr<AsyncPanZoomController> apzc =
+      mApz->GetTargetAPZC(aLayersId, aScrollId, aProofOfMapLock);
+  if (!apzc) {
+    return Nothing();
+  }
+
+  return Some(ScrollOffsetAndRange{
+      // FIXME: Use the one-frame delayed offset now. This doesn't take
+      // scroll-linked effets into accounts, so we have to fix this in the
+      // future.
+      apzc->GetCurrentAsyncVisualViewport(
+              AsyncTransformConsumer::eForCompositing)
+          .TopLeft(),
+      apzc->GetCurrentScrollRangeInCssPixels()});
+}
+
+void APZSampler::AssertOnSamplerThread() const {
+  if (APZThreadUtils::GetThreadAssertionsEnabled()) {
+    MOZ_ASSERT(IsSamplerThread());
+  }
+}
+
+bool APZSampler::IsSamplerThread() const {
+  if (mIsUsingWebRender) {
+    // If the sampler thread id isn't set yet then we cannot be running on the
+    // sampler thread (because we will have the thread id before we run any
+    // other C++ code on it, and this function is only ever invoked from C++
+    // code), so return false in that scenario.
+    MutexAutoLock lock(mThreadIdLock);
+    return mSamplerThreadId && PlatformThread::CurrentId() == *mSamplerThreadId;
+  }
+  return CompositorThreadHolder::IsInCompositorThread();
+}
+
+/*static*/
+already_AddRefed<APZSampler> APZSampler::GetSampler(
+    const wr::WrWindowId& aWindowId) {
+  RefPtr<APZSampler> sampler;
+  StaticMutexAutoLock lock(sWindowIdLock);
+  if (sWindowIdMap) {
+    auto it = sWindowIdMap->find(wr::AsUint64(aWindowId));
+    if (it != sWindowIdMap->end()) {
+      sampler = it->second;
+    }
+  }
+  return sampler.forget();
+}
+
+}  // namespace layers
+}  // namespace mozilla
+
+void apz_register_sampler(mozilla::wr::WrWindowId aWindowId) {
+  mozilla::layers::APZSampler::SetSamplerThread(aWindowId);
+}
+
+void apz_sample_transforms(mozilla::wr::WrWindowId aWindowId,
+                           const uint64_t* aGeneratedFrameId,
+                           mozilla::wr::Transaction* aTransaction) {
+  mozilla::layers::APZSampler::SampleForWebRender(aWindowId, aGeneratedFrameId,
+                                                  aTransaction);
+}
+
+void apz_deregister_sampler(mozilla::wr::WrWindowId aWindowId) {}
diff --git a/gfx/layers/apz/src/APZUpdater.cpp b/gfx/layers/apz/src/APZUpdater.cpp
new file mode 100644
index 0000000000..2bbad6e1a7
--- /dev/null
+++ b/gfx/layers/apz/src/APZUpdater.cpp
@@ -0,0 +1,546 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/APZUpdater.h"
+
+#include "APZCTreeManager.h"
+#include "AsyncPanZoomController.h"
+#include "base/task.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/layers/APZThreadUtils.h"
+#include "mozilla/layers/CompositorThread.h"
+#include "mozilla/layers/SynchronousTask.h"
+#include "mozilla/layers/WebRenderScrollDataWrapper.h"
+#include "mozilla/webrender/WebRenderAPI.h"
+
+namespace mozilla {
+namespace layers {
+
+StaticMutex APZUpdater::sWindowIdLock;
+StaticAutoPtr<std::unordered_map<uint64_t, APZUpdater*>>
+    APZUpdater::sWindowIdMap;
+
+APZUpdater::APZUpdater(const RefPtr<APZCTreeManager>& aApz,
+                       bool aConnectedToWebRender)
+    : mApz(aApz),
+      mDestroyed(false),
+      mConnectedToWebRender(aConnectedToWebRender),
+      mThreadIdLock("APZUpdater::ThreadIdLock"),
+      mQueueLock("APZUpdater::QueueLock") {
+  MOZ_ASSERT(aApz);
+  mApz->SetUpdater(this);
+}
+
+APZUpdater::~APZUpdater() {
+  mApz->SetUpdater(nullptr);
+
+  StaticMutexAutoLock lock(sWindowIdLock);
+  if (mWindowId) {
+    MOZ_ASSERT(sWindowIdMap);
+    // Ensure that ClearTree was called and the task got run
+    MOZ_ASSERT(sWindowIdMap->find(wr::AsUint64(*mWindowId)) ==
+               sWindowIdMap->end());
+  }
+}
+
+bool APZUpdater::HasTreeManager(const RefPtr<APZCTreeManager>& aApz) {
+  return aApz.get() == mApz.get();
+}
+
+void APZUpdater::SetWebRenderWindowId(const wr::WindowId& aWindowId) {
+  StaticMutexAutoLock lock(sWindowIdLock);
+  MOZ_ASSERT(!mWindowId);
+  mWindowId = Some(aWindowId);
+  if (!sWindowIdMap) {
+    sWindowIdMap = new std::unordered_map<uint64_t, APZUpdater*>();
+    NS_DispatchToMainThread(NS_NewRunnableFunction(
+        "APZUpdater::ClearOnShutdown", [] { ClearOnShutdown(&sWindowIdMap); }));
+  }
+  (*sWindowIdMap)[wr::AsUint64(aWindowId)] = this;
+}
+
+/*static*/
+void APZUpdater::SetUpdaterThread(const wr::WrWindowId& aWindowId) {
+  if (RefPtr<APZUpdater> updater = GetUpdater(aWindowId)) {
+    MutexAutoLock lock(updater->mThreadIdLock);
+    updater->mUpdaterThreadId = Some(PlatformThread::CurrentId());
+  }
+}
+
+// Takes a conditional lock!
+/*static*/
+void APZUpdater::PrepareForSceneSwap(const wr::WrWindowId& aWindowId)
+    MOZ_NO_THREAD_SAFETY_ANALYSIS {
+  if (RefPtr<APZUpdater> updater = GetUpdater(aWindowId)) {
+    updater->mApz->LockTree();
+  }
+}
+
+// Assumes we took a conditional lock!
+/*static*/
+void APZUpdater::CompleteSceneSwap(const wr::WrWindowId& aWindowId,
+                                   const wr::WrPipelineInfo& aInfo) {
+  RefPtr<APZUpdater> updater = GetUpdater(aWindowId);
+  if (!updater) {
+    // This should only happen in cases where PrepareForSceneSwap also got a
+    // null updater. No updater-thread tasks get run between PrepareForSceneSwap
+    // and this function, so there is no opportunity for the updater mapping
+    // to have gotten removed from sWindowIdMap in between the two calls.
+    return;
+  }
+  updater->mApz->mTreeLock.AssertCurrentThreadIn();
+
+  for (const auto& removedPipeline : aInfo.removed_pipelines) {
+    LayersId layersId = wr::AsLayersId(removedPipeline.pipeline_id);
+    updater->mEpochData.erase(layersId);
+  }
+  // Reset the built info for all pipelines, then put it back for the ones
+  // that got built in this scene swap.
+  for (auto& i : updater->mEpochData) {
+    i.second.mBuilt = Nothing();
+  }
+  for (const auto& epoch : aInfo.epochs) {
+    LayersId layersId = wr::AsLayersId(epoch.pipeline_id);
+    updater->mEpochData[layersId].mBuilt = Some(epoch.epoch);
+  }
+
+  // Run any tasks that got unblocked, then unlock the tree. The order is
+  // important because we want to run all the tasks up to and including the
+  // UpdateHitTestingTree calls corresponding to the built epochs, and we
+  // want to run those before we release the lock (i.e. atomically with the
+  // scene swap). This ensures that any hit-tests always encounter a consistent
+  // state between the APZ tree and the built scene in WR.
+  //
+  // While we could add additional information to the queued tasks to figure
+  // out the minimal set of tasks we want to run here, it's easier and harmless
+  // to just run all the queued and now-unblocked tasks inside the lock.
+  //
+  // Note that the ProcessQueue here might remove the window id -> APZUpdater
+  // mapping from sWindowIdMap, but we still unlock the tree successfully to
+  // leave things in a good state.
+  updater->ProcessQueue();
+
+  updater->mApz->UnlockTree();
+}
+
+/*static*/
+void APZUpdater::ProcessPendingTasks(const wr::WrWindowId& aWindowId) {
+  if (RefPtr<APZUpdater> updater = GetUpdater(aWindowId)) {
+    updater->ProcessQueue();
+  }
+}
+
+void APZUpdater::ClearTree(LayersId aRootLayersId) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZUpdater> self = this;
+  RunOnUpdaterThread(aRootLayersId,
+                     NS_NewRunnableFunction("APZUpdater::ClearTree", [=]() {
+                       self->mApz->ClearTree();
+                       self->mDestroyed = true;
+
+                       // Once ClearTree is called on the APZCTreeManager, we
+                       // are in a shutdown phase. After this point it's ok if
+                       // WebRender cannot get a hold of the updater via the
+                       // window id, and it's a good point to remove the mapping
+                       // and avoid leaving a dangling pointer to this object.
+                       StaticMutexAutoLock lock(sWindowIdLock);
+                       if (self->mWindowId) {
+                         MOZ_ASSERT(sWindowIdMap);
+                         sWindowIdMap->erase(wr::AsUint64(*(self->mWindowId)));
+                       }
+                     }));
+}
+
+void APZUpdater::UpdateFocusState(LayersId aRootLayerTreeId,
+                                  LayersId aOriginatingLayersId,
+                                  const FocusTarget& aFocusTarget) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RunOnUpdaterThread(aOriginatingLayersId,
+                     NewRunnableMethod<LayersId, LayersId, FocusTarget>(
+                         "APZUpdater::UpdateFocusState", mApz,
+                         &APZCTreeManager::UpdateFocusState, aRootLayerTreeId,
+                         aOriginatingLayersId, aFocusTarget));
+}
+
+void APZUpdater::UpdateScrollDataAndTreeState(
+    LayersId aRootLayerTreeId, LayersId aOriginatingLayersId,
+    const wr::Epoch& aEpoch, WebRenderScrollData&& aScrollData) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZUpdater> self = this;
+  // Insert an epoch requirement update into the queue, so that
+  // tasks inserted into the queue after this point only get executed
+  // once the epoch requirement is satisfied. In particular, the
+  // UpdateHitTestingTree call below needs to wait until the epoch requirement
+  // is satisfied, which is why it is a separate task in the queue.
+  RunOnUpdaterThread(
+      aOriginatingLayersId,
+      NS_NewRunnableFunction("APZUpdater::UpdateEpochRequirement", [=]() {
+        if (aRootLayerTreeId == aOriginatingLayersId) {
+          self->mEpochData[aOriginatingLayersId].mIsRoot = true;
+        }
+        self->mEpochData[aOriginatingLayersId].mRequired = aEpoch;
+      }));
+  RunOnUpdaterThread(
+      aOriginatingLayersId,
+      NS_NewRunnableFunction(
+          "APZUpdater::UpdateHitTestingTree",
+          [=, aScrollData = std::move(aScrollData)]() mutable {
+            auto isFirstPaint = aScrollData.IsFirstPaint();
+            auto paintSequenceNumber = aScrollData.GetPaintSequenceNumber();
+
+            self->mScrollData[aOriginatingLayersId] = std::move(aScrollData);
+            auto root = self->mScrollData.find(aRootLayerTreeId);
+            if (root == self->mScrollData.end()) {
+              return;
+            }
+            self->mApz->UpdateHitTestingTree(
+                WebRenderScrollDataWrapper(*self, &(root->second)),
+                isFirstPaint, aOriginatingLayersId, paintSequenceNumber);
+          }));
+}
+
+void APZUpdater::UpdateScrollOffsets(LayersId aRootLayerTreeId,
+                                     LayersId aOriginatingLayersId,
+                                     ScrollUpdatesMap&& aUpdates,
+                                     uint32_t aPaintSequenceNumber) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZUpdater> self = this;
+  RunOnUpdaterThread(
+      aOriginatingLayersId,
+      NS_NewRunnableFunction(
+          "APZUpdater::UpdateScrollOffsets",
+          [=, updates = std::move(aUpdates)]() mutable {
+            self->mScrollData[aOriginatingLayersId].ApplyUpdates(
+                std::move(updates), aPaintSequenceNumber);
+            auto root = self->mScrollData.find(aRootLayerTreeId);
+            if (root == self->mScrollData.end()) {
+              return;
+            }
+            self->mApz->UpdateHitTestingTree(
+                WebRenderScrollDataWrapper(*self, &(root->second)),
+                /*isFirstPaint*/ false, aOriginatingLayersId,
+                aPaintSequenceNumber);
+          }));
+}
+
+void APZUpdater::NotifyLayerTreeAdopted(LayersId aLayersId,
+                                        const RefPtr<APZUpdater>& aOldUpdater) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RunOnUpdaterThread(aLayersId,
+                     NewRunnableMethod<LayersId, RefPtr<APZCTreeManager>>(
+                         "APZUpdater::NotifyLayerTreeAdopted", mApz,
+                         &APZCTreeManager::NotifyLayerTreeAdopted, aLayersId,
+                         aOldUpdater ? aOldUpdater->mApz : nullptr));
+}
+
+void APZUpdater::NotifyLayerTreeRemoved(LayersId aLayersId) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZUpdater> self = this;
+  RunOnUpdaterThread(
+      aLayersId,
+      NS_NewRunnableFunction("APZUpdater::NotifyLayerTreeRemoved", [=]() {
+        self->mEpochData.erase(aLayersId);
+        self->mScrollData.erase(aLayersId);
+        self->mApz->NotifyLayerTreeRemoved(aLayersId);
+      }));
+}
+
+bool APZUpdater::GetAPZTestData(LayersId aLayersId, APZTestData* aOutData) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+
+  RefPtr<APZCTreeManager> apz = mApz;
+  bool ret = false;
+  SynchronousTask waiter("APZUpdater::GetAPZTestData");
+  RunOnUpdaterThread(
+      aLayersId, NS_NewRunnableFunction("APZUpdater::GetAPZTestData", [&]() {
+        AutoCompleteTask notifier(&waiter);
+        ret = apz->GetAPZTestData(aLayersId, aOutData);
+      }));
+
+  // Wait until the task posted above has run and populated aOutData and ret
+  waiter.Wait();
+
+  return ret;
+}
+
+void APZUpdater::SetTestAsyncScrollOffset(
+    LayersId aLayersId, const ScrollableLayerGuid::ViewID& aScrollId,
+    const CSSPoint& aOffset) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZCTreeManager> apz = mApz;
+  RunOnUpdaterThread(
+      aLayersId,
+      NS_NewRunnableFunction("APZUpdater::SetTestAsyncScrollOffset", [=]() {
+        RefPtr<AsyncPanZoomController> apzc =
+            apz->GetTargetAPZC(aLayersId, aScrollId);
+        if (apzc) {
+          apzc->SetTestAsyncScrollOffset(aOffset);
+        } else {
+          NS_WARNING("Unable to find APZC in SetTestAsyncScrollOffset");
+        }
+      }));
+}
+
+void APZUpdater::SetTestAsyncZoom(LayersId aLayersId,
+                                  const ScrollableLayerGuid::ViewID& aScrollId,
+                                  const LayerToParentLayerScale& aZoom) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  RefPtr<APZCTreeManager> apz = mApz;
+  RunOnUpdaterThread(
+      aLayersId, NS_NewRunnableFunction("APZUpdater::SetTestAsyncZoom", [=]() {
+        RefPtr<AsyncPanZoomController> apzc =
+            apz->GetTargetAPZC(aLayersId, aScrollId);
+        if (apzc) {
+          apzc->SetTestAsyncZoom(aZoom);
+        } else {
+          NS_WARNING("Unable to find APZC in SetTestAsyncZoom");
+        }
+      }));
+}
+
+const WebRenderScrollData* APZUpdater::GetScrollData(LayersId aLayersId) const {
+  AssertOnUpdaterThread();
+  auto it = mScrollData.find(aLayersId);
+  return (it == mScrollData.end() ? nullptr : &(it->second));
+}
+
+void APZUpdater::AssertOnUpdaterThread() const {
+  if (APZThreadUtils::GetThreadAssertionsEnabled()) {
+    MOZ_ASSERT(IsUpdaterThread());
+  }
+}
+
+void APZUpdater::RunOnUpdaterThread(LayersId aLayersId,
+                                    already_AddRefed<Runnable> aTask) {
+  RefPtr<Runnable> task = aTask;
+
+  // In the scenario where IsConnectedToWebRender() is true, this function
+  // might get called early (before mUpdaterThreadId is set). In that case
+  // IsUpdaterThread() will return false and we'll queue the task onto
+  // mUpdaterQueue. This is fine; the task is still guaranteed to run (barring
+  // catastrophic failure) because the WakeSceneBuilder call will still trigger
+  // the callback to run tasks.
+
+  if (IsUpdaterThread()) {
+    // This function should only be called from the updater thread in test
+    // scenarios where we are not connected to WebRender. If it were called from
+    // the updater thread when we are connected to WebRender, running the task
+    // right away would be incorrect (we'd need to check that |aLayersId|
+    // isn't blocked, and if it is then enqueue the task instead).
+    MOZ_ASSERT(!IsConnectedToWebRender());
+    task->Run();
+    return;
+  }
+
+  if (IsConnectedToWebRender()) {
+    // If the updater thread is a WebRender thread, and we're not on it
+    // right now, save the task in the queue. We will run tasks from the queue
+    // during the callback from the updater thread, which we trigger by the
+    // call to WakeSceneBuilder.
+
+    bool sendWakeMessage = true;
+    {  // scope lock
+      MutexAutoLock lock(mQueueLock);
+      for (const auto& queuedTask : mUpdaterQueue) {
+        if (queuedTask.mLayersId == aLayersId) {
+          // If there's already a task in the queue with this layers id, then
+          // we must have previously sent a WakeSceneBuilder message (when
+          // adding the first task with this layers id to the queue). Either
+          // that hasn't been fully processed yet, or the layers id is blocked
+          // waiting for an epoch - in either case there's no point in sending
+          // another WakeSceneBuilder message.
+          sendWakeMessage = false;
+          break;
+        }
+      }
+      mUpdaterQueue.push_back(QueuedTask{aLayersId, task});
+    }
+    if (sendWakeMessage) {
+      RefPtr<wr::WebRenderAPI> api = mApz->GetWebRenderAPI();
+      if (api) {
+        api->WakeSceneBuilder();
+      } else {
+        // Not sure if this can happen, but it might be possible. If it does,
+        // the task is in the queue, but if we didn't get a WebRenderAPI it
+        // might never run, or it might run later if we manage to get a
+        // WebRenderAPI later. For now let's just emit a warning, this can
+        // probably be upgraded to an assert later.
+        NS_WARNING("Possibly dropping task posted to updater thread");
+      }
+    }
+    return;
+  }
+
+  if (CompositorThread()) {
+    CompositorThread()->Dispatch(task.forget());
+  } else {
+    // Could happen during startup
+    NS_WARNING("Dropping task posted to updater thread");
+  }
+}
+
+bool APZUpdater::IsUpdaterThread() const {
+  if (IsConnectedToWebRender()) {
+    // If the updater thread id isn't set yet then we cannot be running on the
+    // updater thread (because we will have the thread id before we run any
+    // C++ code on it, and this function is only ever invoked from C++ code),
+    // so return false in that scenario.
+    MutexAutoLock lock(mThreadIdLock);
+    return mUpdaterThreadId && PlatformThread::CurrentId() == *mUpdaterThreadId;
+  }
+  return CompositorThreadHolder::IsInCompositorThread();
+}
+
+void APZUpdater::RunOnControllerThread(LayersId aLayersId,
+                                       already_AddRefed<Runnable> aTask) {
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+
+  RefPtr<Runnable> task = aTask;
+
+  RunOnUpdaterThread(
+      aLayersId,
+      NewRunnableFunction("APZUpdater::RunOnControllerThread",
+                          &APZThreadUtils::RunOnControllerThread,
+                          std::move(task), nsIThread::DISPATCH_NORMAL));
+}
+
+bool APZUpdater::IsConnectedToWebRender() const {
+  return mConnectedToWebRender;
+}
+
+/*static*/
+already_AddRefed<APZUpdater> APZUpdater::GetUpdater(
+    const wr::WrWindowId& aWindowId) {
+  RefPtr<APZUpdater> updater;
+  StaticMutexAutoLock lock(sWindowIdLock);
+  if (sWindowIdMap) {
+    auto it = sWindowIdMap->find(wr::AsUint64(aWindowId));
+    if (it != sWindowIdMap->end()) {
+      updater = it->second;
+    }
+  }
+  return updater.forget();
+}
+
+void APZUpdater::ProcessQueue() {
+  MOZ_ASSERT(!mDestroyed);
+
+  {  // scope lock to check for emptiness
+    MutexAutoLock lock(mQueueLock);
+    if (mUpdaterQueue.empty()) {
+      return;
+    }
+  }
+
+  std::deque<QueuedTask> blockedTasks;
+  while (true) {
+    QueuedTask task;
+
+    {  // scope lock to extract a task
+      MutexAutoLock lock(mQueueLock);
+      if (mUpdaterQueue.empty()) {
+        // If we're done processing mUpdaterQueue, swap the tasks that are
+        // still blocked back in and finish
+        std::swap(mUpdaterQueue, blockedTasks);
+        break;
+      }
+      task = mUpdaterQueue.front();
+      mUpdaterQueue.pop_front();
+    }
+
+    // We check the task to see if it is blocked. Note that while this
+    // ProcessQueue function is executing, a particular layers id cannot go
+    // from blocked to unblocked, because only CompleteSceneSwap can unblock
+    // a layers id, and that also runs on the updater thread. If somehow
+    // a layers id gets unblocked while we're processing the queue, then it
+    // might result in tasks getting executed out of order.
+
+    auto it = mEpochData.find(task.mLayersId);
+    if (it != mEpochData.end() && it->second.IsBlocked()) {
+      // If this task is blocked, put it into the blockedTasks queue that
+      // we will replace mUpdaterQueue with
+      blockedTasks.push_back(task);
+    } else {
+      // Run and discard the task
+      task.mRunnable->Run();
+    }
+  }
+
+  if (mDestroyed) {
+    // If we get here, then we must have just run the ClearTree task for
+    // this updater. There might be tasks in the queue from content subtrees
+    // of this window that are blocked due to stale epochs. This can happen
+    // if the tasks were queued after the root pipeline was removed in
+    // WebRender, which prevents scene builds (and therefore prevents us
+    // from getting updated epochs via CompleteSceneSwap). See bug 1465658
+    // comment 43 for some more context.
+    // To avoid leaking these tasks, we discard the contents of the queue.
+    // This happens during window shutdown so if we don't run the tasks it's
+    // not going to matter much.
+    MutexAutoLock lock(mQueueLock);
+    if (!mUpdaterQueue.empty()) {
+      mUpdaterQueue.clear();
+    }
+  }
+}
+
+void APZUpdater::MarkAsDetached(LayersId aLayersId) {
+  mApz->MarkAsDetached(aLayersId);
+}
+
+APZUpdater::EpochState::EpochState() : mRequired{0}, mIsRoot(false) {}
+
+bool APZUpdater::EpochState::IsBlocked() const {
+  // The root is a special case because we basically assume it is "visible"
+  // even before it is built for the first time. This is because building the
+  // scene automatically makes it visible, and we need to make sure the APZ
+  // scroll data gets applied atomically with that happening.
+  //
+  // Layer subtrees on the other hand do not automatically become visible upon
+  // being built, because there must be a another layer tree update to change
+  // the visibility (i.e. an ancestor layer tree update that adds the necessary
+  // reflayer to complete the chain of reflayers).
+  //
+  // So in the case of non-visible subtrees, we know that no hit-test will
+  // actually end up hitting that subtree either before or after the scene swap,
+  // because the subtree will remain non-visible. That in turns means that we
+  // can apply the APZ scroll data for that subtree epoch before the scene is
+  // built, because it's not going to get used anyway. And that means we don't
+  // need to block the queue for non-visible subtrees. Which is a good thing,
+  // because in practice it seems like we often have non-visible subtrees sent
+  // to the compositor from content.
+  if (mIsRoot && !mBuilt) {
+    return true;
+  }
+  return mBuilt && (*mBuilt < mRequired);
+}
+
+}  // namespace layers
+}  // namespace mozilla
+
+// Rust callback implementations
+
+void apz_register_updater(mozilla::wr::WrWindowId aWindowId) {
+  mozilla::layers::APZUpdater::SetUpdaterThread(aWindowId);
+}
+
+void apz_pre_scene_swap(mozilla::wr::WrWindowId aWindowId) {
+  mozilla::layers::APZUpdater::PrepareForSceneSwap(aWindowId);
+}
+
+void apz_post_scene_swap(mozilla::wr::WrWindowId aWindowId,
+                         const mozilla::wr::WrPipelineInfo* aInfo) {
+  mozilla::layers::APZUpdater::CompleteSceneSwap(aWindowId, *aInfo);
+}
+
+void apz_run_updater(mozilla::wr::WrWindowId aWindowId) {
+  mozilla::layers::APZUpdater::ProcessPendingTasks(aWindowId);
+}
+
+void apz_deregister_updater(mozilla::wr::WrWindowId aWindowId) {
+  // Run anything that's still left.
+  mozilla::layers::APZUpdater::ProcessPendingTasks(aWindowId);
+}
diff --git a/gfx/layers/apz/src/APZUtils.cpp b/gfx/layers/apz/src/APZUtils.cpp
new file mode 100644
index 0000000000..843046c34a
--- /dev/null
+++ b/gfx/layers/apz/src/APZUtils.cpp
@@ -0,0 +1,118 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/APZUtils.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layers.h"
+
+namespace mozilla {
+namespace layers {
+
+namespace apz {
+
+bool IsCloseToHorizontal(float aAngle, float aThreshold) {
+  return (aAngle < aThreshold || aAngle > (M_PI - aThreshold));
+}
+
+bool IsCloseToVertical(float aAngle, float aThreshold) {
+  return (fabs(aAngle - (M_PI / 2)) < aThreshold);
+}
+
+bool IsStuckAtBottom(gfxFloat aTranslation,
+                     const LayerRectAbsolute& aInnerRange,
+                     const LayerRectAbsolute& aOuterRange) {
+  // The item will be stuck at the bottom if the async scroll delta is in
+  // the range [aOuterRange.Y(), aInnerRange.Y()]. Since the translation
+  // is negated with repect to the async scroll delta (i.e. scrolling down
+  // produces a positive scroll delta and negative translation), we invert it
+  // and check to see if it falls in the specified range.
+  return aOuterRange.Y() <= -aTranslation && -aTranslation <= aInnerRange.Y();
+}
+
+bool IsStuckAtTop(gfxFloat aTranslation, const LayerRectAbsolute& aInnerRange,
+                  const LayerRectAbsolute& aOuterRange) {
+  // Same as IsStuckAtBottom, except we want to check for the range
+  // [aInnerRange.YMost(), aOuterRange.YMost()].
+  return aInnerRange.YMost() <= -aTranslation &&
+         -aTranslation <= aOuterRange.YMost();
+}
+
+ScreenPoint ComputeFixedMarginsOffset(
+    const ScreenMargin& aCompositorFixedLayerMargins, SideBits aFixedSides,
+    const ScreenMargin& aGeckoFixedLayerMargins) {
+  // Work out the necessary translation, in screen space.
+  ScreenPoint translation;
+
+  ScreenMargin effectiveMargin =
+      aCompositorFixedLayerMargins - aGeckoFixedLayerMargins;
+  if ((aFixedSides & SideBits::eLeftRight) == SideBits::eLeftRight) {
+    translation.x += (effectiveMargin.left - effectiveMargin.right) / 2;
+  } else if (aFixedSides & SideBits::eRight) {
+    translation.x -= effectiveMargin.right;
+  } else if (aFixedSides & SideBits::eLeft) {
+    translation.x += effectiveMargin.left;
+  }
+
+  if ((aFixedSides & SideBits::eTopBottom) == SideBits::eTopBottom) {
+    translation.y += (effectiveMargin.top - effectiveMargin.bottom) / 2;
+  } else if (aFixedSides & SideBits::eBottom) {
+    translation.y -= effectiveMargin.bottom;
+  } else if (aFixedSides & SideBits::eTop) {
+    translation.y += effectiveMargin.top;
+  }
+
+  return translation;
+}
+
+bool AboutToCheckerboard(const FrameMetrics& aPaintedMetrics,
+                         const FrameMetrics& aCompositorMetrics) {
+  // The main-thread code to compute the painted area can introduce some
+  // rounding error due to multiple unit conversions, so we inflate the rect by
+  // one app unit to account for that.
+  CSSRect painted = aPaintedMetrics.GetDisplayPort() +
+                    aPaintedMetrics.GetLayoutScrollOffset();
+  painted.Inflate(CSSMargin::FromAppUnits(nsMargin(1, 1, 1, 1)));
+
+  // Inflate the rect by the danger zone. See the description of the danger zone
+  // prefs in AsyncPanZoomController.cpp for an explanation of this.
+  CSSRect visible =
+      CSSRect(aCompositorMetrics.GetVisualScrollOffset(),
+              aCompositorMetrics.CalculateBoundedCompositedSizeInCssPixels());
+  visible.Inflate(ScreenSize(StaticPrefs::apz_danger_zone_x(),
+                             StaticPrefs::apz_danger_zone_y()) /
+                  aCompositorMetrics.DisplayportPixelsPerCSSPixel());
+
+  // Clamp both rects to the scrollable rect, because having either of those
+  // exceed the scrollable rect doesn't make sense, and could lead to false
+  // positives.
+  painted = painted.Intersect(aPaintedMetrics.GetScrollableRect());
+  visible = visible.Intersect(aPaintedMetrics.GetScrollableRect());
+
+  return !painted.Contains(visible);
+}
+
+SideBits GetOverscrollSideBits(const ParentLayerPoint& aOverscrollAmount) {
+  SideBits sides = SideBits::eNone;
+
+  if (aOverscrollAmount.x < 0) {
+    sides |= SideBits::eLeft;
+  } else if (aOverscrollAmount.x > 0) {
+    sides |= SideBits::eRight;
+  }
+
+  if (aOverscrollAmount.y < 0) {
+    sides |= SideBits::eTop;
+  } else if (aOverscrollAmount.y > 0) {
+    sides |= SideBits::eBottom;
+  }
+
+  return sides;
+}
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/APZUtils.h b/gfx/layers/apz/src/APZUtils.h
new file mode 100644
index 0000000000..6614b6eeae
--- /dev/null
+++ b/gfx/layers/apz/src/APZUtils.h
@@ -0,0 +1,241 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_APZUtils_h
+#define mozilla_layers_APZUtils_h
+
+// This file is for APZ-related utilities that are used by code in gfx/layers
+// only. For APZ-related utilities used by the Rest of the World (widget/,
+// layout/, dom/, IPDL protocols, etc.), use APZPublicUtils.h.
+// Do not include this header from source files outside of gfx/layers.
+
+#include <stdint.h>  // for uint32_t
+#include <type_traits>
+#include "gfxTypes.h"
+#include "FrameMetrics.h"
+#include "LayersTypes.h"
+#include "UnitTransforms.h"
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/gfx/Point.h"
+#include "mozilla/DefineEnum.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/FloatingPoint.h"
+
+namespace mozilla {
+
+namespace layers {
+
+enum CancelAnimationFlags : uint32_t {
+  Default = 0x0,             /* Cancel all animations */
+  ExcludeOverscroll = 0x1,   /* Don't clear overscroll */
+  ScrollSnap = 0x2,          /* Snap to snap points */
+  ExcludeWheel = 0x4,        /* Don't stop wheel smooth-scroll animations */
+  TriggeredExternally = 0x8, /* Cancellation was not triggered by APZ in
+                                response to an input event */
+};
+
+inline CancelAnimationFlags operator|(CancelAnimationFlags a,
+                                      CancelAnimationFlags b) {
+  return static_cast<CancelAnimationFlags>(static_cast<int>(a) |
+                                           static_cast<int>(b));
+}
+
+// clang-format off
+enum class ScrollSource {
+  // Touch-screen.
+  Touchscreen,
+
+  // Touchpad with gesture support.
+  Touchpad,
+
+  // Mouse wheel.
+  Wheel,
+
+  // Keyboard
+  Keyboard,
+};
+// clang-format on
+
+inline bool ScrollSourceRespectsDisregardedDirections(ScrollSource aSource) {
+  return aSource == ScrollSource::Wheel || aSource == ScrollSource::Touchpad;
+}
+
+inline bool ScrollSourceAllowsOverscroll(ScrollSource aSource) {
+  return aSource == ScrollSource::Touchpad ||
+         aSource == ScrollSource::Touchscreen;
+}
+
+// Epsilon to be used when comparing 'float' coordinate values
+// with FuzzyEqualsAdditive. The rationale is that 'float' has 7 decimal
+// digits of precision, and coordinate values should be no larger than in the
+// ten thousands. Note also that the smallest legitimate difference in page
+// coordinates is 1 app unit, which is 1/60 of a (CSS pixel), so this epsilon
+// isn't too large.
+const CSSCoord COORDINATE_EPSILON = 0.01f;
+
+inline bool IsZero(const CSSPoint& aPoint) {
+  return FuzzyEqualsAdditive(aPoint.x, CSSCoord(), COORDINATE_EPSILON) &&
+         FuzzyEqualsAdditive(aPoint.y, CSSCoord(), COORDINATE_EPSILON);
+}
+
+// Represents async transforms consisting of a scale and a translation.
+struct AsyncTransform {
+  explicit AsyncTransform(
+      LayerToParentLayerScale aScale = LayerToParentLayerScale(),
+      ParentLayerPoint aTranslation = ParentLayerPoint())
+      : mScale(aScale), mTranslation(aTranslation) {}
+
+  operator AsyncTransformComponentMatrix() const {
+    return AsyncTransformComponentMatrix::Scaling(mScale.scale, mScale.scale, 1)
+        .PostTranslate(mTranslation.x, mTranslation.y, 0);
+  }
+
+  bool operator==(const AsyncTransform& rhs) const {
+    return mTranslation == rhs.mTranslation && mScale == rhs.mScale;
+  }
+
+  bool operator!=(const AsyncTransform& rhs) const { return !(*this == rhs); }
+
+  LayerToParentLayerScale mScale;
+  ParentLayerPoint mTranslation;
+};
+
+// Deem an AsyncTransformComponentMatrix (obtained by multiplying together
+// one or more AsyncTransformComponentMatrix objects) as constituting a
+// complete async transform.
+inline AsyncTransformMatrix CompleteAsyncTransform(
+    const AsyncTransformComponentMatrix& aMatrix) {
+  return ViewAs<AsyncTransformMatrix>(
+      aMatrix, PixelCastJustification::MultipleAsyncTransforms);
+}
+
+struct TargetConfirmationFlags final {
+  explicit TargetConfirmationFlags(bool aTargetConfirmed)
+      : mTargetConfirmed(aTargetConfirmed),
+        mRequiresTargetConfirmation(false),
+        mHitScrollbar(false),
+        mHitScrollThumb(false),
+        mDispatchToContent(false) {}
+
+  explicit TargetConfirmationFlags(
+      const gfx::CompositorHitTestInfo& aHitTestInfo)
+      : mTargetConfirmed(
+            (aHitTestInfo != gfx::CompositorHitTestInvisibleToHit) &&
+            (aHitTestInfo & gfx::CompositorHitTestDispatchToContent).isEmpty()),
+        mRequiresTargetConfirmation(aHitTestInfo.contains(
+            gfx::CompositorHitTestFlags::eRequiresTargetConfirmation)),
+        mHitScrollbar(
+            aHitTestInfo.contains(gfx::CompositorHitTestFlags::eScrollbar)),
+        mHitScrollThumb(aHitTestInfo.contains(
+            gfx::CompositorHitTestFlags::eScrollbarThumb)),
+        mDispatchToContent(
+            !(aHitTestInfo & gfx::CompositorHitTestDispatchToContent)
+                 .isEmpty()) {}
+
+  bool mTargetConfirmed : 1;
+  bool mRequiresTargetConfirmation : 1;
+  bool mHitScrollbar : 1;
+  bool mHitScrollThumb : 1;
+  bool mDispatchToContent : 1;
+};
+
+enum class AsyncTransformComponent { eLayout, eVisual };
+
+using AsyncTransformComponents = EnumSet<AsyncTransformComponent>;
+
+constexpr AsyncTransformComponents LayoutAndVisual(
+    AsyncTransformComponent::eLayout, AsyncTransformComponent::eVisual);
+
+/**
+ * Allows consumers of async transforms to specify for what purpose they are
+ * using the async transform:
+ *
+ *   |eForEventHandling| is intended for event handling and other uses that
+ *                       need the most up-to-date transform, reflecting all
+ *                       events that have been processed so far, even if the
+ *                       transform is not yet reflected visually.
+ *   |eForCompositing| is intended for the transform that should be reflected
+ *                     visually.
+ *
+ * For example, if an APZC has metrics with the mForceDisableApz flag set,
+ * then the |eForCompositing| async transform will be empty, while the
+ * |eForEventHandling| async transform will reflect processed input events
+ * regardless of mForceDisableApz.
+ */
+enum class AsyncTransformConsumer {
+  eForEventHandling,
+  eForCompositing,
+};
+
+/**
+ * Metrics that GeckoView wants to know at every composite.
+ * These are the effective visual scroll offset and zoom level of
+ * the root content APZC at composition time.
+ */
+struct GeckoViewMetrics {
+  CSSPoint mVisualScrollOffset;
+  CSSToParentLayerScale mZoom;
+};
+
+namespace apz {
+
+/**
+ * Is aAngle within the given threshold of the horizontal axis?
+ * @param aAngle an angle in radians in the range [0, pi]
+ * @param aThreshold an angle in radians in the range [0, pi/2]
+ */
+bool IsCloseToHorizontal(float aAngle, float aThreshold);
+
+// As above, but for the vertical axis.
+bool IsCloseToVertical(float aAngle, float aThreshold);
+
+// Returns true if a sticky layer with async translation |aTranslation| is
+// stuck with a bottom margin. The inner/outer ranges are produced by the main
+// thread at the last paint, and so |aTranslation| only needs to be the
+// async translation from the last paint.
+bool IsStuckAtBottom(gfxFloat aTranslation,
+                     const LayerRectAbsolute& aInnerRange,
+                     const LayerRectAbsolute& aOuterRange);
+
+// Returns true if a sticky layer with async translation |aTranslation| is
+// stuck with a top margin.
+bool IsStuckAtTop(gfxFloat aTranslation, const LayerRectAbsolute& aInnerRange,
+                  const LayerRectAbsolute& aOuterRange);
+
+/**
+ * Compute the translation that should be applied to a layer that's fixed
+ * at |eFixedSides|, to respect the fixed layer margins |aFixedMargins|.
+ */
+ScreenPoint ComputeFixedMarginsOffset(
+    const ScreenMargin& aCompositorFixedLayerMargins, SideBits aFixedSides,
+    const ScreenMargin& aGeckoFixedLayerMargins);
+
+/**
+ * Takes the visible rect from the compositor metrics, adds a pref-based
+ * margin around it, and checks to see if it is contained inside the painted
+ * rect from the painted metrics. Returns true if it is contained, or false
+ * if not. Returning false means that a (relatively) small amount of async
+ * scrolling/zooming can result in the visible area going outside the painted
+ * area and resulting in visual checkerboarding.
+ * Note that this may return false positives for cases where the scrollframe
+ * in question is nested inside other scrollframes, as the composition bounds
+ * used to determine the visible rect may in fact be clipped by enclosing
+ * scrollframes, but that is not accounted for in this function.
+ */
+bool AboutToCheckerboard(const FrameMetrics& aPaintedMetrics,
+                         const FrameMetrics& aCompositorMetrics);
+
+/**
+ * Returns SideBits where the given |aOverscrollAmount| overscrolls.
+ */
+SideBits GetOverscrollSideBits(const ParentLayerPoint& aOverscrollAmount);
+
+}  // namespace apz
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_APZUtils_h
diff --git a/gfx/layers/apz/src/AndroidAPZ.cpp b/gfx/layers/apz/src/AndroidAPZ.cpp
new file mode 100644
index 0000000000..4895c893de
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidAPZ.cpp
@@ -0,0 +1,36 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AndroidAPZ.h"
+
+#include "AndroidFlingPhysics.h"
+#include "AndroidVelocityTracker.h"
+#include "AsyncPanZoomController.h"
+#include "GenericFlingAnimation.h"
+#include "OverscrollHandoffState.h"
+
+namespace mozilla {
+namespace layers {
+
+AsyncPanZoomAnimation* AndroidSpecificState::CreateFlingAnimation(
+    AsyncPanZoomController& aApzc, const FlingHandoffState& aHandoffState,
+    float aPLPPI) {
+  return new GenericFlingAnimation<AndroidFlingPhysics>(aApzc, aHandoffState,
+                                                        aPLPPI);
+}
+
+UniquePtr<VelocityTracker> AndroidSpecificState::CreateVelocityTracker(
+    Axis* aAxis) {
+  return MakeUnique<AndroidVelocityTracker>();
+}
+
+/* static */
+void AndroidSpecificState::InitializeGlobalState() {
+  AndroidFlingPhysics::InitializeGlobalState();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/AndroidAPZ.h b/gfx/layers/apz/src/AndroidAPZ.h
new file mode 100644
index 0000000000..ab30b4e612
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidAPZ.h
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AndroidAPZ_h_
+#define mozilla_layers_AndroidAPZ_h_
+
+#include "AsyncPanZoomAnimation.h"
+#include "AsyncPanZoomController.h"
+
+namespace mozilla {
+namespace layers {
+
+class AndroidSpecificState : public PlatformSpecificStateBase {
+ public:
+  virtual AndroidSpecificState* AsAndroidSpecificState() override {
+    return this;
+  }
+
+  virtual AsyncPanZoomAnimation* CreateFlingAnimation(
+      AsyncPanZoomController& aApzc, const FlingHandoffState& aHandoffState,
+      float aPLPPI) override;
+  virtual UniquePtr<VelocityTracker> CreateVelocityTracker(
+      Axis* aAxis) override;
+
+  static void InitializeGlobalState();
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_AndroidAPZ_h_
diff --git a/gfx/layers/apz/src/AndroidFlingPhysics.cpp b/gfx/layers/apz/src/AndroidFlingPhysics.cpp
new file mode 100644
index 0000000000..d18f4be4d4
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidFlingPhysics.cpp
@@ -0,0 +1,218 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AndroidFlingPhysics.h"
+
+#include <cmath>
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPtr.h"
+
+namespace mozilla {
+namespace layers {
+
+// The fling physics calculations implemented here are adapted from
+// Chrome's implementation of fling physics on Android:
+// https://cs.chromium.org/chromium/src/ui/events/android/scroller.cc?rcl=3ae3aaff927038a5c644926842cb0c31dea60c79
+
+static double ComputeDeceleration(float aDPI) {
+  const float kFriction = 0.84f;
+  const float kGravityEarth = 9.80665f;
+  return kGravityEarth  // g (m/s^2)
+         * 39.37f       // inch/meter
+         * aDPI         // pixels/inch
+         * kFriction;
+}
+
+// == std::log(0.78f) / std::log(0.9f)
+const float kDecelerationRate = 2.3582018f;
+
+// Default friction constant in android.view.ViewConfiguration.
+static float GetFlingFriction() {
+  return StaticPrefs::apz_android_chrome_fling_physics_friction();
+}
+
+// Tension lines cross at (GetInflexion(), 1).
+static float GetInflexion() {
+  // Clamp the inflexion to the range [0,1]. Values outside of this range
+  // do not make sense in the physics model, and for negative values the
+  // approximation used to compute the spline curve does not converge.
+  const float inflexion =
+      StaticPrefs::apz_android_chrome_fling_physics_inflexion();
+  if (inflexion < 0.0f) {
+    return 0.0f;
+  }
+  if (inflexion > 1.0f) {
+    return 1.0f;
+  }
+  return inflexion;
+}
+
+// Fling scroll is stopped when the scroll position is |kThresholdForFlingEnd|
+// pixels or closer from the end.
+static float GetThresholdForFlingEnd() {
+  return StaticPrefs::apz_android_chrome_fling_physics_stop_threshold();
+}
+
+static double ComputeSplineDeceleration(ParentLayerCoord aVelocity,
+                                        double aTuningCoeff) {
+  float velocityPerSec = aVelocity * 1000.0f;
+  return std::log(GetInflexion() * velocityPerSec /
+                  (GetFlingFriction() * aTuningCoeff));
+}
+
+static TimeDuration ComputeFlingDuration(ParentLayerCoord aVelocity,
+                                         double aTuningCoeff) {
+  const double splineDecel = ComputeSplineDeceleration(aVelocity, aTuningCoeff);
+  const double timeSeconds = std::exp(splineDecel / (kDecelerationRate - 1.0));
+  return TimeDuration::FromSeconds(timeSeconds);
+}
+
+static ParentLayerCoord ComputeFlingDistance(ParentLayerCoord aVelocity,
+                                             double aTuningCoeff) {
+  const double splineDecel = ComputeSplineDeceleration(aVelocity, aTuningCoeff);
+  return GetFlingFriction() * aTuningCoeff *
+         std::exp(kDecelerationRate / (kDecelerationRate - 1.0) * splineDecel);
+}
+
+struct SplineConstants {
+ public:
+  SplineConstants() {
+    const float kStartTension = 0.5f;
+    const float kEndTension = 1.0f;
+    const float kP1 = kStartTension * GetInflexion();
+    const float kP2 = 1.0f - kEndTension * (1.0f - GetInflexion());
+
+    float xMin = 0.0f;
+    for (int i = 0; i < kNumSamples; i++) {
+      const float alpha = static_cast<float>(i) / kNumSamples;
+
+      float xMax = 1.0f;
+      float x, tx, coef;
+      // While the inflexion can be overridden by the user, it's clamped to
+      // [0,1]. For values in this range, the approximation algorithm below
+      // should converge in < 20 iterations. For good measure, we impose an
+      // iteration limit as well.
+      static const int sIterationLimit = 100;
+      int iterations = 0;
+      while (iterations++ < sIterationLimit) {
+        x = xMin + (xMax - xMin) / 2.0f;
+        coef = 3.0f * x * (1.0f - x);
+        tx = coef * ((1.0f - x) * kP1 + x * kP2) + x * x * x;
+        if (FuzzyEqualsAdditive(tx, alpha)) {
+          break;
+        }
+        if (tx > alpha) {
+          xMax = x;
+        } else {
+          xMin = x;
+        }
+      }
+      mSplinePositions[i] = coef * ((1.0f - x) * kStartTension + x) + x * x * x;
+    }
+    mSplinePositions[kNumSamples] = 1.0f;
+  }
+
+  void CalculateCoefficients(float aTime, float* aOutDistanceCoef,
+                             float* aOutVelocityCoef) {
+    *aOutDistanceCoef = 1.0f;
+    *aOutVelocityCoef = 0.0f;
+    const int index = static_cast<int>(kNumSamples * aTime);
+    if (index < kNumSamples) {
+      const float tInf = static_cast<float>(index) / kNumSamples;
+      const float dInf = mSplinePositions[index];
+      const float tSup = static_cast<float>(index + 1) / kNumSamples;
+      const float dSup = mSplinePositions[index + 1];
+      *aOutVelocityCoef = (dSup - dInf) / (tSup - tInf);
+      *aOutDistanceCoef = dInf + (aTime - tInf) * *aOutVelocityCoef;
+    }
+  }
+
+ private:
+  static const int kNumSamples = 100;
+  float mSplinePositions[kNumSamples + 1];
+};
+
+StaticAutoPtr<SplineConstants> gSplineConstants;
+
+/* static */
+void AndroidFlingPhysics::InitializeGlobalState() {
+  gSplineConstants = new SplineConstants();
+  ClearOnShutdown(&gSplineConstants);
+}
+
+void AndroidFlingPhysics::Init(const ParentLayerPoint& aStartingVelocity,
+                               float aPLPPI) {
+  mVelocity = aStartingVelocity.Length();
+  // We should not have created a fling animation if there is no velocity.
+  MOZ_ASSERT(mVelocity != 0.0f);
+  const double tuningCoeff = ComputeDeceleration(aPLPPI);
+  mTargetDuration = ComputeFlingDuration(mVelocity, tuningCoeff);
+  MOZ_ASSERT(!mTargetDuration.IsZero());
+  mDurationSoFar = TimeDuration();
+  mLastPos = ParentLayerPoint();
+  mCurrentPos = ParentLayerPoint();
+  float coeffX =
+      mVelocity == 0 ? 1.0f : aStartingVelocity.x.value / mVelocity.value;
+  float coeffY =
+      mVelocity == 0 ? 1.0f : aStartingVelocity.y.value / mVelocity.value;
+  mTargetDistance = ComputeFlingDistance(mVelocity, tuningCoeff);
+  mTargetPos =
+      ParentLayerPoint(mTargetDistance * coeffX, mTargetDistance * coeffY);
+  const float hyp = mTargetPos.Length();
+  if (FuzzyEqualsAdditive(hyp, 0.0f)) {
+    mDeltaNorm = ParentLayerPoint(1, 1);
+  } else {
+    mDeltaNorm = ParentLayerPoint(mTargetPos.x / hyp, mTargetPos.y / hyp);
+  }
+}
+void AndroidFlingPhysics::Sample(const TimeDuration& aDelta,
+                                 ParentLayerPoint* aOutVelocity,
+                                 ParentLayerPoint* aOutOffset) {
+  float newVelocity;
+  if (SampleImpl(aDelta, &newVelocity)) {
+    *aOutOffset = (mCurrentPos - mLastPos);
+    *aOutVelocity = ParentLayerPoint(mDeltaNorm.x * newVelocity,
+                                     mDeltaNorm.y * newVelocity);
+    mLastPos = mCurrentPos;
+  } else {
+    *aOutOffset = (mTargetPos - mLastPos);
+    *aOutVelocity = ParentLayerPoint();
+  }
+}
+
+bool AndroidFlingPhysics::SampleImpl(const TimeDuration& aDelta,
+                                     float* aOutVelocity) {
+  mDurationSoFar += aDelta;
+  if (mDurationSoFar >= mTargetDuration) {
+    return false;
+  }
+
+  const float timeRatio =
+      mDurationSoFar.ToSeconds() / mTargetDuration.ToSeconds();
+  float distanceCoef = 1.0f;
+  float velocityCoef = 0.0f;
+  gSplineConstants->CalculateCoefficients(timeRatio, &distanceCoef,
+                                          &velocityCoef);
+
+  // The caller expects the velocity in pixels per _millisecond_.
+  *aOutVelocity =
+      velocityCoef * mTargetDistance / mTargetDuration.ToMilliseconds();
+
+  mCurrentPos = mTargetPos * distanceCoef;
+
+  ParentLayerPoint remainder = mTargetPos - mCurrentPos;
+  const float threshold = GetThresholdForFlingEnd();
+  if (fabsf(remainder.x) < threshold && fabsf(remainder.y) < threshold) {
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/AndroidFlingPhysics.h b/gfx/layers/apz/src/AndroidFlingPhysics.h
new file mode 100644
index 0000000000..68fb53e804
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidFlingPhysics.h
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AndroidFlingPhysics_h_
+#define mozilla_layers_AndroidFlingPhysics_h_
+
+#include "AsyncPanZoomController.h"
+#include "Units.h"
+#include "mozilla/Assertions.h"
+
+namespace mozilla {
+namespace layers {
+
+class AndroidFlingPhysics {
+ public:
+  void Init(const ParentLayerPoint& aVelocity, float aPLPPI);
+  void Sample(const TimeDuration& aDelta, ParentLayerPoint* aOutVelocity,
+              ParentLayerPoint* aOutOffset);
+
+  static void InitializeGlobalState();
+
+ private:
+  // Returns false if the animation should end.
+  bool SampleImpl(const TimeDuration& aDelta, float* aOutVelocity);
+
+  // Information pertaining to the current fling.
+  // This is initialized on each call to Init().
+  ParentLayerCoord mVelocity;  // diagonal velocity (length of velocity vector)
+  TimeDuration mTargetDuration;
+  TimeDuration mDurationSoFar;
+  ParentLayerPoint mLastPos;
+  ParentLayerPoint mCurrentPos;
+  ParentLayerCoord mTargetDistance;  // diagonal distance
+  ParentLayerPoint mTargetPos;       // really a target *offset* relative to the
+                                     // start position, which we don't track
+  ParentLayerPoint mDeltaNorm;       // mTargetPos with length normalized to 1
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_AndroidFlingPhysics_h_
diff --git a/gfx/layers/apz/src/AndroidVelocityTracker.cpp b/gfx/layers/apz/src/AndroidVelocityTracker.cpp
new file mode 100644
index 0000000000..a355811a00
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidVelocityTracker.cpp
@@ -0,0 +1,288 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AndroidVelocityTracker.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+
+namespace mozilla {
+namespace layers {
+
+// This velocity tracker implementation was adapted from Chromium's
+// second-order unweighted least-squares velocity tracker strategy
+// (https://cs.chromium.org/chromium/src/ui/events/gesture_detection/velocity_tracker.cc?l=101&rcl=9ea9a086d4f54c702ec9a38e55fb3eb8bbc2401b).
+
+// Threshold between position updates for determining that a pointer has
+// stopped moving. Some input devices do not send move events in the
+// case where a pointer has stopped.  We need to detect this case so that we can
+// accurately predict the velocity after the pointer starts moving again.
+static const TimeDuration kAssumePointerMoveStoppedTime =
+    TimeDuration::FromMilliseconds(40);
+
+// The degree of the approximation.
+static const uint8_t kDegree = 2;
+
+// The degree of the polynomial used in SolveLeastSquares().
+// This should be the degree of the approximation plus one.
+static const uint8_t kPolyDegree = kDegree + 1;
+
+// Maximum size of position history.
+static const uint8_t kHistorySize = 20;
+
+AndroidVelocityTracker::AndroidVelocityTracker() {}
+
+void AndroidVelocityTracker::StartTracking(ParentLayerCoord aPos,
+                                           TimeStamp aTimestamp) {
+  Clear();
+  mHistory.AppendElement(std::make_pair(aTimestamp, aPos));
+  mLastEventTime = aTimestamp;
+}
+
+Maybe<float> AndroidVelocityTracker::AddPosition(ParentLayerCoord aPos,
+                                                 TimeStamp aTimestamp) {
+  if ((aTimestamp - mLastEventTime) >= kAssumePointerMoveStoppedTime) {
+    Clear();
+  }
+
+  if ((aTimestamp - mLastEventTime).ToMilliseconds() < 1.0) {
+    // If we get a sample within a millisecond of the previous one,
+    // just update its position. Two samples in the history with the
+    // same timestamp can lead to things like infinite velocities.
+    if (mHistory.Length() > 0) {
+      mHistory[mHistory.Length() - 1].second = aPos;
+    }
+  } else {
+    mHistory.AppendElement(std::make_pair(aTimestamp, aPos));
+    if (mHistory.Length() > kHistorySize) {
+      mHistory.RemoveElementAt(0);
+    }
+  }
+
+  mLastEventTime = aTimestamp;
+
+  if (mHistory.Length() < 2) {
+    return Nothing();
+  }
+
+  auto start = mHistory[mHistory.Length() - 2];
+  auto end = mHistory[mHistory.Length() - 1];
+  auto velocity =
+      (end.second - start.second) / (end.first - start.first).ToMilliseconds();
+  // The velocity needs to be negated because the positions represent
+  // touch positions, and the direction of scrolling is opposite to the
+  // direction of the finger's movement.
+  return Some(-velocity);
+}
+
+static float VectorDot(const float* a, const float* b, uint32_t m) {
+  float r = 0;
+  while (m--) {
+    r += *(a++) * *(b++);
+  }
+  return r;
+}
+
+static float VectorNorm(const float* a, uint32_t m) {
+  float r = 0;
+  while (m--) {
+    float t = *(a++);
+    r += t * t;
+  }
+  return sqrtf(r);
+}
+
+/**
+ * Solves a linear least squares problem to obtain a N degree polynomial that
+ * fits the specified input data as nearly as possible.
+ *
+ * Returns true if a solution is found, false otherwise.
+ *
+ * The input consists of two vectors of data points X and Y with indices 0..m-1
+ * along with a weight vector W of the same size.
+ *
+ * The output is a vector B with indices 0..n that describes a polynomial
+ * that fits the data, such the sum of W[i] * W[i] * abs(Y[i] - (B[0] + B[1]
+ * X[i] * + B[2] X[i]^2 ... B[n] X[i]^n)) for all i between 0 and m-1 is
+ * minimized.
+ *
+ * Accordingly, the weight vector W should be initialized by the caller with the
+ * reciprocal square root of the variance of the error in each input data point.
+ * In other words, an ideal choice for W would be W[i] = 1 / var(Y[i]) = 1 /
+ * stddev(Y[i]).
+ * The weights express the relative importance of each data point.  If the
+ * weights are* all 1, then the data points are considered to be of equal
+ * importance when fitting the polynomial.  It is a good idea to choose weights
+ * that diminish the importance of data points that may have higher than usual
+ * error margins.
+ *
+ * Errors among data points are assumed to be independent.  W is represented
+ * here as a vector although in the literature it is typically taken to be a
+ * diagonal matrix.
+ *
+ * That is to say, the function that generated the input data can be
+ * approximated by y(x) ~= B[0] + B[1] x + B[2] x^2 + ... + B[n] x^n.
+ *
+ * The coefficient of determination (R^2) is also returned to describe the
+ * goodness of fit of the model for the given data.  It is a value between 0
+ * and 1, where 1 indicates perfect correspondence.
+ *
+ * This function first expands the X vector to a m by n matrix A such that
+ * A[i][0] = 1, A[i][1] = X[i], A[i][2] = X[i]^2, ..., A[i][n] = X[i]^n, then
+ * multiplies it by w[i].
+ *
+ * Then it calculates the QR decomposition of A yielding an m by m orthonormal
+ * matrix Q and an m by n upper triangular matrix R.  Because R is upper
+ * triangular (lower part is all zeroes), we can simplify the decomposition into
+ * an m by n matrix Q1 and a n by n matrix R1 such that A = Q1 R1.
+ *
+ * Finally we solve the system of linear equations given by
+ * R1 B = (Qtranspose W Y) to find B.
+ *
+ * For efficiency, we lay out A and Q column-wise in memory because we
+ * frequently operate on the column vectors.  Conversely, we lay out R row-wise.
+ *
+ * http://en.wikipedia.org/wiki/Numerical_methods_for_linear_least_squares
+ * http://en.wikipedia.org/wiki/Gram-Schmidt
+ */
+static bool SolveLeastSquares(const float* x, const float* y, const float* w,
+                              uint32_t m, uint32_t n, float* out_b) {
+  // MSVC does not support variable-length arrays (used by the original Android
+  // implementation of this function).
+#if defined(COMPILER_MSVC)
+  const uint32_t M_ARRAY_LENGTH = VelocityTracker::kHistorySize;
+  const uint32_t N_ARRAY_LENGTH = VelocityTracker::kPolyDegree;
+  DCHECK_LE(m, M_ARRAY_LENGTH);
+  DCHECK_LE(n, N_ARRAY_LENGTH);
+#else
+  const uint32_t M_ARRAY_LENGTH = m;
+  const uint32_t N_ARRAY_LENGTH = n;
+#endif
+
+  // Expand the X vector to a matrix A, pre-multiplied by the weights.
+  float a[N_ARRAY_LENGTH][M_ARRAY_LENGTH];  // column-major order
+  for (uint32_t h = 0; h < m; h++) {
+    a[0][h] = w[h];
+    for (uint32_t i = 1; i < n; i++) {
+      a[i][h] = a[i - 1][h] * x[h];
+    }
+  }
+
+  // Apply the Gram-Schmidt process to A to obtain its QR decomposition.
+
+  // Orthonormal basis, column-major order.
+  float q[N_ARRAY_LENGTH][M_ARRAY_LENGTH];
+  // Upper triangular matrix, row-major order.
+  float r[N_ARRAY_LENGTH][N_ARRAY_LENGTH];
+  for (uint32_t j = 0; j < n; j++) {
+    for (uint32_t h = 0; h < m; h++) {
+      q[j][h] = a[j][h];
+    }
+    for (uint32_t i = 0; i < j; i++) {
+      float dot = VectorDot(&q[j][0], &q[i][0], m);
+      for (uint32_t h = 0; h < m; h++) {
+        q[j][h] -= dot * q[i][h];
+      }
+    }
+
+    float norm = VectorNorm(&q[j][0], m);
+    if (norm < 0.000001f) {
+      // vectors are linearly dependent or zero so no solution
+      return false;
+    }
+
+    float invNorm = 1.0f / norm;
+    for (uint32_t h = 0; h < m; h++) {
+      q[j][h] *= invNorm;
+    }
+    for (uint32_t i = 0; i < n; i++) {
+      r[j][i] = i < j ? 0 : VectorDot(&q[j][0], &a[i][0], m);
+    }
+  }
+
+  // Solve R B = Qt W Y to find B.  This is easy because R is upper triangular.
+  // We just work from bottom-right to top-left calculating B's coefficients.
+  float wy[M_ARRAY_LENGTH];
+  for (uint32_t h = 0; h < m; h++) {
+    wy[h] = y[h] * w[h];
+  }
+  for (uint32_t i = n; i-- != 0;) {
+    out_b[i] = VectorDot(&q[i][0], wy, m);
+    for (uint32_t j = n - 1; j > i; j--) {
+      out_b[i] -= r[i][j] * out_b[j];
+    }
+    out_b[i] /= r[i][i];
+  }
+
+  return true;
+}
+
+Maybe<float> AndroidVelocityTracker::ComputeVelocity(TimeStamp aTimestamp) {
+  if (mHistory.IsEmpty()) {
+    return Nothing{};
+  }
+
+  // Polynomial coefficients describing motion along the axis.
+  float xcoeff[kPolyDegree + 1];
+  for (size_t i = 0; i <= kPolyDegree; i++) {
+    xcoeff[i] = 0;
+  }
+
+  // Iterate over movement samples in reverse time order and collect samples.
+  float pos[kHistorySize];
+  float w[kHistorySize];
+  float time[kHistorySize];
+  uint32_t m = 0;
+  int index = mHistory.Length() - 1;
+  const TimeDuration horizon = TimeDuration::FromMilliseconds(
+      StaticPrefs::apz_velocity_relevance_time_ms());
+  const auto& newest_movement = mHistory[index];
+
+  do {
+    const auto& movement = mHistory[index];
+    TimeDuration age = newest_movement.first - movement.first;
+    if (age > horizon) break;
+
+    ParentLayerCoord position = movement.second;
+    pos[m] = position;
+    w[m] = 1.0f;
+    time[m] =
+        -static_cast<float>(age.ToMilliseconds()) / 1000.0f;  // in seconds
+    index--;
+    m++;
+  } while (index >= 0);
+
+  if (m == 0) {
+    return Nothing{};  // no data
+  }
+
+  // Calculate a least squares polynomial fit.
+
+  // Polynomial degree (number of coefficients), or zero if no information is
+  // available.
+  uint32_t degree = kDegree;
+  if (degree > m - 1) {
+    degree = m - 1;
+  }
+
+  if (degree >= 1) {  // otherwise, no velocity data available
+    uint32_t n = degree + 1;
+    if (SolveLeastSquares(time, pos, w, m, n, xcoeff)) {
+      float velocity = xcoeff[1];
+
+      // The velocity needs to be negated because the positions represent
+      // touch positions, and the direction of scrolling is opposite to the
+      // direction of the finger's movement.
+      return Some(-velocity / 1000.0f);  // convert to pixels per millisecond
+    }
+  }
+
+  return Nothing{};
+}
+
+void AndroidVelocityTracker::Clear() { mHistory.Clear(); }
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/AndroidVelocityTracker.h b/gfx/layers/apz/src/AndroidVelocityTracker.h
new file mode 100644
index 0000000000..40e346a9ea
--- /dev/null
+++ b/gfx/layers/apz/src/AndroidVelocityTracker.h
@@ -0,0 +1,42 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AndroidVelocityTracker_h
+#define mozilla_layers_AndroidVelocityTracker_h
+
+#include <utility>
+#include <cstdint>
+
+#include "Axis.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace layers {
+
+class AndroidVelocityTracker : public VelocityTracker {
+ public:
+  explicit AndroidVelocityTracker();
+  void StartTracking(ParentLayerCoord aPos, TimeStamp aTimestamp) override;
+  Maybe<float> AddPosition(ParentLayerCoord aPos,
+                           TimeStamp aTimestamp) override;
+  Maybe<float> ComputeVelocity(TimeStamp aTimestamp) override;
+  void Clear() override;
+
+ private:
+  // A queue of (timestamp, position) pairs; these are the historical
+  // positions at the given timestamps.
+  nsTArray<std::pair<TimeStamp, ParentLayerCoord>> mHistory;
+  // The last time an event was added to the tracker, or the null moment if no
+  // events have been added.
+  TimeStamp mLastEventTime;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/AsyncDragMetrics.h b/gfx/layers/apz/src/AsyncDragMetrics.h
new file mode 100644
index 0000000000..5374818a06
--- /dev/null
+++ b/gfx/layers/apz/src/AsyncDragMetrics.h
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_DragMetrics_h
+#define mozilla_layers_DragMetrics_h
+
+#include "mozilla/layers/ScrollableLayerGuid.h"
+#include "LayersTypes.h"
+#include "mozilla/Maybe.h"
+
+namespace IPC {
+template <typename T>
+struct ParamTraits;
+}  // namespace IPC
+
+namespace mozilla {
+
+namespace layers {
+
+class AsyncDragMetrics {
+  friend struct IPC::ParamTraits<mozilla::layers::AsyncDragMetrics>;
+
+ public:
+  // IPC constructor
+  AsyncDragMetrics()
+      : mViewId(0),
+        mPresShellId(0),
+        mDragStartSequenceNumber(0),
+        mScrollbarDragOffset(0) {}
+
+  AsyncDragMetrics(const ScrollableLayerGuid::ViewID& aViewId,
+                   uint32_t aPresShellId, uint64_t aDragStartSequenceNumber,
+                   OuterCSSCoord aScrollbarDragOffset,
+                   ScrollDirection aDirection)
+      : mViewId(aViewId),
+        mPresShellId(aPresShellId),
+        mDragStartSequenceNumber(aDragStartSequenceNumber),
+        mScrollbarDragOffset(aScrollbarDragOffset),
+        mDirection(Some(aDirection)) {}
+
+  ScrollableLayerGuid::ViewID mViewId;
+  uint32_t mPresShellId;
+  uint64_t mDragStartSequenceNumber;
+  OuterCSSCoord mScrollbarDragOffset;  // relative to the thumb's start offset
+  Maybe<ScrollDirection> mDirection;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/AsyncPanZoomAnimation.h b/gfx/layers/apz/src/AsyncPanZoomAnimation.h
new file mode 100644
index 0000000000..127667afd9
--- /dev/null
+++ b/gfx/layers/apz/src/AsyncPanZoomAnimation.h
@@ -0,0 +1,101 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AsyncPanZoomAnimation_h_
+#define mozilla_layers_AsyncPanZoomAnimation_h_
+
+#include "APZUtils.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "nsISupportsImpl.h"
+#include "nsTArray.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+namespace layers {
+
+struct FrameMetrics;
+
+class WheelScrollAnimation;
+class OverscrollAnimation;
+class SmoothMsdScrollAnimation;
+class SmoothScrollAnimation;
+
+class AsyncPanZoomAnimation {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AsyncPanZoomAnimation)
+
+ public:
+  explicit AsyncPanZoomAnimation() = default;
+
+  virtual bool DoSample(FrameMetrics& aFrameMetrics,
+                        const TimeDuration& aDelta) = 0;
+
+  /**
+   * Attempt to handle a main-thread scroll offset update without cancelling
+   * the animation. This may or may not make sense depending on the type of
+   * the animation and whether the scroll update is relative or absolute.
+   *
+   * If the scroll update is relative, |aRelativeDelta| will contain the
+   * delta of the relative update. If the scroll update is absolute,
+   * |aRelativeDelta| will be Nothing() (the animation can check the APZC's
+   * FrameMetrics for the new absolute scroll offset if it wants to handle
+   * and absolute update).
+   *
+   * Returns whether the animation could handle the scroll update. If the
+   * return value is false, the animation will be cancelled.
+   */
+  virtual bool HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta) {
+    return false;
+  }
+
+  bool Sample(FrameMetrics& aFrameMetrics, const TimeDuration& aDelta) {
+    // In some situations, particularly when handoff is involved, it's possible
+    // for |aDelta| to be negative on the first call to sample. Ignore such a
+    // sample here, to avoid each derived class having to deal with this case.
+    if (aDelta.ToMilliseconds() <= 0) {
+      return true;
+    }
+
+    return DoSample(aFrameMetrics, aDelta);
+  }
+
+  /**
+   * Get the deferred tasks in |mDeferredTasks| and place them in |aTasks|. See
+   * |mDeferredTasks| for more information.  Clears |mDeferredTasks|.
+   */
+  nsTArray<RefPtr<Runnable>> TakeDeferredTasks() {
+    return std::move(mDeferredTasks);
+  }
+
+  virtual WheelScrollAnimation* AsWheelScrollAnimation() { return nullptr; }
+  virtual SmoothMsdScrollAnimation* AsSmoothMsdScrollAnimation() {
+    return nullptr;
+  }
+  virtual SmoothScrollAnimation* AsSmoothScrollAnimation() { return nullptr; }
+  virtual OverscrollAnimation* AsOverscrollAnimation() { return nullptr; }
+
+  virtual bool WantsRepaints() { return true; }
+
+  virtual void Cancel(CancelAnimationFlags aFlags) {}
+
+  virtual bool WasTriggeredByScript() const { return false; }
+
+ protected:
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~AsyncPanZoomAnimation() = default;
+
+  /**
+   * Tasks scheduled for execution after the APZC's mMonitor is released.
+   * Derived classes can add tasks here in Sample(), and the APZC can call
+   * ExecuteDeferredTasks() to execute them.
+   */
+  nsTArray<RefPtr<Runnable>> mDeferredTasks;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_AsyncPanZoomAnimation_h_
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
new file mode 100644
index 0000000000..342375c019
--- /dev/null
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -0,0 +1,6848 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AsyncPanZoomController.h"  // for AsyncPanZoomController, etc
+
+#include <math.h>       // for fabsf, fabs, atan2
+#include <stdint.h>     // for uint32_t, uint64_t
+#include <sys/types.h>  // for int32_t
+#include <algorithm>    // for max, min
+#include <utility>      // for std::make_pair
+
+#include "APZCTreeManager.h"            // for APZCTreeManager
+#include "AsyncPanZoomAnimation.h"      // for AsyncPanZoomAnimation
+#include "AutoDirWheelDeltaAdjuster.h"  // for APZAutoDirWheelDeltaAdjuster
+#include "AutoscrollAnimation.h"        // for AutoscrollAnimation
+#include "Axis.h"                       // for AxisX, AxisY, Axis, etc
+#include "CheckerboardEvent.h"          // for CheckerboardEvent
+#include "Compositor.h"                 // for Compositor
+#include "DesktopFlingPhysics.h"        // for DesktopFlingPhysics
+#include "FrameMetrics.h"               // for FrameMetrics, etc
+#include "GenericFlingAnimation.h"      // for GenericFlingAnimation
+#include "GestureEventListener.h"       // for GestureEventListener
+#include "HitTestingTreeNode.h"         // for HitTestingTreeNode
+#include "InputData.h"                  // for MultiTouchInput, etc
+#include "InputBlockState.h"            // for InputBlockState, TouchBlockState
+#include "InputQueue.h"                 // for InputQueue
+#include "Overscroll.h"                 // for OverscrollAnimation
+#include "OverscrollHandoffState.h"     // for OverscrollHandoffState
+#include "SimpleVelocityTracker.h"      // for SimpleVelocityTracker
+#include "Units.h"                      // for CSSRect, CSSPoint, etc
+#include "UnitTransforms.h"             // for TransformTo
+#include "base/message_loop.h"          // for MessageLoop
+#include "base/task.h"                  // for NewRunnableMethod, etc
+#include "gfxTypes.h"                   // for gfxFloat
+#include "mozilla/Assertions.h"         // for MOZ_ASSERT, etc
+#include "mozilla/BasicEvents.h"        // for Modifiers, MODIFIER_*
+#include "mozilla/ClearOnShutdown.h"    // for ClearOnShutdown
+#include "mozilla/ServoStyleConsts.h"   // for StyleComputedTimingFunction
+#include "mozilla/EventForwards.h"      // for nsEventStatus_*
+#include "mozilla/EventStateManager.h"  // for EventStateManager
+#include "mozilla/glean/GleanMetrics.h"
+#include "mozilla/MouseEvents.h"     // for WidgetWheelEvent
+#include "mozilla/Preferences.h"     // for Preferences
+#include "mozilla/RecursiveMutex.h"  // for RecursiveMutexAutoLock, etc
+#include "mozilla/RefPtr.h"          // for RefPtr
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_general.h"
+#include "mozilla/StaticPrefs_gfx.h"
+#include "mozilla/StaticPrefs_mousewheel.h"
+#include "mozilla/StaticPrefs_layers.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_slider.h"
+#include "mozilla/StaticPrefs_test.h"
+#include "mozilla/StaticPrefs_toolkit.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+#include "mozilla/TimeStamp.h"  // for TimeDuration, TimeStamp
+#include "mozilla/dom/CheckerboardReportService.h"  // for CheckerboardEventStorage
+// note: CheckerboardReportService.h actually lives in gfx/layers/apz/util/
+#include "mozilla/dom/Touch.h"              // for Touch
+#include "mozilla/gfx/gfxVars.h"            // for gfxVars
+#include "mozilla/gfx/BasePoint.h"          // for BasePoint
+#include "mozilla/gfx/BaseRect.h"           // for BaseRect
+#include "mozilla/gfx/Point.h"              // for Point, RoundedToInt, etc
+#include "mozilla/gfx/Rect.h"               // for RoundedIn
+#include "mozilla/gfx/ScaleFactor.h"        // for ScaleFactor
+#include "mozilla/layers/APZThreadUtils.h"  // for AssertOnControllerThread, etc
+#include "mozilla/layers/APZUtils.h"        // for AsyncTransform
+#include "mozilla/layers/CompositorController.h"  // for CompositorController
+#include "mozilla/layers/DirectionUtils.h"  // for GetAxis{Start,End,Length,Scale}
+#include "mozilla/layers/APZPublicUtils.h"   // for GetScrollMode
+#include "mozilla/webrender/WebRenderAPI.h"  // for MinimapData
+#include "mozilla/mozalloc.h"                // for operator new, etc
+#include "mozilla/Unused.h"                  // for unused
+#include "mozilla/webrender/WebRenderTypes.h"
+#include "nsAlgorithm.h"  // for clamped
+#include "nsCOMPtr.h"     // for already_AddRefed
+#include "nsDebug.h"      // for NS_WARNING
+#include "nsLayoutUtils.h"
+#include "nsMathUtils.h"  // for NS_hypot
+#include "nsPoint.h"      // for nsIntPoint
+#include "nsStyleConsts.h"
+#include "nsTArray.h"        // for nsTArray, nsTArray_Impl, etc
+#include "nsThreadUtils.h"   // for NS_IsMainThread
+#include "nsViewportInfo.h"  // for ViewportMinScale(), ViewportMaxScale()
+#include "prsystem.h"        // for PR_GetPhysicalMemorySize
+#include "mozilla/ipc/SharedMemoryBasic.h"  // for SharedMemoryBasic
+#include "ScrollSnap.h"                     // for ScrollSnapUtils
+#include "ScrollAnimationPhysics.h"         // for ComputeAcceleratedWheelDelta
+#include "SmoothMsdScrollAnimation.h"
+#include "SmoothScrollAnimation.h"
+#include "WheelScrollAnimation.h"
+#if defined(MOZ_WIDGET_ANDROID)
+#  include "AndroidAPZ.h"
+#endif  // defined(MOZ_WIDGET_ANDROID)
+
+static mozilla::LazyLogModule sApzCtlLog("apz.controller");
+#define APZC_LOG(...) MOZ_LOG(sApzCtlLog, LogLevel::Debug, (__VA_ARGS__))
+#define APZC_LOGV(...) MOZ_LOG(sApzCtlLog, LogLevel::Verbose, (__VA_ARGS__))
+
+// Log to the apz.controller log with additional info from the APZC
+#define APZC_LOG_DETAIL(fmt, apzc, ...)                   \
+  APZC_LOG("%p(%s scrollId=%" PRIu64 "): " fmt, (apzc),   \
+           (apzc)->IsRootContent() ? "root" : "subframe", \
+           (apzc)->GetScrollId(), ##__VA_ARGS__)
+
+#define APZC_LOG_FM_COMMON(fm, prefix, level, ...)                 \
+  if (MOZ_LOG_TEST(sApzCtlLog, level)) {                           \
+    std::stringstream ss;                                          \
+    ss << nsPrintfCString(prefix, __VA_ARGS__).get() << ":" << fm; \
+    MOZ_LOG(sApzCtlLog, level, ("%s\n", ss.str().c_str()));        \
+  }
+#define APZC_LOG_FM(fm, prefix, ...) \
+  APZC_LOG_FM_COMMON(fm, prefix, LogLevel::Debug, __VA_ARGS__)
+#define APZC_LOGV_FM(fm, prefix, ...) \
+  APZC_LOG_FM_COMMON(fm, prefix, LogLevel::Verbose, __VA_ARGS__)
+
+namespace mozilla {
+namespace layers {
+
+typedef mozilla::layers::AllowedTouchBehavior AllowedTouchBehavior;
+typedef GeckoContentController::APZStateChange APZStateChange;
+typedef GeckoContentController::TapType TapType;
+typedef mozilla::gfx::Point Point;
+typedef mozilla::gfx::Matrix4x4 Matrix4x4;
+
+// Choose between platform-specific implementations.
+#ifdef MOZ_WIDGET_ANDROID
+typedef WidgetOverscrollEffect OverscrollEffect;
+typedef AndroidSpecificState PlatformSpecificState;
+#else
+typedef GenericOverscrollEffect OverscrollEffect;
+typedef PlatformSpecificStateBase
+    PlatformSpecificState;  // no extra state, just use the base class
+#endif
+
+/**
+ * \page APZCPrefs APZ preferences
+ *
+ * The following prefs are used to control the behaviour of the APZC.
+ * The default values are provided in StaticPrefList.yaml.
+ *
+ * \li\b apz.allow_double_tap_zooming
+ * Pref that allows or disallows double tap to zoom
+ *
+ * \li\b apz.allow_immediate_handoff
+ * If set to true, scroll can be handed off from one APZC to another within
+ * a single input block. If set to false, a single input block can only
+ * scroll one APZC.
+ *
+ * \li\b apz.allow_zooming_out
+ * If set to true, APZ will allow zooming out past the initial scale on
+ * desktop. This is false by default to match Chrome's behaviour.
+ *
+ * \li\b apz.android.chrome_fling_physics.friction
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * how quickly a fling animation slows down due to friction (and therefore
+ * also how far it reaches). Should be in the range [0-1].
+ *
+ * \li\b apz.android.chrome_fling_physics.inflexion
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * the shape of the fling curve. Should be in the range [0-1].
+ *
+ * \li\b apz.android.chrome_fling_physics.stop_threshold
+ * A tunable parameter for Chrome fling physics on Android that governs
+ * how close the fling animation has to get to its target destination
+ * before it stops.
+ * Units: ParentLayer pixels
+ *
+ * \li\b apz.autoscroll.enabled
+ * If set to true, autoscrolling is driven by APZ rather than the content
+ * process main thread.
+ *
+ * \li\b apz.axis_lock.mode
+ * The preferred axis locking style. See AxisLockMode for possible values.
+ *
+ * \li\b apz.axis_lock.lock_angle
+ * Angle from axis within which we stay axis-locked.\n
+ * Units: radians
+ *
+ * \li\b apz.axis_lock.breakout_threshold
+ * Distance in inches the user must pan before axis lock can be broken.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.axis_lock.breakout_angle
+ * Angle at which axis lock can be broken.\n
+ * Units: radians
+ *
+ * \li\b apz.axis_lock.direct_pan_angle
+ * If the angle from an axis to the line drawn by a pan move is less than
+ * this value, we can assume that panning can be done in the allowed direction
+ * (horizontal or vertical).\n
+ * Currently used only for touch-action css property stuff and was addded to
+ * keep behaviour consistent with IE.\n
+ * Units: radians
+ *
+ * \li\b apz.content_response_timeout
+ * Amount of time before we timeout response from content. For example, if
+ * content is being unruly/slow and we don't get a response back within this
+ * time, we will just pretend that content did not preventDefault any touch
+ * events we dispatched to it.\n
+ * Units: milliseconds
+ *
+ * \li\b apz.danger_zone_x
+ * \li\b apz.danger_zone_y
+ * When drawing high-res tiles, we drop down to drawing low-res tiles
+ * when we know we can't keep up with the scrolling. The way we determine
+ * this is by checking if we are entering the "danger zone", which is the
+ * boundary of the painted content. For example, if the painted content
+ * goes from y=0...1000 and the visible portion is y=250...750 then
+ * we're far from checkerboarding. If we get to y=490...990 though then we're
+ * only 10 pixels away from showing checkerboarding so we are probably in
+ * a state where we can't keep up with scrolling. The danger zone prefs specify
+ * how wide this margin is; in the above example a y-axis danger zone of 10
+ * pixels would make us drop to low-res at y=490...990.\n
+ * This value is in screen pixels.
+ *
+ * \li\b apz.disable_for_scroll_linked_effects
+ * Setting this pref to true will disable APZ scrolling on documents where
+ * scroll-linked effects are detected. A scroll linked effect is detected if
+ * positioning or transform properties are updated inside a scroll event
+ * dispatch; we assume that such an update is in response to the scroll event
+ * and is therefore a scroll-linked effect which will be laggy with APZ
+ * scrolling.
+ *
+ * \li\b apz.displayport_expiry_ms
+ * While a scrollable frame is scrolling async, we set a displayport on it
+ * to make sure it is layerized. However this takes up memory, so once the
+ * scrolling stops we want to remove the displayport. This pref controls how
+ * long after scrolling stops the displayport is removed. A value of 0 will
+ * disable the expiry behavior entirely.
+ * Units: milliseconds
+ *
+ * \li\b apz.drag.enabled
+ * Setting this pref to true will cause APZ to handle mouse-dragging of
+ * scrollbar thumbs.
+ *
+ * \li\b apz.drag.initial.enabled
+ * Setting this pref to true will cause APZ to try to handle mouse-dragging
+ * of scrollbar thumbs without an initial round-trip to content to start it
+ * if possible. Only has an effect if apz.drag.enabled is also true.
+ *
+ * \li\b apz.drag.touch.enabled
+ * Setting this pref to true will cause APZ to handle touch-dragging of
+ * scrollbar thumbs. Only has an effect if apz.drag.enabled is also true.
+ *
+ * \li\b apz.enlarge_displayport_when_clipped
+ * Pref that enables enlarging of the displayport along one axis when the
+ * generated displayport's size is beyond that of the scrollable rect on the
+ * opposite axis.
+ *
+ * \li\b apz.fling_accel_min_fling_velocity
+ * The minimum velocity of the second fling, and the minimum velocity of the
+ * previous fling animation at the point of interruption, for the new fling to
+ * be considered for fling acceleration.
+ * Units: screen pixels per milliseconds
+ *
+ * \li\b apz.fling_accel_min_pan_velocity
+ * The minimum velocity during the pan gesture that causes a fling for that
+ * fling to be considered for fling acceleration.
+ * Units: screen pixels per milliseconds
+ *
+ * \li\b apz.fling_accel_max_pause_interval_ms
+ * The maximum time that is allowed to elapse between the touch start event that
+ * interrupts the previous fling, and the touch move that initiates panning for
+ * the current fling, for that fling to be considered for fling acceleration.
+ * Units: milliseconds
+ *
+ * \li\b apz.fling_accel_base_mult
+ * \li\b apz.fling_accel_supplemental_mult
+ * When applying an acceleration on a fling, the new computed velocity is
+ * (new_fling_velocity * base_mult) + (old_velocity * supplemental_mult).
+ * The base_mult and supplemental_mult multiplier values are controlled by
+ * these prefs. Note that "old_velocity" here is the initial velocity of the
+ * previous fling _after_ acceleration was applied to it (if applicable).
+ *
+ * \li\b apz.fling_curve_function_x1
+ * \li\b apz.fling_curve_function_y1
+ * \li\b apz.fling_curve_function_x2
+ * \li\b apz.fling_curve_function_y2
+ * \li\b apz.fling_curve_threshold_inches_per_ms
+ * These five parameters define a Bezier curve function and threshold used to
+ * increase the actual velocity relative to the user's finger velocity. When the
+ * finger velocity is below the threshold (or if the threshold is not positive),
+ * the velocity is used as-is. If the finger velocity exceeds the threshold
+ * velocity, then the function defined by the curve is applied on the part of
+ * the velocity that exceeds the threshold. Note that the upper bound of the
+ * velocity is still specified by the \b apz.max_velocity_inches_per_ms pref,
+ * and the function will smoothly curve the velocity from the threshold to the
+ * max. In general the function parameters chosen should define an ease-out
+ * curve in order to increase the velocity in this range, or an ease-in curve to
+ * decrease the velocity. A straight-line curve is equivalent to disabling the
+ * curve entirely by setting the threshold to -1. The max velocity pref must
+ * also be set in order for the curving to take effect, as it defines the upper
+ * bound of the velocity curve.\n
+ * The points (x1, y1) and (x2, y2) used as the two intermediate control points
+ * in the cubic bezier curve; the first and last points are (0,0) and (1,1).\n
+ * Some example values for these prefs can be found at\n
+ * https://searchfox.org/mozilla-central/rev/f82d5c549f046cb64ce5602bfd894b7ae807c8f8/dom/animation/ComputedTimingFunction.cpp#27-33
+ *
+ * \li\b apz.fling_friction
+ * Amount of friction applied during flings. This is used in the following
+ * formula: v(t1) = v(t0) * (1 - f)^(t1 - t0), where v(t1) is the velocity
+ * for a new sample, v(t0) is the velocity at the previous sample, f is the
+ * value of this pref, and (t1 - t0) is the amount of time, in milliseconds,
+ * that has elapsed between the two samples.\n
+ * NOTE: Not currently used in Android fling calculations.
+ *
+ * \li\b apz.fling_min_velocity_threshold
+ * Minimum velocity for a fling to actually kick off. If the user pans and lifts
+ * their finger such that the velocity is smaller than or equal to this amount,
+ * no fling is initiated.\n
+ * Units: screen pixels per millisecond
+ *
+ * \li\b apz.fling_stop_on_tap_threshold
+ * When flinging, if the velocity is above this number, then a tap on the
+ * screen will stop the fling without dispatching a tap to content. If the
+ * velocity is below this threshold a tap will also be dispatched.
+ * Note: when modifying this pref be sure to run the APZC gtests as some of
+ * them depend on the value of this pref.\n
+ * Units: screen pixels per millisecond
+ *
+ * \li\b apz.fling_stopped_threshold
+ * When flinging, if the velocity goes below this number, we just stop the
+ * animation completely. This is to prevent asymptotically approaching 0
+ * velocity and rerendering unnecessarily.\n
+ * Units: screen pixels per millisecond.\n
+ * NOTE: Should not be set to anything
+ * other than 0.0 for Android except for tests to disable flings.
+ *
+ * \li\b apz.keyboard.enabled
+ * Determines whether scrolling with the keyboard will be allowed to be handled
+ * by APZ.
+ *
+ * \li\b apz.keyboard.passive-listeners
+ * When enabled, APZ will interpret the passive event listener flag to mean
+ * that the event listener won't change the focused element or selection of
+ * the page. With this, web content can use passive key listeners and not have
+ * keyboard APZ disabled.
+ *
+ * \li\b apz.max_tap_time
+ * Maximum time for a touch on the screen and corresponding lift of the finger
+ * to be considered a tap. This also applies to double taps, except that it is
+ * used both for the interval between the first touchdown and first touchup,
+ * and for the interval between the first touchup and the second touchdown.\n
+ * Units: milliseconds.
+ *
+ * \li\b apz.max_velocity_inches_per_ms
+ * Maximum velocity.  Velocity will be capped at this value if a faster fling
+ * occurs.  Negative values indicate unlimited velocity.\n
+ * Units: (real-world, i.e. screen) inches per millisecond
+ *
+ * \li\b apz.max_velocity_queue_size
+ * Maximum size of velocity queue. The queue contains last N velocity records.
+ * On touch end we calculate the average velocity in order to compensate
+ * touch/mouse drivers misbehaviour.
+ *
+ * \li\b apz.min_skate_speed
+ * Minimum amount of speed along an axis before we switch to "skate" multipliers
+ * rather than using the "stationary" multipliers.\n
+ * Units: CSS pixels per millisecond
+ *
+ * \li\b apz.one_touch_pinch.enabled
+ * Whether or not the "one-touch-pinch" gesture (for zooming with one finger)
+ * is enabled or not.
+ *
+ * \li\b apz.overscroll.enabled
+ * Pref that enables overscrolling. If this is disabled, excess scroll that
+ * cannot be handed off is discarded.
+ *
+ * \li\b apz.overscroll.min_pan_distance_ratio
+ * The minimum ratio of the pan distance along one axis to the pan distance
+ * along the other axis needed to initiate overscroll along the first axis
+ * during panning.
+ *
+ * \li\b apz.overscroll.stretch_factor
+ * How much overscrolling can stretch content along an axis.
+ * The maximum stretch along an axis is a factor of (1 + kStretchFactor).
+ * (So if kStretchFactor is 0, you can't stretch at all; if kStretchFactor
+ * is 1, you can stretch at most by a factor of 2).
+ *
+ * \li\b apz.overscroll.stop_distance_threshold
+ * \li\b apz.overscroll.stop_velocity_threshold
+ * Thresholds for stopping the overscroll animation. When both the distance
+ * and the velocity fall below their thresholds, we stop oscillating.\n
+ * Units: screen pixels (for distance)
+ *        screen pixels per millisecond (for velocity)
+ *
+ * \li\b apz.overscroll.spring_stiffness
+ * The spring stiffness constant for the overscroll mass-spring-damper model.
+ *
+ * \li\b apz.overscroll.damping
+ * The damping constant for the overscroll mass-spring-damper model.
+ *
+ * \li\b apz.overscroll.max_velocity
+ * The maximum velocity (in ParentLayerPixels per millisecond) allowed when
+ * initiating the overscroll snap-back animation.
+ *
+ * \li\b apz.paint_skipping.enabled
+ * When APZ is scrolling and sending repaint requests to the main thread, often
+ * the main thread doesn't actually need to do a repaint. This pref allows the
+ * main thread to skip doing those repaints in cases where it doesn't need to.
+ *
+ * \li\b apz.pinch_lock.mode
+ * The preferred pinch locking style. See PinchLockMode for possible values.
+ *
+ * \li\b apz.pinch_lock.scroll_lock_threshold
+ * Pinch locking is triggered if the user scrolls more than this distance
+ * and pinches less than apz.pinch_lock.span_lock_threshold.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.pinch_lock.span_breakout_threshold
+ * Distance in inches the user must pinch before lock can be broken.\n
+ * Units: (real-world, i.e. screen) inches measured between two touch points
+ *
+ * \li\b apz.pinch_lock.span_lock_threshold
+ * Pinch locking is triggered if the user pinches less than this distance
+ * and scrolls more than apz.pinch_lock.scroll_lock_threshold.\n
+ * Units: (real-world, i.e. screen) inches measured between two touch points
+ *
+ * \li\b apz.pinch_lock.buffer_max_age
+ * To ensure that pinch locking threshold calculations are not affected by
+ * variations in touch screen sensitivity, calculations draw from a buffer of
+ * recent events. This preference specifies the maximum time that events are
+ * held in this buffer.
+ * Units: milliseconds
+ *
+ * \li\b apz.popups.enabled
+ * Determines whether APZ is used for XUL popup widgets with remote content.
+ * Ideally, this should always be true, but it is currently not well tested, and
+ * has known issues, so needs to be prefable.
+ *
+ * \li\b apz.record_checkerboarding
+ * Whether or not to record detailed info on checkerboarding events.
+ *
+ * \li\b apz.second_tap_tolerance
+ * Constant describing the tolerance in distance we use, multiplied by the
+ * device DPI, within which a second tap is counted as part of a gesture
+ * continuing from the first tap. Making this larger allows the user more
+ * distance between the first and second taps in a "double tap" or "one touch
+ * pinch" gesture.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.test.logging_enabled
+ * Enable logging of APZ test data (see bug 961289).
+ *
+ * \li\b apz.touch_move_tolerance
+ * See the description for apz.touch_start_tolerance below. This is a similar
+ * threshold, except it is used to suppress touchmove events from being
+ * delivered to content for NON-scrollable frames (or more precisely, for APZCs
+ * where ArePointerEventsConsumable returns false).\n Units: (real-world, i.e.
+ * screen) inches
+ *
+ * \li\b apz.touch_start_tolerance
+ * Constant describing the tolerance in distance we use, multiplied by the
+ * device DPI, before we start panning the screen. This is to prevent us from
+ * accidentally processing taps as touch moves, and from very short/accidental
+ * touches moving the screen. touchmove events are also not delivered to content
+ * within this distance on scrollable frames.\n
+ * Units: (real-world, i.e. screen) inches
+ *
+ * \li\b apz.velocity_bias
+ * How much to adjust the displayport in the direction of scrolling. This value
+ * is multiplied by the velocity and added to the displayport offset.
+ *
+ * \li\b apz.velocity_relevance_time_ms
+ * When computing a fling velocity from the most recently stored velocity
+ * information, only velocities within the most X milliseconds are used.
+ * This pref controls the value of X.\n
+ * Units: ms
+ *
+ * \li\b apz.x_skate_size_multiplier
+ * \li\b apz.y_skate_size_multiplier
+ * The multiplier we apply to the displayport size if it is skating (current
+ * velocity is above \b apz.min_skate_speed). We prefer to increase the size of
+ * the Y axis because it is more natural in the case that a user is reading a
+ * page page that scrolls up/down. Note that one, both or neither of these may
+ * be used at any instant.\n In general we want \b
+ * apz.[xy]_skate_size_multiplier to be smaller than the corresponding
+ * stationary size multiplier because when panning fast we would like to paint
+ * less and get faster, more predictable paint times. When panning slowly we
+ * can afford to paint more even though it's slower.
+ *
+ * \li\b apz.x_stationary_size_multiplier
+ * \li\b apz.y_stationary_size_multiplier
+ * The multiplier we apply to the displayport size if it is not skating (see
+ * documentation for the skate size multipliers above).
+ *
+ * \li\b apz.x_skate_highmem_adjust
+ * \li\b apz.y_skate_highmem_adjust
+ * On high memory systems, we adjust the displayport during skating
+ * to be larger so we can reduce checkerboarding.
+ *
+ * \li\b apz.zoom_animation_duration_ms
+ * This controls how long the zoom-to-rect animation takes.\n
+ * Units: ms
+ *
+ * \li\b apz.scale_repaint_delay_ms
+ * How long to delay between repaint requests during a scale.
+ * A negative number prevents repaint requests during a scale.\n
+ * Units: ms
+ */
+
+/**
+ * Computed time function used for sampling frames of a zoom to animation.
+ */
+StaticAutoPtr<StyleComputedTimingFunction> gZoomAnimationFunction;
+
+/**
+ * Computed time function used for curving up velocity when it gets high.
+ */
+StaticAutoPtr<StyleComputedTimingFunction> gVelocityCurveFunction;
+
+/**
+ * The estimated duration of a paint for the purposes of calculating a new
+ * displayport, in milliseconds.
+ */
+static const double kDefaultEstimatedPaintDurationMs = 50;
+
+/**
+ * Returns true if this is a high memory system and we can use
+ * extra memory for a larger displayport to reduce checkerboarding.
+ */
+static bool gIsHighMemSystem = false;
+static bool IsHighMemSystem() { return gIsHighMemSystem; }
+
+AsyncPanZoomAnimation* PlatformSpecificStateBase::CreateFlingAnimation(
+    AsyncPanZoomController& aApzc, const FlingHandoffState& aHandoffState,
+    float aPLPPI) {
+  return new GenericFlingAnimation<DesktopFlingPhysics>(aApzc, aHandoffState,
+                                                        aPLPPI);
+}
+
+UniquePtr<VelocityTracker> PlatformSpecificStateBase::CreateVelocityTracker(
+    Axis* aAxis) {
+  return MakeUnique<SimpleVelocityTracker>(aAxis);
+}
+
+SampleTime AsyncPanZoomController::GetFrameTime() const {
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  return treeManagerLocal ? treeManagerLocal->GetFrameTime()
+                          : SampleTime::FromNow();
+}
+
+bool AsyncPanZoomController::IsZero(const ParentLayerPoint& aPoint) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return layers::IsZero(aPoint / zoom);
+}
+
+bool AsyncPanZoomController::IsZero(ParentLayerCoord aCoord) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return FuzzyEqualsAdditive((aCoord / zoom), CSSCoord(), COORDINATE_EPSILON);
+}
+
+bool AsyncPanZoomController::FuzzyGreater(ParentLayerCoord aCoord1,
+                                          ParentLayerCoord aCoord2) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const auto zoom = Metrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return false;
+  }
+
+  return (aCoord1 - aCoord2) / zoom > COORDINATE_EPSILON;
+}
+
+class MOZ_STACK_CLASS StateChangeNotificationBlocker final {
+ public:
+  explicit StateChangeNotificationBlocker(AsyncPanZoomController* aApzc)
+      : mApzc(aApzc) {
+    RecursiveMutexAutoLock lock(mApzc->mRecursiveMutex);
+    mInitialState = mApzc->mState;
+    mApzc->mNotificationBlockers++;
+  }
+
+  ~StateChangeNotificationBlocker() {
+    AsyncPanZoomController::PanZoomState newState;
+    {
+      RecursiveMutexAutoLock lock(mApzc->mRecursiveMutex);
+      mApzc->mNotificationBlockers--;
+      newState = mApzc->mState;
+    }
+    mApzc->DispatchStateChangeNotification(mInitialState, newState);
+  }
+
+ private:
+  AsyncPanZoomController* mApzc;
+  AsyncPanZoomController::PanZoomState mInitialState;
+};
+
+/**
+ * An RAII class to temporarily apply async test attributes to the provided
+ * AsyncPanZoomController.
+ *
+ * This class should be used in the implementation of any AsyncPanZoomController
+ * method that queries the async scroll offset or async zoom (this includes
+ * the async layout viewport offset, since modifying the async scroll offset
+ * may result in the layout viewport moving as well).
+ */
+class MOZ_RAII AutoApplyAsyncTestAttributes final {
+ public:
+  explicit AutoApplyAsyncTestAttributes(
+      const AsyncPanZoomController*,
+      const RecursiveMutexAutoLock& aProofOfLock);
+  ~AutoApplyAsyncTestAttributes();
+
+ private:
+  AsyncPanZoomController* mApzc;
+  FrameMetrics mPrevFrameMetrics;
+  ParentLayerPoint mPrevOverscroll;
+  const RecursiveMutexAutoLock& mProofOfLock;
+};
+
+AutoApplyAsyncTestAttributes::AutoApplyAsyncTestAttributes(
+    const AsyncPanZoomController* aApzc,
+    const RecursiveMutexAutoLock& aProofOfLock)
+    // Having to use const_cast here seems less ugly than the alternatives
+    // of making several members of AsyncPanZoomController that
+    // ApplyAsyncTestAttributes() modifies |mutable|, or several methods that
+    // query the async transforms non-const.
+    : mApzc(const_cast<AsyncPanZoomController*>(aApzc)),
+      mPrevFrameMetrics(aApzc->Metrics()),
+      mPrevOverscroll(aApzc->GetOverscrollAmountInternal()),
+      mProofOfLock(aProofOfLock) {
+  mApzc->ApplyAsyncTestAttributes(aProofOfLock);
+}
+
+AutoApplyAsyncTestAttributes::~AutoApplyAsyncTestAttributes() {
+  mApzc->UnapplyAsyncTestAttributes(mProofOfLock, mPrevFrameMetrics,
+                                    mPrevOverscroll);
+}
+
+class ZoomAnimation : public AsyncPanZoomAnimation {
+ public:
+  ZoomAnimation(AsyncPanZoomController& aApzc, const CSSPoint& aStartOffset,
+                const CSSToParentLayerScale& aStartZoom,
+                const CSSPoint& aEndOffset,
+                const CSSToParentLayerScale& aEndZoom)
+      : mApzc(aApzc),
+        mTotalDuration(TimeDuration::FromMilliseconds(
+            StaticPrefs::apz_zoom_animation_duration_ms())),
+        mStartOffset(aStartOffset),
+        mStartZoom(aStartZoom),
+        mEndOffset(aEndOffset),
+        mEndZoom(aEndZoom) {}
+
+  virtual bool DoSample(FrameMetrics& aFrameMetrics,
+                        const TimeDuration& aDelta) override {
+    mDuration += aDelta;
+    double animPosition = mDuration / mTotalDuration;
+
+    if (animPosition >= 1.0) {
+      aFrameMetrics.SetZoom(mEndZoom);
+      mApzc.SetVisualScrollOffset(mEndOffset);
+      return false;
+    }
+
+    // Sample the zoom at the current time point.  The sampled zoom
+    // will affect the final computed resolution.
+    float sampledPosition =
+        gZoomAnimationFunction->At(animPosition, /* aBeforeFlag = */ false);
+
+    // We scale the scrollOffset linearly with sampledPosition, so the zoom
+    // needs to scale inversely to match.
+    if (mStartZoom == CSSToParentLayerScale(0) ||
+        mEndZoom == CSSToParentLayerScale(0)) {
+      return false;
+    }
+
+    aFrameMetrics.SetZoom(
+        CSSToParentLayerScale(1 / (sampledPosition / mEndZoom.scale +
+                                   (1 - sampledPosition) / mStartZoom.scale)));
+
+    mApzc.SetVisualScrollOffset(CSSPoint::FromUnknownPoint(gfx::Point(
+        mEndOffset.x * sampledPosition + mStartOffset.x * (1 - sampledPosition),
+        mEndOffset.y * sampledPosition +
+            mStartOffset.y * (1 - sampledPosition))));
+    return true;
+  }
+
+  virtual bool WantsRepaints() override { return true; }
+
+ private:
+  AsyncPanZoomController& mApzc;
+
+  TimeDuration mDuration;
+  const TimeDuration mTotalDuration;
+
+  // Old metrics from before we started a zoom animation. This is only valid
+  // when we are in the "ANIMATED_ZOOM" state. This is used so that we can
+  // interpolate between the start and end frames. We only use the
+  // |mViewportScrollOffset| and |mResolution| fields on this.
+  CSSPoint mStartOffset;
+  CSSToParentLayerScale mStartZoom;
+
+  // Target metrics for a zoom to animation. This is only valid when we are in
+  // the "ANIMATED_ZOOM" state. We only use the |mViewportScrollOffset| and
+  // |mResolution| fields on this.
+  CSSPoint mEndOffset;
+  CSSToParentLayerScale mEndZoom;
+};
+
+/*static*/
+void AsyncPanZoomController::InitializeGlobalState() {
+  static bool sInitialized = false;
+  if (sInitialized) return;
+  sInitialized = true;
+
+  MOZ_ASSERT(NS_IsMainThread());
+
+  gZoomAnimationFunction = new StyleComputedTimingFunction(
+      StyleComputedTimingFunction::Keyword(StyleTimingKeyword::Ease));
+  ClearOnShutdown(&gZoomAnimationFunction);
+  gVelocityCurveFunction =
+      new StyleComputedTimingFunction(StyleComputedTimingFunction::CubicBezier(
+          StaticPrefs::apz_fling_curve_function_x1_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_y1_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_x2_AtStartup(),
+          StaticPrefs::apz_fling_curve_function_y2_AtStartup()));
+  ClearOnShutdown(&gVelocityCurveFunction);
+
+  uint64_t sysmem = PR_GetPhysicalMemorySize();
+  uint64_t threshold = 1LL << 32;  // 4 GB in bytes
+  gIsHighMemSystem = sysmem >= threshold;
+
+  PlatformSpecificState::InitializeGlobalState();
+}
+
+AsyncPanZoomController::AsyncPanZoomController(
+    LayersId aLayersId, APZCTreeManager* aTreeManager,
+    const RefPtr<InputQueue>& aInputQueue,
+    GeckoContentController* aGeckoContentController, GestureBehavior aGestures)
+    : mLayersId(aLayersId),
+      mGeckoContentController(aGeckoContentController),
+      mRefPtrMonitor("RefPtrMonitor"),
+      // mTreeManager must be initialized before GetFrameTime() is called
+      mTreeManager(aTreeManager),
+      mRecursiveMutex("AsyncPanZoomController"),
+      mLastContentPaintMetrics(mLastContentPaintMetadata.GetMetrics()),
+      mPanDirRestricted(false),
+      mPinchLocked(false),
+      mPinchEventBuffer(TimeDuration::FromMilliseconds(
+          StaticPrefs::apz_pinch_lock_buffer_max_age_AtStartup())),
+      mZoomConstraints(false, false,
+                       mScrollMetadata.GetMetrics().GetDevPixelsPerCSSPixel() *
+                           ViewportMinScale() / ParentLayerToScreenScale(1),
+                       mScrollMetadata.GetMetrics().GetDevPixelsPerCSSPixel() *
+                           ViewportMaxScale() / ParentLayerToScreenScale(1)),
+      mLastSampleTime(GetFrameTime()),
+      mLastCheckerboardReport(GetFrameTime()),
+      mOverscrollEffect(MakeUnique<OverscrollEffect>(*this)),
+      mState(NOTHING),
+      mX(this),
+      mY(this),
+      mNotificationBlockers(0),
+      mInputQueue(aInputQueue),
+      mPinchPaintTimerSet(false),
+      mDelayedTransformEnd(false),
+      mTestAttributeAppliers(0),
+      mTestHasAsyncKeyScrolled(false),
+      mCheckerboardEventLock("APZCBELock") {
+  if (aGestures == USE_GESTURE_DETECTOR) {
+    mGestureEventListener = new GestureEventListener(this);
+  }
+  // Put one default-constructed sampled state in the queue.
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mSampledState.emplace_back();
+}
+
+AsyncPanZoomController::~AsyncPanZoomController() { MOZ_ASSERT(IsDestroyed()); }
+
+PlatformSpecificStateBase* AsyncPanZoomController::GetPlatformSpecificState() {
+  if (!mPlatformSpecificState) {
+    mPlatformSpecificState = MakeUnique<PlatformSpecificState>();
+  }
+  return mPlatformSpecificState.get();
+}
+
+already_AddRefed<GeckoContentController>
+AsyncPanZoomController::GetGeckoContentController() const {
+  MonitorAutoLock lock(mRefPtrMonitor);
+  RefPtr<GeckoContentController> controller = mGeckoContentController;
+  return controller.forget();
+}
+
+already_AddRefed<GestureEventListener>
+AsyncPanZoomController::GetGestureEventListener() const {
+  MonitorAutoLock lock(mRefPtrMonitor);
+  RefPtr<GestureEventListener> listener = mGestureEventListener;
+  return listener.forget();
+}
+
+const RefPtr<InputQueue>& AsyncPanZoomController::GetInputQueue() const {
+  return mInputQueue;
+}
+
+void AsyncPanZoomController::Destroy() {
+  AssertOnUpdaterThread();
+
+  CancelAnimation(CancelAnimationFlags::ScrollSnap);
+
+  {  // scope the lock
+    MonitorAutoLock lock(mRefPtrMonitor);
+    mGeckoContentController = nullptr;
+    mGestureEventListener = nullptr;
+  }
+  mParent = nullptr;
+  mTreeManager = nullptr;
+}
+
+bool AsyncPanZoomController::IsDestroyed() const {
+  return mTreeManager == nullptr;
+}
+
+float AsyncPanZoomController::GetDPI() const {
+  if (APZCTreeManager* localPtr = mTreeManager) {
+    return localPtr->GetDPI();
+  }
+  // If this APZC has been destroyed then this value is not going to be
+  // used for anything that the user will end up seeing, so we can just
+  // return 0.
+  return 0.0;
+}
+
+ScreenCoord AsyncPanZoomController::GetTouchStartTolerance() const {
+  return (StaticPrefs::apz_touch_start_tolerance() * GetDPI());
+}
+
+ScreenCoord AsyncPanZoomController::GetTouchMoveTolerance() const {
+  return (StaticPrefs::apz_touch_move_tolerance() * GetDPI());
+}
+
+ScreenCoord AsyncPanZoomController::GetSecondTapTolerance() const {
+  return (StaticPrefs::apz_second_tap_tolerance() * GetDPI());
+}
+
+/* static */ AsyncPanZoomController::AxisLockMode
+AsyncPanZoomController::GetAxisLockMode() {
+  return static_cast<AxisLockMode>(StaticPrefs::apz_axis_lock_mode());
+}
+
+bool AsyncPanZoomController::UsingStatefulAxisLock() const {
+  return (GetAxisLockMode() == STANDARD || GetAxisLockMode() == STICKY);
+}
+
+/* static */ AsyncPanZoomController::PinchLockMode
+AsyncPanZoomController::GetPinchLockMode() {
+  return static_cast<PinchLockMode>(StaticPrefs::apz_pinch_lock_mode());
+}
+
+PointerEventsConsumableFlags AsyncPanZoomController::ArePointerEventsConsumable(
+    TouchBlockState* aBlock, const MultiTouchInput& aInput) {
+  uint32_t touchPoints = aInput.mTouches.Length();
+  if (touchPoints == 0) {
+    // Cant' do anything with zero touch points
+    return {false, false};
+  }
+
+  // This logic is simplified, erring on the side of returning true if we're
+  // not sure. It's safer to pretend that we can consume the event and then
+  // not be able to than vice-versa. But at the same time, we should try hard
+  // to return an accurate result, because returning true can trigger a
+  // pointercancel event to web content, which can break certain features
+  // that are using touch-action and handling the pointermove events.
+  //
+  // Note that in particular this function can return true if APZ is waiting on
+  // the main thread for touch-action information. In this scenario, the
+  // APZEventState::MainThreadAgreesEventsAreConsumableByAPZ() function tries
+  // to use the main-thread touch-action information to filter out false
+  // positives.
+  //
+  // We could probably enhance this logic to determine things like "we're
+  // not pannable, so we can only zoom in, and the zoom is already maxed
+  // out, so we're not zoomable either" but no need for that at this point.
+
+  bool pannableX = aBlock->GetOverscrollHandoffChain()->CanScrollInDirection(
+      this, ScrollDirection::eHorizontal);
+  bool touchActionAllowsX = aBlock->TouchActionAllowsPanningX();
+  bool pannableY = (aBlock->GetOverscrollHandoffChain()->CanScrollInDirection(
+                        this, ScrollDirection::eVertical) ||
+                    // In the case of the root APZC with any dynamic toolbar, it
+                    // shoule be pannable if there is room moving the dynamic
+                    // toolbar.
+                    (IsRootContent() && CanVerticalScrollWithDynamicToolbar()));
+  bool touchActionAllowsY = aBlock->TouchActionAllowsPanningY();
+
+  bool pannable;
+  bool touchActionAllowsPanning;
+
+  Maybe<ScrollDirection> panDirection =
+      aBlock->GetBestGuessPanDirection(aInput);
+  if (panDirection == Some(ScrollDirection::eVertical)) {
+    pannable = pannableY;
+    touchActionAllowsPanning = touchActionAllowsY;
+  } else if (panDirection == Some(ScrollDirection::eHorizontal)) {
+    pannable = pannableX;
+    touchActionAllowsPanning = touchActionAllowsX;
+  } else {
+    // If we don't have a guessed pan direction, err on the side of returning
+    // true.
+    pannable = pannableX || pannableY;
+    touchActionAllowsPanning = touchActionAllowsX || touchActionAllowsY;
+  }
+
+  if (touchPoints == 1) {
+    return {pannable, touchActionAllowsPanning};
+  }
+
+  bool zoomable = ZoomConstraintsAllowZoom();
+  bool touchActionAllowsZoom = aBlock->TouchActionAllowsPinchZoom();
+
+  return {pannable || zoomable,
+          touchActionAllowsPanning || touchActionAllowsZoom};
+}
+
+nsEventStatus AsyncPanZoomController::HandleDragEvent(
+    const MouseInput& aEvent, const AsyncDragMetrics& aDragMetrics,
+    OuterCSSCoord aInitialThumbPos, const CSSRect& aInitialScrollableRect) {
+  // RDM is a special case where touch events will be synthesized in response
+  // to mouse events, and APZ will receive both even though RDM prevent-defaults
+  // the mouse events. This is because mouse events don't opt into APZ waiting
+  // to check if the event has been prevent-defaulted and are still processed
+  // as a result. To handle this, have APZ ignore mouse events when RDM and
+  // touch simulation are active.
+  bool isRDMTouchSimulationActive = false;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    isRDMTouchSimulationActive =
+        mScrollMetadata.GetIsRDMTouchSimulationActive();
+  }
+
+  if (!StaticPrefs::apz_drag_enabled() || isRDMTouchSimulationActive) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (!GetApzcTreeManager()) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    if (aEvent.mType == MouseInput::MouseType::MOUSE_UP) {
+      if (mState == SCROLLBAR_DRAG) {
+        APZC_LOG("%p ending drag\n", this);
+        SetState(NOTHING);
+      }
+
+      SnapBackIfOverscrolled();
+
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+
+  HitTestingTreeNodeAutoLock node;
+  GetApzcTreeManager()->FindScrollThumbNode(aDragMetrics, mLayersId, node);
+  if (!node) {
+    APZC_LOG("%p unable to find scrollthumb node with viewid %" PRIu64 "\n",
+             this, aDragMetrics.mViewId);
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  if (aEvent.mType == MouseInput::MouseType::MOUSE_DOWN) {
+    APZC_LOG("%p starting scrollbar drag\n", this);
+    SetState(SCROLLBAR_DRAG);
+  }
+
+  if (aEvent.mType != MouseInput::MouseType::MOUSE_MOVE) {
+    APZC_LOG("%p discarding event of type %d\n", this, aEvent.mType);
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  const ScrollbarData& scrollbarData = node->GetScrollbarData();
+  MOZ_ASSERT(scrollbarData.mScrollbarLayerType ==
+             layers::ScrollbarLayerType::Thumb);
+  MOZ_ASSERT(scrollbarData.mDirection.isSome());
+  ScrollDirection direction = *scrollbarData.mDirection;
+
+  bool isMouseAwayFromThumb = false;
+  if (int snapMultiplier = StaticPrefs::slider_snapMultiplier()) {
+    // It's fine to ignore the async component of the thumb's transform,
+    // because any async transform of the thumb will be in the direction of
+    // scrolling, but here we're interested in the other direction.
+    ParentLayerRect thumbRect =
+        (node->GetTransform() * AsyncTransformMatrix())
+            .TransformBounds(LayerRect(node->GetVisibleRect()));
+    ScrollDirection otherDirection = GetPerpendicularDirection(direction);
+    ParentLayerCoord distance =
+        GetAxisStart(otherDirection, thumbRect.DistanceTo(aEvent.mLocalOrigin));
+    ParentLayerCoord thumbWidth = GetAxisLength(otherDirection, thumbRect);
+    // Avoid triggering this condition spuriously when the thumb is
+    // offscreen and its visible region is therefore empty.
+    if (thumbWidth > 0 && thumbWidth * snapMultiplier < distance) {
+      isMouseAwayFromThumb = true;
+      APZC_LOG("%p determined mouse is away from thumb, will snap\n", this);
+    }
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  OuterCSSCoord thumbPosition;
+  if (isMouseAwayFromThumb) {
+    thumbPosition = aInitialThumbPos;
+  } else {
+    thumbPosition = ConvertScrollbarPoint(aEvent.mLocalOrigin, scrollbarData) -
+                    aDragMetrics.mScrollbarDragOffset;
+  }
+
+  OuterCSSCoord maxThumbPos = scrollbarData.mScrollTrackLength;
+  maxThumbPos -= scrollbarData.mThumbLength;
+
+  float scrollPercent =
+      maxThumbPos.value == 0.0f ? 0.0f : (float)(thumbPosition / maxThumbPos);
+  APZC_LOG("%p scrollbar dragged to %f percent\n", this, scrollPercent);
+
+  CSSCoord minScrollPosition =
+      GetAxisStart(direction, aInitialScrollableRect.TopLeft());
+  CSSCoord maxScrollPosition =
+      GetAxisStart(direction, aInitialScrollableRect.BottomRight()) -
+      GetAxisLength(direction, Metrics().CalculateCompositedSizeInCssPixels());
+  CSSCoord scrollPosition =
+      minScrollPosition +
+      (scrollPercent * (maxScrollPosition - minScrollPosition));
+
+  scrollPosition = std::max(scrollPosition, minScrollPosition);
+  scrollPosition = std::min(scrollPosition, maxScrollPosition);
+
+  CSSPoint scrollOffset = Metrics().GetVisualScrollOffset();
+  if (direction == ScrollDirection::eHorizontal) {
+    scrollOffset.x = scrollPosition;
+  } else {
+    scrollOffset.y = scrollPosition;
+  }
+  APZC_LOG("%p set scroll offset to %s from scrollbar drag\n", this,
+           ToString(scrollOffset).c_str());
+  // Since the scroll position was calculated based on the scrollable rect at
+  // the start of the drag, we need to clamp the scroll position in case the
+  // scrollable rect has since shrunk.
+  ClampAndSetVisualScrollOffset(scrollOffset);
+  ScheduleCompositeAndMaybeRepaint();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::HandleInputEvent(
+    const InputData& aEvent,
+    const ScreenToParentLayerMatrix4x4& aTransformToApzc) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (aEvent.mInputType) {
+    case MULTITOUCH_INPUT: {
+      MultiTouchInput multiTouchInput = aEvent.AsMultiTouchInput();
+      RefPtr<GestureEventListener> listener = GetGestureEventListener();
+      if (listener) {
+        // We only care about screen coordinates in the gesture listener,
+        // so we don't bother transforming the event to parent layer coordinates
+        rv = listener->HandleInputEvent(multiTouchInput);
+        if (rv == nsEventStatus_eConsumeNoDefault) {
+          return rv;
+        }
+      }
+
+      if (!multiTouchInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      switch (multiTouchInput.mType) {
+        case MultiTouchInput::MULTITOUCH_START:
+          rv = OnTouchStart(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_MOVE:
+          rv = OnTouchMove(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_END:
+          rv = OnTouchEnd(multiTouchInput);
+          break;
+        case MultiTouchInput::MULTITOUCH_CANCEL:
+          rv = OnTouchCancel(multiTouchInput);
+          break;
+      }
+      break;
+    }
+    case PANGESTURE_INPUT: {
+      PanGestureInput panGestureInput = aEvent.AsPanGestureInput();
+      if (!panGestureInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      switch (panGestureInput.mType) {
+        case PanGestureInput::PANGESTURE_MAYSTART:
+          rv = OnPanMayBegin(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_CANCELLED:
+          rv = OnPanCancelled(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_START:
+          rv = OnPanBegin(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_PAN:
+          rv = OnPan(panGestureInput, FingersOnTouchpad::Yes);
+          break;
+        case PanGestureInput::PANGESTURE_END:
+          rv = OnPanEnd(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMSTART:
+          rv = OnPanMomentumStart(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMPAN:
+          rv = OnPan(panGestureInput, FingersOnTouchpad::No);
+          break;
+        case PanGestureInput::PANGESTURE_MOMENTUMEND:
+          rv = OnPanMomentumEnd(panGestureInput);
+          break;
+        case PanGestureInput::PANGESTURE_INTERRUPTED:
+          rv = OnPanInterrupted(panGestureInput);
+          break;
+      }
+      break;
+    }
+    case MOUSE_INPUT: {
+      MouseInput mouseInput = aEvent.AsMouseInput();
+      if (!mouseInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+      break;
+    }
+    case SCROLLWHEEL_INPUT: {
+      ScrollWheelInput scrollInput = aEvent.AsScrollWheelInput();
+      if (!scrollInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = OnScrollWheel(scrollInput);
+      break;
+    }
+    case PINCHGESTURE_INPUT: {
+      // The APZCTreeManager should take care of ensuring that only root-content
+      // APZCs get pinch inputs.
+      MOZ_ASSERT(IsRootContent());
+      PinchGestureInput pinchInput = aEvent.AsPinchGestureInput();
+      if (!pinchInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = HandleGestureEvent(pinchInput);
+      break;
+    }
+    case TAPGESTURE_INPUT: {
+      TapGestureInput tapInput = aEvent.AsTapGestureInput();
+      if (!tapInput.TransformToLocal(aTransformToApzc)) {
+        return rv;
+      }
+
+      rv = HandleGestureEvent(tapInput);
+      break;
+    }
+    case KEYBOARD_INPUT: {
+      const KeyboardInput& keyInput = aEvent.AsKeyboardInput();
+      rv = OnKeyboard(keyInput);
+      break;
+    }
+  }
+
+  return rv;
+}
+
+nsEventStatus AsyncPanZoomController::HandleGestureEvent(
+    const InputData& aEvent) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (aEvent.mInputType) {
+    case PINCHGESTURE_INPUT: {
+      // This may be invoked via a one-touch-pinch gesture from
+      // GestureEventListener. In that case we want redirect it to the enclosing
+      // root-content APZC.
+      if (!IsRootContent()) {
+        if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+          if (RefPtr<AsyncPanZoomController> root =
+                  treeManagerLocal->FindZoomableApzc(this)) {
+            rv = root->HandleGestureEvent(aEvent);
+          }
+        }
+        break;
+      }
+      PinchGestureInput pinchGestureInput = aEvent.AsPinchGestureInput();
+      pinchGestureInput.TransformToLocal(GetTransformToThis());
+      switch (pinchGestureInput.mType) {
+        case PinchGestureInput::PINCHGESTURE_START:
+          rv = OnScaleBegin(pinchGestureInput);
+          break;
+        case PinchGestureInput::PINCHGESTURE_SCALE:
+          rv = OnScale(pinchGestureInput);
+          break;
+        case PinchGestureInput::PINCHGESTURE_FINGERLIFTED:
+        case PinchGestureInput::PINCHGESTURE_END:
+          rv = OnScaleEnd(pinchGestureInput);
+          break;
+      }
+      break;
+    }
+    case TAPGESTURE_INPUT: {
+      TapGestureInput tapGestureInput = aEvent.AsTapGestureInput();
+      tapGestureInput.TransformToLocal(GetTransformToThis());
+      switch (tapGestureInput.mType) {
+        case TapGestureInput::TAPGESTURE_LONG:
+          rv = OnLongPress(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_LONG_UP:
+          rv = OnLongPressUp(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_UP:
+          rv = OnSingleTapUp(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_CONFIRMED:
+          rv = OnSingleTapConfirmed(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_DOUBLE:
+          if (!IsRootContent()) {
+            if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+              if (AsyncPanZoomController* apzc =
+                      treeManagerLocal->FindRootApzcFor(GetLayersId())) {
+                rv = apzc->OnDoubleTap(tapGestureInput);
+              }
+            }
+            break;
+          }
+          rv = OnDoubleTap(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_SECOND:
+          rv = OnSecondTap(tapGestureInput);
+          break;
+        case TapGestureInput::TAPGESTURE_CANCEL:
+          rv = OnCancelTap(tapGestureInput);
+          break;
+      }
+      break;
+    }
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unhandled input event");
+      break;
+  }
+
+  return rv;
+}
+
+void AsyncPanZoomController::StartAutoscroll(const ScreenPoint& aPoint) {
+  // Cancel any existing animation.
+  CancelAnimation();
+
+  SetState(AUTOSCROLL);
+  StartAnimation(do_AddRef(new AutoscrollAnimation(*this, aPoint)));
+}
+
+void AsyncPanZoomController::StopAutoscroll() {
+  if (mState == AUTOSCROLL) {
+    CancelAnimation(TriggeredExternally);
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchStart(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-start in state %s\n", this,
+                  ToString(mState).c_str());
+  mPanDirRestricted = false;
+
+  switch (mState) {
+    case FLING:
+    case ANIMATING_ZOOM:
+    case SMOOTH_SCROLL:
+    case SMOOTHMSD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case PAN_MOMENTUM:
+    case AUTOSCROLL:
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      GetCurrentTouchBlock()->GetOverscrollHandoffChain()->CancelAnimations(
+          ExcludeOverscroll);
+      [[fallthrough]];
+    case SCROLLBAR_DRAG:
+    case NOTHING: {
+      ParentLayerPoint point = GetFirstTouchPoint(aEvent);
+      mLastTouch.mPosition = mStartTouch = GetFirstExternalTouchPoint(aEvent);
+      StartTouch(point, aEvent.mTimeStamp);
+      if (RefPtr<GeckoContentController> controller =
+              GetGeckoContentController()) {
+        MOZ_ASSERT(GetCurrentTouchBlock());
+        controller->NotifyAPZStateChange(
+            GetGuid(), APZStateChange::eStartTouch,
+            GetCurrentTouchBlock()->GetOverscrollHandoffChain()->CanBePanned(
+                this),
+            Some(GetCurrentTouchBlock()->GetBlockId()));
+      }
+      mLastTouch.mTimeStamp = mTouchStartTime = aEvent.mTimeStamp;
+      SetState(TOUCHING);
+      break;
+    }
+    case TOUCHING:
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PINCHING:
+      NS_WARNING("Received impossible touch in OnTouchStart");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchMove(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-move in state %s\n", this,
+                  ToString(mState).c_str());
+  switch (mState) {
+    case FLING:
+    case SMOOTHMSD_SCROLL:
+    case NOTHING:
+    case ANIMATING_ZOOM:
+      // May happen if the user double-taps and drags without lifting after the
+      // second tap. Ignore the move if this happens.
+      return nsEventStatus_eIgnore;
+
+    case TOUCHING: {
+      ScreenCoord panThreshold = GetTouchStartTolerance();
+      ExternalPoint extPoint = GetFirstExternalTouchPoint(aEvent);
+      Maybe<std::pair<MultiTouchInput, MultiTouchInput>> splitEvent;
+
+      // We intentionally skip the UpdateWithTouchAtDevicePoint call when the
+      // panThreshold is zero. This ensures more deterministic behaviour during
+      // testing. If we call that, Axis::mPos gets updated to the point of this
+      // touchmove event, but we "consume" the move to overcome the
+      // panThreshold, so it's hard to pan a specific amount reliably from a
+      // mochitest.
+      if (panThreshold > 0.0f) {
+        const float vectorLength = PanVector(extPoint).Length();
+
+        if (vectorLength < panThreshold) {
+          UpdateWithTouchAtDevicePoint(aEvent);
+          mLastTouch = {extPoint, aEvent.mTimeStamp};
+
+          return nsEventStatus_eIgnore;
+        }
+
+        splitEvent = MaybeSplitTouchMoveEvent(aEvent, panThreshold,
+                                              vectorLength, extPoint);
+
+        UpdateWithTouchAtDevicePoint(splitEvent ? splitEvent->first : aEvent);
+      }
+
+      nsEventStatus result;
+      const MultiTouchInput& firstEvent =
+          splitEvent ? splitEvent->first : aEvent;
+
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      if (GetCurrentTouchBlock()->TouchActionAllowsPanningXY()) {
+        // In the calls to StartPanning() below, the first argument needs to be
+        // the External position of |firstEvent|.
+        // However, instead of computing that using
+        // GetFirstExternalTouchPoint(firstEvent), we pass |extPoint| which
+        // has been modified by MaybeSplitTouchMoveEvent() to the desired
+        // value. This is a workaround for the fact that recomputing the
+        // External point would require a round-trip through |mScreenPoint|
+        // which is an integer.
+
+        // User tries to trigger a touch behavior. If allowed touch behavior is
+        // vertical pan + horizontal pan (touch-action value is equal to AUTO)
+        // we can return ConsumeNoDefault status immediately to trigger cancel
+        // event further.
+        // It should happen independent of the parent type (whether it is
+        // scrolling or not).
+        StartPanning(extPoint, firstEvent.mTimeStamp);
+        result = nsEventStatus_eConsumeNoDefault;
+      } else {
+        result = StartPanning(extPoint, firstEvent.mTimeStamp);
+      }
+
+      if (splitEvent && IsInPanningState()) {
+        TrackTouch(splitEvent->second);
+        return nsEventStatus_eConsumeNoDefault;
+      }
+
+      return result;
+    }
+
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PAN_MOMENTUM:
+      TrackTouch(aEvent);
+      return nsEventStatus_eConsumeNoDefault;
+
+    case PINCHING:
+      // The scale gesture listener should have handled this.
+      NS_WARNING(
+          "Gesture listener should have handled pinching in OnTouchMove.");
+      return nsEventStatus_eIgnore;
+
+    case SMOOTH_SCROLL:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case AUTOSCROLL:
+    case SCROLLBAR_DRAG:
+      // Should not receive a touch-move in the OVERSCROLL_ANIMATION state
+      // as touch blocks that begin in an overscrolled state cancel the
+      // animation. The same is true for wheel scroll animations.
+      NS_WARNING("Received impossible touch in OnTouchMove");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchEnd(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-end in state %s\n", this,
+                  ToString(mState).c_str());
+  OnTouchEndOrCancel();
+
+  // In case no touch behavior triggered previously we can avoid sending
+  // scroll events or requesting content repaint. This condition is added
+  // to make tests consistent - in case touch-action is NONE (and therefore
+  // no pans/zooms can be performed) we expected neither scroll or repaint
+  // events.
+  if (mState != NOTHING) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+  }
+
+  switch (mState) {
+    case FLING:
+      // Should never happen.
+      NS_WARNING("Received impossible touch end in OnTouchEnd.");
+      [[fallthrough]];
+    case ANIMATING_ZOOM:
+    case SMOOTHMSD_SCROLL:
+    case NOTHING:
+      // May happen if the user double-taps and drags without lifting after the
+      // second tap. Ignore if this happens.
+      return nsEventStatus_eIgnore;
+
+    case TOUCHING:
+      // We may have some velocity stored on the axis from move events
+      // that were not big enough to trigger scrolling. Clear that out.
+      SetVelocityVector(ParentLayerPoint(0, 0));
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      APZC_LOG("%p still has %u touch points active\n", this,
+               GetCurrentTouchBlock()->GetActiveTouchCount());
+      // In cases where the user is panning, then taps the second finger without
+      // entering a pinch, we will arrive here when the second finger is lifted.
+      // However the first finger is still down so we want to remain in state
+      // TOUCHING.
+      if (GetCurrentTouchBlock()->GetActiveTouchCount() == 0) {
+        // It's possible we may be overscrolled if the user tapped during a
+        // previous overscroll pan. Make sure to snap back in this situation.
+        // An ancestor APZC could be overscrolled instead of this APZC, so
+        // walk the handoff chain as well.
+        GetCurrentTouchBlock()
+            ->GetOverscrollHandoffChain()
+            ->SnapBackOverscrolledApzc(this);
+        mFlingAccelerator.Reset();
+        // SnapBackOverscrolledApzc() will put any APZC it causes to snap back
+        // into the OVERSCROLL_ANIMATION state. If that's not us, since we're
+        // done TOUCHING enter the NOTHING state.
+        if (mState != OVERSCROLL_ANIMATION) {
+          SetState(NOTHING);
+        }
+      }
+      return nsEventStatus_eIgnore;
+
+    case PANNING:
+    case PANNING_LOCKED_X:
+    case PANNING_LOCKED_Y:
+    case PAN_MOMENTUM: {
+      MOZ_ASSERT(GetCurrentTouchBlock());
+      EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::Yes);
+      return HandleEndOfPan();
+    }
+    case PINCHING:
+      SetState(NOTHING);
+      // Scale gesture listener should have handled this.
+      NS_WARNING(
+          "Gesture listener should have handled pinching in OnTouchEnd.");
+      return nsEventStatus_eIgnore;
+
+    case SMOOTH_SCROLL:
+    case WHEEL_SCROLL:
+    case KEYBOARD_SCROLL:
+    case OVERSCROLL_ANIMATION:
+    case AUTOSCROLL:
+    case SCROLLBAR_DRAG:
+      // Should not receive a touch-end in the OVERSCROLL_ANIMATION state
+      // as touch blocks that begin in an overscrolled state cancel the
+      // animation. The same is true for WHEEL_SCROLL.
+      NS_WARNING("Received impossible touch in OnTouchEnd");
+      break;
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnTouchCancel(
+    const MultiTouchInput& aEvent) {
+  APZC_LOG_DETAIL("got a touch-cancel in state %s\n", this,
+                  ToString(mState).c_str());
+  OnTouchEndOrCancel();
+  CancelAnimationAndGestureState();
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScaleBegin(
+    const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale-begin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mPinchLocked = false;
+  mPinchPaintTimerSet = false;
+  // Note that there may not be a touch block at this point, if we received the
+  // PinchGestureEvent directly from widget code without any touch events.
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  // For platforms that don't support APZ zooming, dispatch a message to the
+  // content controller, it may want to do something else with this gesture.
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      APZC_LOG("%p notifying controller of pinch gesture start\n", this);
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          0, aEvent.modifiers);
+    }
+  }
+
+  SetState(PINCHING);
+  Telemetry::Accumulate(Telemetry::APZ_ZOOM_PINCHSOURCE, (int)aEvent.mSource);
+  SetVelocityVector(ParentLayerPoint(0, 0));
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mLastZoomFocus =
+      aEvent.mLocalFocusPoint - Metrics().GetCompositionBounds().TopLeft();
+
+  mPinchEventBuffer.push(aEvent);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScale(const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale in state %s\n", this, ToString(mState).c_str());
+
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (mState != PINCHING) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  mPinchEventBuffer.push(aEvent);
+  HandlePinchLocking(aEvent);
+  bool allowZoom = ZoomConstraintsAllowZoom() && !mPinchLocked;
+
+  // If we are pinch-locked, this is a two-finger pan.
+  // Tracking panning distance and velocity.
+  // UpdateWithTouchAtDevicePoint() acquires the tree lock, so
+  // it cannot be called while the mRecursiveMutex lock is held.
+  if (mPinchLocked) {
+    mX.UpdateWithTouchAtDevicePoint(aEvent.mLocalFocusPoint.x,
+                                    aEvent.mTimeStamp);
+    mY.UpdateWithTouchAtDevicePoint(aEvent.mLocalFocusPoint.y,
+                                    aEvent.mTimeStamp);
+  }
+
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      APZC_LOG("%p notifying controller of pinch gesture\n", this);
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mCurrentSpan - aEvent.mPreviousSpan,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          aEvent.modifiers);
+    }
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    // Only the root APZC is zoomable, and the root APZC is not allowed to have
+    // different x and y scales. If it did, the calculations in this function
+    // would have to be adjusted (as e.g. it would no longer be valid to take
+    // the minimum or maximum of the ratios of the widths and heights of the
+    // page rect and the composition bounds).
+    MOZ_ASSERT(Metrics().IsRootContent());
+
+    CSSToParentLayerScale userZoom = Metrics().GetZoom();
+    ParentLayerPoint focusPoint =
+        aEvent.mLocalFocusPoint - Metrics().GetCompositionBounds().TopLeft();
+    CSSPoint cssFocusPoint;
+    if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+      cssFocusPoint = focusPoint / Metrics().GetZoom();
+    }
+
+    ParentLayerPoint focusChange = mLastZoomFocus - focusPoint;
+    mLastZoomFocus = focusPoint;
+    // If displacing by the change in focus point will take us off page bounds,
+    // then reduce the displacement such that it doesn't.
+    focusChange.x -= mX.DisplacementWillOverscrollAmount(focusChange.x);
+    focusChange.y -= mY.DisplacementWillOverscrollAmount(focusChange.y);
+    if (userZoom != CSSToParentLayerScale(0)) {
+      ScrollBy(focusChange / userZoom);
+    }
+
+    // If the span is zero or close to it, we don't want to process this zoom
+    // change because we're going to get wonky numbers for the spanRatio. So
+    // let's bail out here. Note that we do this after the focus-change-scroll
+    // above, so that if we have a pinch with zero span but changing focus,
+    // such as generated by some Synaptics touchpads on Windows, we still
+    // scroll properly.
+    float prevSpan = aEvent.mPreviousSpan;
+    if (fabsf(prevSpan) <= EPSILON || fabsf(aEvent.mCurrentSpan) <= EPSILON) {
+      // We might have done a nonzero ScrollBy above, so update metrics and
+      // repaint/recomposite
+      ScheduleCompositeAndMaybeRepaint();
+      return nsEventStatus_eConsumeNoDefault;
+    }
+    float spanRatio = aEvent.mCurrentSpan / aEvent.mPreviousSpan;
+
+    // When we zoom in with focus, we can zoom too much towards the boundaries
+    // that we actually go over them. These are the needed displacements along
+    // either axis such that we don't overscroll the boundaries when zooming.
+    CSSPoint neededDisplacement;
+
+    CSSToParentLayerScale realMinZoom = mZoomConstraints.mMinZoom;
+    CSSToParentLayerScale realMaxZoom = mZoomConstraints.mMaxZoom;
+    realMinZoom.scale =
+        std::max(realMinZoom.scale, Metrics().GetCompositionBounds().Width() /
+                                        Metrics().GetScrollableRect().Width());
+    realMinZoom.scale =
+        std::max(realMinZoom.scale, Metrics().GetCompositionBounds().Height() /
+                                        Metrics().GetScrollableRect().Height());
+    if (realMaxZoom < realMinZoom) {
+      realMaxZoom = realMinZoom;
+    }
+
+    bool doScale = allowZoom && ((spanRatio > 1.0 && userZoom < realMaxZoom) ||
+                                 (spanRatio < 1.0 && userZoom > realMinZoom));
+
+    if (doScale) {
+      spanRatio = clamped(spanRatio, realMinZoom.scale / userZoom.scale,
+                          realMaxZoom.scale / userZoom.scale);
+
+      // Note that the spanRatio here should never put us into OVERSCROLL_BOTH
+      // because up above we clamped it.
+      neededDisplacement.x =
+          -mX.ScaleWillOverscrollAmount(spanRatio, cssFocusPoint.x);
+      neededDisplacement.y =
+          -mY.ScaleWillOverscrollAmount(spanRatio, cssFocusPoint.y);
+
+      ScaleWithFocus(spanRatio, cssFocusPoint);
+
+      if (neededDisplacement != CSSPoint()) {
+        ScrollBy(neededDisplacement);
+      }
+
+      // We don't want to redraw on every scale, so throttle it.
+      if (!mPinchPaintTimerSet) {
+        const int delay = StaticPrefs::apz_scale_repaint_delay_ms();
+        if (delay >= 0) {
+          if (RefPtr<GeckoContentController> controller =
+                  GetGeckoContentController()) {
+            mPinchPaintTimerSet = true;
+            controller->PostDelayedTask(
+                NewRunnableMethod(
+                    "layers::AsyncPanZoomController::"
+                    "DoDelayedRequestContentRepaint",
+                    this,
+                    &AsyncPanZoomController::DoDelayedRequestContentRepaint),
+                delay);
+          }
+        }
+      } else if (apz::AboutToCheckerboard(mLastContentPaintMetrics,
+                                          Metrics())) {
+        // If we already scheduled a throttled repaint request but are also
+        // in danger of checkerboarding soon, trigger the repaint request to
+        // go out immediately. This should reduce the amount of time we spend
+        // checkerboarding.
+        //
+        // Note that if we remain in this "about to
+        // checkerboard" state over a period of time with multiple pinch input
+        // events (which is quite likely), then we will flip-flop between taking
+        // the above branch (!mPinchPaintTimerSet) and this branch (which will
+        // flush the repaint request and reset mPinchPaintTimerSet to false).
+        // This is sort of desirable because it halves the number of repaint
+        // requests we send, and therefore reduces IPC traffic.
+        // Keep in mind that many of these repaint requests will be ignored on
+        // the main-thread anyway due to the resolution mismatch - the first
+        // repaint request will be honored because APZ's notion of the painted
+        // resolution matches the actual main thread resolution, but that first
+        // repaint request will change the resolution on the main thread.
+        // Subsequent repaint requests will be ignored in APZCCallbackHelper, at
+        // https://searchfox.org/mozilla-central/rev/e0eb861a187f0bb6d994228f2e0e49b2c9ee455e/gfx/layers/apz/util/APZCCallbackHelper.cpp#331-338,
+        // until we receive a NotifyLayersUpdated call that re-syncs APZ's
+        // notion of the painted resolution to the main thread. These ignored
+        // repaint requests are contributing to IPC traffic needlessly, and so
+        // halving the number of repaint requests (as mentioned above) seems
+        // desirable.
+        DoDelayedRequestContentRepaint();
+      }
+    } else {
+      // Trigger a repaint request after scrolling.
+      RequestContentRepaint();
+    }
+
+    // We did a ScrollBy call above even if we didn't do a scale, so we
+    // should composite for that.
+    ScheduleComposite();
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnScaleEnd(
+    const PinchGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a scale-end in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mPinchPaintTimerSet = false;
+
+  if (HasReadyTouchBlock() &&
+      !GetCurrentTouchBlock()->TouchActionAllowsPinchZoom()) {
+    return nsEventStatus_eIgnore;
+  }
+
+  // FIXME: bug 1525793 -- this may need to handle zooming or not on a
+  // per-document basis.
+  if (!StaticPrefs::apz_allow_zooming()) {
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      controller->NotifyPinchGesture(
+          aEvent.mType, GetGuid(),
+          ViewAs<LayoutDevicePixel>(
+              aEvent.mFocusPoint,
+              PixelCastJustification::
+                  LayoutDeviceIsScreenForUntransformedEvent),
+          0, aEvent.modifiers);
+    }
+  }
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    ScheduleComposite();
+    RequestContentRepaint();
+  }
+
+  mPinchEventBuffer.clear();
+
+  if (aEvent.mType == PinchGestureInput::PINCHGESTURE_FINGERLIFTED) {
+    // One finger is still down, so transition to a TOUCHING state
+    if (!mPinchLocked) {
+      mPanDirRestricted = false;
+      mLastTouch.mPosition = mStartTouch =
+          ToExternalPoint(aEvent.mScreenOffset, aEvent.mFocusPoint);
+      mLastTouch.mTimeStamp = mTouchStartTime = aEvent.mTimeStamp;
+      StartTouch(aEvent.mLocalFocusPoint, aEvent.mTimeStamp);
+      SetState(TOUCHING);
+    } else {
+      // If we are pinch locked, StartTouch() was already called
+      // when we entered the pinch lock.
+      StartPanning(ToExternalPoint(aEvent.mScreenOffset, aEvent.mFocusPoint),
+                   aEvent.mTimeStamp);
+    }
+  } else {
+    // Otherwise, handle the gesture being completely done.
+
+    // Some of the code paths below, like ScrollSnap() or HandleEndOfPan(),
+    // may start an animation, but otherwise we want to end up in the NOTHING
+    // state. To avoid state change notification churn, we use a
+    // notification blocker.
+    bool stateWasPinching = (mState == PINCHING);
+    StateChangeNotificationBlocker blocker(this);
+    SetState(NOTHING);
+
+    if (ZoomConstraintsAllowZoom()) {
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      // We can get into a situation where we are overscrolled at the end of a
+      // pinch if we go into overscroll with a two-finger pan, and then turn
+      // that into a pinch by increasing the span sufficiently. In such a case,
+      // there is no snap-back animation to get us out of overscroll, so we need
+      // to get out of it somehow.
+      // Moreover, in cases of scroll handoff, the overscroll can be on an APZC
+      // further up in the handoff chain rather than on the current APZC, so
+      // we need to clear overscroll along the entire handoff chain.
+      if (HasReadyTouchBlock()) {
+        GetCurrentTouchBlock()->GetOverscrollHandoffChain()->ClearOverscroll();
+      } else {
+        ClearOverscroll();
+      }
+      // Along with clearing the overscroll, we also want to snap to the nearest
+      // snap point as appropriate.
+      ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+    } else {
+      // when zoom is not allowed
+      EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::Yes);
+      if (stateWasPinching) {
+        // still pinching
+        if (HasReadyTouchBlock()) {
+          return HandleEndOfPan();
+        }
+      }
+    }
+  }
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::HandleEndOfPan() {
+  MOZ_ASSERT(!mAnimation);
+  MOZ_ASSERT(GetCurrentTouchBlock() || GetCurrentPanGestureBlock());
+  GetCurrentInputBlock()->GetOverscrollHandoffChain()->FlushRepaints();
+  ParentLayerPoint flingVelocity = GetVelocityVector();
+
+  // Clear our velocities; if DispatchFling() gives the fling to us,
+  // the fling velocity gets *added* to our existing velocity in
+  // AcceptFling().
+  SetVelocityVector(ParentLayerPoint(0, 0));
+  // Clear our state so that we don't stay in the PANNING state
+  // if DispatchFling() gives the fling to somone else. However,
+  // don't send the state change notification until we've determined
+  // what our final state is to avoid notification churn.
+  StateChangeNotificationBlocker blocker(this);
+  SetState(NOTHING);
+
+  APZC_LOG("%p starting a fling animation if %f > %f\n", this,
+           flingVelocity.Length().value,
+           StaticPrefs::apz_fling_min_velocity_threshold());
+
+  if (flingVelocity.Length() <=
+      StaticPrefs::apz_fling_min_velocity_threshold()) {
+    // Relieve overscroll now if needed, since we will not transition to a fling
+    // animation and then an overscroll animation, and relieve it then.
+    GetCurrentInputBlock()
+        ->GetOverscrollHandoffChain()
+        ->SnapBackOverscrolledApzc(this);
+    mFlingAccelerator.Reset();
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  // Make a local copy of the tree manager pointer and check that it's not
+  // null before calling DispatchFling(). This is necessary because Destroy(),
+  // which nulls out mTreeManager, could be called concurrently.
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    const FlingHandoffState handoffState{
+        flingVelocity,
+        GetCurrentInputBlock()->GetOverscrollHandoffChain(),
+        Some(mTouchStartRestingTimeBeforePan),
+        mMinimumVelocityDuringPan.valueOr(0),
+        false /* not handoff */,
+        GetCurrentInputBlock()->GetScrolledApzc()};
+    treeManagerLocal->DispatchFling(this, handoffState);
+  }
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+Maybe<LayoutDevicePoint> AsyncPanZoomController::ConvertToGecko(
+    const ScreenIntPoint& aPoint) {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    if (Maybe<ScreenIntPoint> layoutPoint =
+            treeManagerLocal->ConvertToGecko(aPoint, this)) {
+      return Some(LayoutDevicePoint(ViewAs<LayoutDevicePixel>(
+          *layoutPoint,
+          PixelCastJustification::LayoutDeviceIsScreenForUntransformedEvent)));
+    }
+  }
+  return Nothing();
+}
+
+OuterCSSCoord AsyncPanZoomController::ConvertScrollbarPoint(
+    const ParentLayerPoint& aScrollbarPoint,
+    const ScrollbarData& aThumbData) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  CSSPoint scrollbarPoint;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    // First, get it into the right coordinate space.
+    scrollbarPoint = aScrollbarPoint / Metrics().GetZoom();
+  }
+
+  // The scrollbar can be transformed with the frame but the pres shell
+  // resolution is only applied to the scroll frame.
+  OuterCSSPoint outerScrollbarPoint =
+      scrollbarPoint * Metrics().GetCSSToOuterCSSScale();
+
+  // Now, get it to be relative to the beginning of the scroll track.
+  OuterCSSRect cssCompositionBound =
+      Metrics().CalculateCompositionBoundsInOuterCssPixels();
+  return GetAxisStart(*aThumbData.mDirection, outerScrollbarPoint) -
+         GetAxisStart(*aThumbData.mDirection, cssCompositionBound) -
+         aThumbData.mScrollTrackStart;
+}
+
+static bool AllowsScrollingMoreThanOnePage(double aMultiplier) {
+  return Abs(aMultiplier) >=
+         EventStateManager::MIN_MULTIPLIER_VALUE_ALLOWING_OVER_ONE_PAGE_SCROLL;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetScrollWheelDelta(
+    const ScrollWheelInput& aEvent) const {
+  return GetScrollWheelDelta(aEvent, aEvent.mDeltaX, aEvent.mDeltaY,
+                             aEvent.mUserDeltaMultiplierX,
+                             aEvent.mUserDeltaMultiplierY);
+}
+
+ParentLayerPoint AsyncPanZoomController::GetScrollWheelDelta(
+    const ScrollWheelInput& aEvent, double aDeltaX, double aDeltaY,
+    double aMultiplierX, double aMultiplierY) const {
+  ParentLayerSize scrollAmount;
+  ParentLayerSize pageScrollSize;
+
+  {
+    // Grab the lock to access the frame metrics.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    LayoutDeviceIntSize scrollAmountLD = mScrollMetadata.GetLineScrollAmount();
+    LayoutDeviceIntSize pageScrollSizeLD =
+        mScrollMetadata.GetPageScrollAmount();
+    scrollAmount = scrollAmountLD / Metrics().GetDevPixelsPerCSSPixel() *
+                   Metrics().GetZoom();
+    pageScrollSize = pageScrollSizeLD / Metrics().GetDevPixelsPerCSSPixel() *
+                     Metrics().GetZoom();
+  }
+
+  ParentLayerPoint delta;
+  switch (aEvent.mDeltaType) {
+    case ScrollWheelInput::SCROLLDELTA_LINE: {
+      delta.x = aDeltaX * scrollAmount.width;
+      delta.y = aDeltaY * scrollAmount.height;
+      break;
+    }
+    case ScrollWheelInput::SCROLLDELTA_PAGE: {
+      delta.x = aDeltaX * pageScrollSize.width;
+      delta.y = aDeltaY * pageScrollSize.height;
+      break;
+    }
+    case ScrollWheelInput::SCROLLDELTA_PIXEL: {
+      delta = ToParentLayerCoordinates(ScreenPoint(aDeltaX, aDeltaY),
+                                       aEvent.mOrigin);
+      break;
+    }
+  }
+
+  // Apply user-set multipliers.
+  delta.x *= aMultiplierX;
+  delta.y *= aMultiplierY;
+
+  // For the conditions under which we allow system scroll overrides, see
+  // WidgetWheelEvent::OverriddenDelta{X,Y}.
+  // Note that we do *not* restrict this to the root content, see bug 1217715
+  // for discussion on this.
+  if (StaticPrefs::mousewheel_system_scroll_override_enabled() &&
+      !aEvent.IsCustomizedByUserPrefs() &&
+      aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_LINE &&
+      aEvent.mAllowToOverrideSystemScrollSpeed) {
+    delta.x = WidgetWheelEvent::ComputeOverriddenDelta(delta.x, false);
+    delta.y = WidgetWheelEvent::ComputeOverriddenDelta(delta.y, true);
+  }
+
+  // If this is a line scroll, and this event was part of a scroll series, then
+  // it might need extra acceleration. See WheelHandlingHelper.cpp.
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_LINE &&
+      aEvent.mScrollSeriesNumber > 0) {
+    int32_t start = StaticPrefs::mousewheel_acceleration_start();
+    if (start >= 0 && aEvent.mScrollSeriesNumber >= uint32_t(start)) {
+      int32_t factor = StaticPrefs::mousewheel_acceleration_factor();
+      if (factor > 0) {
+        delta.x = ComputeAcceleratedWheelDelta(
+            delta.x, aEvent.mScrollSeriesNumber, factor);
+        delta.y = ComputeAcceleratedWheelDelta(
+            delta.y, aEvent.mScrollSeriesNumber, factor);
+      }
+    }
+  }
+
+  // We shouldn't scroll more than one page at once except when the
+  // user preference is large.
+  if (!AllowsScrollingMoreThanOnePage(aMultiplierX) &&
+      Abs(delta.x) > pageScrollSize.width) {
+    delta.x = (delta.x >= 0) ? pageScrollSize.width : -pageScrollSize.width;
+  }
+  if (!AllowsScrollingMoreThanOnePage(aMultiplierY) &&
+      Abs(delta.y) > pageScrollSize.height) {
+    delta.y = (delta.y >= 0) ? pageScrollSize.height : -pageScrollSize.height;
+  }
+
+  return delta;
+}
+
+nsEventStatus AsyncPanZoomController::OnKeyboard(const KeyboardInput& aEvent) {
+  // Mark that this APZC has async key scrolled
+  mTestHasAsyncKeyScrolled = true;
+
+  // Calculate the destination for this keyboard scroll action
+  CSSPoint destination = GetKeyboardDestination(aEvent.mAction);
+  ScrollOrigin scrollOrigin =
+      SmoothScrollAnimation::GetScrollOriginForAction(aEvent.mAction.mType);
+  Maybe<CSSSnapDestination> snapDestination =
+      MaybeAdjustDestinationForScrollSnapping(
+          aEvent, destination,
+          GetScrollSnapFlagsForKeyboardAction(aEvent.mAction));
+  ScrollMode scrollMode = apz::GetScrollModeForOrigin(scrollOrigin);
+
+  RecordScrollPayload(aEvent.mTimeStamp);
+  // If the scrolling is instant, then scroll immediately to the destination
+  if (scrollMode == ScrollMode::Instant) {
+    CancelAnimation();
+
+    ParentLayerPoint startPoint, endPoint;
+
+    {
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      // CallDispatchScroll interprets the start and end points as the start and
+      // end of a touch scroll so they need to be reversed.
+      startPoint = destination * Metrics().GetZoom();
+      endPoint = Metrics().GetVisualScrollOffset() * Metrics().GetZoom();
+    }
+
+    ParentLayerPoint delta = endPoint - startPoint;
+
+    ScreenPoint distance = ToScreenCoordinates(
+        ParentLayerPoint(fabs(delta.x), fabs(delta.y)), startPoint);
+
+    OverscrollHandoffState handoffState(
+        *mInputQueue->GetCurrentKeyboardBlock()->GetOverscrollHandoffChain(),
+        distance, ScrollSource::Keyboard);
+
+    CallDispatchScroll(startPoint, endPoint, handoffState);
+    ParentLayerPoint remainingDelta = endPoint - startPoint;
+    if (remainingDelta != delta) {
+      // If any scrolling happened, set KEYBOARD_SCROLL explicitly so that it
+      // will trigger a TransformEnd notification.
+      SetState(KEYBOARD_SCROLL);
+    }
+
+    if (snapDestination) {
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        mLastSnapTargetIds = std::move(snapDestination->mTargetIds);
+      }
+    }
+    SetState(NOTHING);
+
+    return nsEventStatus_eConsumeDoDefault;
+  }
+
+  // The lock must be held across the entire update operation, so the
+  // compositor doesn't end the animation before we get a chance to
+  // update it.
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (snapDestination) {
+    // If we're scroll snapping, use a smooth scroll animation to get
+    // the desired physics. Note that SmoothMsdScrollTo() will re-use an
+    // existing smooth scroll animation if there is one.
+    APZC_LOG("%p keyboard scrolling to snap point %s\n", this,
+             ToString(destination).c_str());
+    SmoothMsdScrollTo(std::move(*snapDestination), ScrollTriggeredByScript::No);
+    return nsEventStatus_eConsumeDoDefault;
+  }
+
+  // Use a keyboard scroll animation to scroll, reusing an existing one if it
+  // exists
+  if (mState != KEYBOARD_SCROLL) {
+    CancelAnimation();
+
+    // Keyboard input that does not change the scroll position should not
+    // cause a TransformBegin state change, in order to avoid firing a
+    // scrollend event when no scrolling occurred.
+    if (!CanScroll(ConvertDestinationToDelta(destination))) {
+      return nsEventStatus_eConsumeDoDefault;
+    }
+    SetState(KEYBOARD_SCROLL);
+
+    nsPoint initialPosition =
+        CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+    StartAnimation(do_AddRef(
+        new SmoothScrollAnimation(*this, initialPosition, scrollOrigin)));
+  }
+
+  // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and then
+  // to appunits/second.
+  nsPoint velocity;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    velocity =
+        CSSPoint::ToAppUnits(ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                              mY.GetVelocity() * 1000.0f) /
+                             Metrics().GetZoom());
+  }
+
+  SmoothScrollAnimation* animation = mAnimation->AsSmoothScrollAnimation();
+  MOZ_ASSERT(animation);
+
+  animation->UpdateDestination(aEvent.mTimeStamp,
+                               CSSPixel::ToAppUnits(destination),
+                               nsSize(velocity.x, velocity.y));
+
+  return nsEventStatus_eConsumeDoDefault;
+}
+
+CSSPoint AsyncPanZoomController::GetKeyboardDestination(
+    const KeyboardScrollAction& aAction) const {
+  CSSSize lineScrollSize;
+  CSSSize pageScrollSize;
+  CSSPoint scrollOffset;
+  CSSRect scrollRect;
+  ParentLayerRect compositionBounds;
+
+  {
+    // Grab the lock to access the frame metrics.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    lineScrollSize = mScrollMetadata.GetLineScrollAmount() /
+                     Metrics().GetDevPixelsPerCSSPixel();
+    pageScrollSize = mScrollMetadata.GetPageScrollAmount() /
+                     Metrics().GetDevPixelsPerCSSPixel();
+
+    scrollOffset = GetCurrentAnimationDestination(lock).valueOr(
+        Metrics().GetVisualScrollOffset());
+
+    scrollRect = Metrics().GetScrollableRect();
+    compositionBounds = Metrics().GetCompositionBounds();
+  }
+
+  // Calculate the scroll destination based off of the scroll type and direction
+  CSSPoint scrollDestination = scrollOffset;
+
+  switch (aAction.mType) {
+    case KeyboardScrollAction::eScrollCharacter: {
+      int32_t scrollDistance =
+          StaticPrefs::toolkit_scrollbox_horizontalScrollDistance();
+
+      if (aAction.mForward) {
+        scrollDestination.x += scrollDistance * lineScrollSize.width;
+      } else {
+        scrollDestination.x -= scrollDistance * lineScrollSize.width;
+      }
+      break;
+    }
+    case KeyboardScrollAction::eScrollLine: {
+      int32_t scrollDistance =
+          StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+      if (scrollDistance * lineScrollSize.height <=
+          compositionBounds.Height()) {
+        if (aAction.mForward) {
+          scrollDestination.y += scrollDistance * lineScrollSize.height;
+        } else {
+          scrollDestination.y -= scrollDistance * lineScrollSize.height;
+        }
+        break;
+      }
+      [[fallthrough]];
+    }
+    case KeyboardScrollAction::eScrollPage: {
+      if (aAction.mForward) {
+        scrollDestination.y += pageScrollSize.height;
+      } else {
+        scrollDestination.y -= pageScrollSize.height;
+      }
+      break;
+    }
+    case KeyboardScrollAction::eScrollComplete: {
+      if (aAction.mForward) {
+        scrollDestination.y = scrollRect.YMost();
+      } else {
+        scrollDestination.y = scrollRect.Y();
+      }
+      break;
+    }
+  }
+
+  return scrollDestination;
+}
+
+ScrollSnapFlags AsyncPanZoomController::GetScrollSnapFlagsForKeyboardAction(
+    const KeyboardScrollAction& aAction) const {
+  switch (aAction.mType) {
+    case KeyboardScrollAction::eScrollCharacter:
+    case KeyboardScrollAction::eScrollLine:
+      return ScrollSnapFlags::IntendedDirection;
+    case KeyboardScrollAction::eScrollPage:
+      return ScrollSnapFlags::IntendedDirection |
+             ScrollSnapFlags::IntendedEndPosition;
+    case KeyboardScrollAction::eScrollComplete:
+      return ScrollSnapFlags::IntendedEndPosition;
+  }
+  return ScrollSnapFlags::Disabled;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetDeltaForEvent(
+    const InputData& aEvent) const {
+  ParentLayerPoint delta;
+  if (aEvent.mInputType == SCROLLWHEEL_INPUT) {
+    delta = GetScrollWheelDelta(aEvent.AsScrollWheelInput());
+  } else if (aEvent.mInputType == PANGESTURE_INPUT) {
+    const PanGestureInput& panInput = aEvent.AsPanGestureInput();
+    delta = ToParentLayerCoordinates(panInput.UserMultipliedPanDisplacement(),
+                                     panInput.mPanStartPoint);
+  }
+  return delta;
+}
+
+CSSRect AsyncPanZoomController::GetCurrentScrollRangeInCssPixels() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().CalculateScrollRange();
+}
+
+// Return whether or not the underlying layer can be scrolled on either axis.
+bool AsyncPanZoomController::CanScroll(const InputData& aEvent) const {
+  ParentLayerPoint delta = GetDeltaForEvent(aEvent);
+  if (!delta.x && !delta.y) {
+    return false;
+  }
+
+  if (SCROLLWHEEL_INPUT == aEvent.mInputType) {
+    const ScrollWheelInput& scrollWheelInput = aEvent.AsScrollWheelInput();
+    // If it's a wheel scroll, we first check if it is an auto-dir scroll.
+    // 1. For an auto-dir scroll, check if it's delta should be adjusted, if it
+    //    is, then we can conclude it must be scrollable; otherwise, fall back
+    //    to checking if it is scrollable without adjusting its delta.
+    // 2. For a non-auto-dir scroll, simply check if it is scrollable without
+    //    adjusting its delta.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (scrollWheelInput.IsAutoDir(mScrollMetadata.ForceMousewheelAutodir())) {
+      auto deltaX = scrollWheelInput.mDeltaX;
+      auto deltaY = scrollWheelInput.mDeltaY;
+      bool isRTL =
+          IsContentOfHonouredTargetRightToLeft(scrollWheelInput.HonoursRoot(
+              mScrollMetadata.ForceMousewheelAutodirHonourRoot()));
+      APZAutoDirWheelDeltaAdjuster adjuster(deltaX, deltaY, mX, mY, isRTL);
+      if (adjuster.ShouldBeAdjusted()) {
+        // If we detect that the delta values should be adjusted for an auto-dir
+        // wheel scroll, then it is impossible to be an unscrollable scroll.
+        return true;
+      }
+    }
+    return CanScrollWithWheel(delta);
+  }
+  return CanScroll(delta);
+}
+
+ScrollDirections AsyncPanZoomController::GetAllowedHandoffDirections() const {
+  ScrollDirections result;
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // In Fission there can be non-scrollable APZCs. It's unclear whether
+  // overscroll-behavior should be respected for these
+  // (see https://github.com/w3c/csswg-drafts/issues/6523) but
+  // we currently don't, to match existing practice.
+  const bool isScrollable = mX.CanScroll() || mY.CanScroll();
+  const bool isRoot = IsRootContent();
+  if ((!isScrollable && !isRoot) || mX.OverscrollBehaviorAllowsHandoff()) {
+    result += ScrollDirection::eHorizontal;
+  }
+  if ((!isScrollable && !isRoot) || mY.OverscrollBehaviorAllowsHandoff()) {
+    result += ScrollDirection::eVertical;
+  }
+  return result;
+}
+
+bool AsyncPanZoomController::CanScroll(const ParentLayerPoint& aDelta) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.CanScroll(ParentLayerCoord(aDelta.x)) ||
+         mY.CanScroll(ParentLayerCoord(aDelta.y));
+}
+
+bool AsyncPanZoomController::CanScrollWithWheel(
+    const ParentLayerPoint& aDelta) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // For more details about the concept of a disregarded direction, refer to the
+  // code in struct ScrollMetadata which defines mDisregardedDirection.
+  Maybe<ScrollDirection> disregardedDirection =
+      mScrollMetadata.GetDisregardedDirection();
+  if (mX.CanScroll(ParentLayerCoord(aDelta.x)) &&
+      disregardedDirection != Some(ScrollDirection::eHorizontal)) {
+    return true;
+  }
+  if (mY.CanScroll(ParentLayerCoord(aDelta.y)) &&
+      disregardedDirection != Some(ScrollDirection::eVertical)) {
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::CanScroll(ScrollDirection aDirection) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  switch (aDirection) {
+    case ScrollDirection::eHorizontal:
+      return mX.CanScroll();
+    case ScrollDirection::eVertical:
+      return mY.CanScroll();
+  }
+  MOZ_ASSERT_UNREACHABLE("Invalid value");
+  return false;
+}
+
+bool AsyncPanZoomController::CanVerticalScrollWithDynamicToolbar() const {
+  MOZ_ASSERT(IsRootContent());
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mY.CanVerticalScrollWithDynamicToolbar();
+}
+
+bool AsyncPanZoomController::CanOverscrollUpwards() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return !mY.CanScrollTo(eSideTop) && mY.OverscrollBehaviorAllowsHandoff();
+}
+
+bool AsyncPanZoomController::CanScrollDownwards() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mY.CanScrollTo(eSideBottom);
+}
+
+SideBits AsyncPanZoomController::ScrollableDirections() const {
+  SideBits result;
+  {  // scope lock to respect lock ordering with APZCTreeManager::mTreeLock
+    // which will be acquired in the `GetCompositorFixedLayerMargins` below.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    result = mX.ScrollableDirections() | mY.ScrollableDirections();
+  }
+
+  if (IsRootContent()) {
+    if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+      ScreenMargin fixedLayerMargins =
+          treeManagerLocal->GetCompositorFixedLayerMargins();
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        result |= mY.ScrollableDirectionsWithDynamicToolbar(fixedLayerMargins);
+      }
+    }
+  }
+
+  return result;
+}
+
+bool AsyncPanZoomController::IsContentOfHonouredTargetRightToLeft(
+    bool aHonoursRoot) const {
+  if (aHonoursRoot) {
+    return mScrollMetadata.IsAutoDirRootContentRTL();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().IsHorizontalContentRightToLeft();
+}
+
+bool AsyncPanZoomController::AllowScrollHandoffInCurrentBlock() const {
+  bool result = mInputQueue->AllowScrollHandoff();
+  if (!StaticPrefs::apz_allow_immediate_handoff()) {
+    if (InputBlockState* currentBlock = GetCurrentInputBlock()) {
+      // Do not allow handoff beyond the first APZC to scroll.
+      if (currentBlock->GetScrolledApzc() == this) {
+        result = false;
+        APZC_LOG("%p dropping handoff; AllowImmediateHandoff=false\n", this);
+      }
+    }
+  }
+  return result;
+}
+
+void AsyncPanZoomController::DoDelayedRequestContentRepaint() {
+  if (!IsDestroyed() && mPinchPaintTimerSet) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    RequestContentRepaint();
+  }
+  mPinchPaintTimerSet = false;
+}
+
+void AsyncPanZoomController::DoDelayedTransformEndNotification(
+    PanZoomState aOldState) {
+  if (!IsDestroyed() && IsDelayedTransformEndSet()) {
+    DispatchStateChangeNotification(aOldState, NOTHING);
+  }
+  SetDelayedTransformEnd(false);
+}
+
+static void AdjustDeltaForAllowedScrollDirections(
+    ParentLayerPoint& aDelta,
+    const ScrollDirections& aAllowedScrollDirections) {
+  if (!aAllowedScrollDirections.contains(ScrollDirection::eHorizontal)) {
+    aDelta.x = 0;
+  }
+  if (!aAllowedScrollDirections.contains(ScrollDirection::eVertical)) {
+    aDelta.y = 0;
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnScrollWheel(
+    const ScrollWheelInput& aEvent) {
+  // Get the scroll wheel's delta values in parent-layer pixels. But before
+  // getting the values, we need to check if it is an auto-dir scroll and if it
+  // should be adjusted, if both answers are yes, let's adjust X and Y values
+  // first, and then get the delta values in parent-layer pixels based on the
+  // adjusted values.
+  bool adjustedByAutoDir = false;
+  auto deltaX = aEvent.mDeltaX;
+  auto deltaY = aEvent.mDeltaY;
+  ParentLayerPoint delta;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (aEvent.IsAutoDir(mScrollMetadata.ForceMousewheelAutodir())) {
+      // It's an auto-dir scroll, so check if its delta should be adjusted, if
+      // so, adjust it.
+      bool isRTL = IsContentOfHonouredTargetRightToLeft(aEvent.HonoursRoot(
+          mScrollMetadata.ForceMousewheelAutodirHonourRoot()));
+      APZAutoDirWheelDeltaAdjuster adjuster(deltaX, deltaY, mX, mY, isRTL);
+      if (adjuster.ShouldBeAdjusted()) {
+        adjuster.Adjust();
+        adjustedByAutoDir = true;
+      }
+    }
+  }
+  // Ensure the calls to GetScrollWheelDelta are outside the mRecursiveMutex
+  // lock since these calls may acquire the APZ tree lock. Holding
+  // mRecursiveMutex while acquiring the APZ tree lock is lock ordering
+  // violation.
+  if (adjustedByAutoDir) {
+    // If the original delta values have been adjusted, we pass them to
+    // replace the original delta values in |aEvent| so that the delta values
+    // in parent-layer pixels are caculated based on the adjusted values, not
+    // the original ones.
+    // Pay special attention to the last two parameters. They are in a swaped
+    // order so that they still correspond to their delta after adjustment.
+    delta = GetScrollWheelDelta(aEvent, deltaX, deltaY,
+                                aEvent.mUserDeltaMultiplierY,
+                                aEvent.mUserDeltaMultiplierX);
+  } else {
+    // If the original delta values haven't been adjusted by auto-dir, just pass
+    // the |aEvent| and caculate the delta values in parent-layer pixels based
+    // on the original delta values from |aEvent|.
+    delta = GetScrollWheelDelta(aEvent);
+  }
+
+  APZC_LOG("%p got a scroll-wheel with delta in parent-layer pixels: %s\n",
+           this, ToString(delta).c_str());
+
+  if (adjustedByAutoDir) {
+    MOZ_ASSERT(delta.x || delta.y,
+               "Adjusted auto-dir delta values can never be all-zero.");
+    APZC_LOG("%p got a scroll-wheel with adjusted auto-dir delta values\n",
+             this);
+  } else if ((delta.x || delta.y) && !CanScrollWithWheel(delta)) {
+    // We can't scroll this apz anymore, so we simply drop the event.
+    if (mInputQueue->GetActiveWheelTransaction() &&
+        StaticPrefs::test_mousescroll()) {
+      if (RefPtr<GeckoContentController> controller =
+              GetGeckoContentController()) {
+        controller->NotifyMozMouseScrollEvent(GetScrollId(),
+                                              u"MozMouseScrollFailed"_ns);
+      }
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  MOZ_ASSERT(mInputQueue->GetCurrentWheelBlock());
+  AdjustDeltaForAllowedScrollDirections(
+      delta, mInputQueue->GetCurrentWheelBlock()->GetAllowedScrollDirections());
+
+  if (delta.x == 0 && delta.y == 0) {
+    // Avoid spurious state changes and unnecessary work
+    return nsEventStatus_eIgnore;
+  }
+
+  switch (aEvent.mScrollMode) {
+    case ScrollWheelInput::SCROLLMODE_INSTANT: {
+      // Wheel events from "clicky" mouse wheels trigger scroll snapping to the
+      // next snap point. Check for this, and adjust the delta to take into
+      // account the snap point.
+      CSSPoint startPosition;
+      {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        startPosition = Metrics().GetVisualScrollOffset();
+      }
+      Maybe<CSSSnapDestination> snapDestination =
+          MaybeAdjustDeltaForScrollSnappingOnWheelInput(aEvent, delta,
+                                                        startPosition);
+
+      ScreenPoint distance = ToScreenCoordinates(
+          ParentLayerPoint(fabs(delta.x), fabs(delta.y)), aEvent.mLocalOrigin);
+
+      CancelAnimation();
+
+      OverscrollHandoffState handoffState(
+          *mInputQueue->GetCurrentWheelBlock()->GetOverscrollHandoffChain(),
+          distance, ScrollSource::Wheel);
+      ParentLayerPoint startPoint = aEvent.mLocalOrigin;
+      ParentLayerPoint endPoint = aEvent.mLocalOrigin - delta;
+      RecordScrollPayload(aEvent.mTimeStamp);
+
+      CallDispatchScroll(startPoint, endPoint, handoffState);
+      ParentLayerPoint remainingDelta = endPoint - startPoint;
+      if (remainingDelta != delta) {
+        // If any scrolling happened, set WHEEL_SCROLL explicitly so that it
+        // will trigger a TransformEnd notification.
+        SetState(WHEEL_SCROLL);
+      }
+
+      if (snapDestination) {
+        {
+          RecursiveMutexAutoLock lock(mRecursiveMutex);
+          mLastSnapTargetIds = std::move(snapDestination->mTargetIds);
+        }
+      }
+      SetState(NOTHING);
+
+      // The calls above handle their own locking; moreover,
+      // ToScreenCoordinates() and CallDispatchScroll() can grab the tree lock.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      RequestContentRepaint();
+
+      break;
+    }
+
+    case ScrollWheelInput::SCROLLMODE_SMOOTH: {
+      // The lock must be held across the entire update operation, so the
+      // compositor doesn't end the animation before we get a chance to
+      // update it.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+      RecordScrollPayload(aEvent.mTimeStamp);
+      // Perform scroll snapping if appropriate.
+      // If we're already in a wheel scroll or smooth scroll animation,
+      // the delta is applied to its destination, not to the current
+      // scroll position. Take this into account when finding a snap point.
+      CSSPoint startPosition = GetCurrentAnimationDestination(lock).valueOr(
+          Metrics().GetVisualScrollOffset());
+
+      if (Maybe<CSSSnapDestination> snapDestination =
+              MaybeAdjustDeltaForScrollSnappingOnWheelInput(aEvent, delta,
+                                                            startPosition)) {
+        // If we're scroll snapping, use a smooth scroll animation to get
+        // the desired physics. Note that SmoothMsdScrollTo() will re-use an
+        // existing smooth scroll animation if there is one.
+        APZC_LOG("%p wheel scrolling to snap point %s\n", this,
+                 ToString(startPosition).c_str());
+        SmoothMsdScrollTo(std::move(*snapDestination),
+                          ScrollTriggeredByScript::No);
+        break;
+      }
+
+      // Otherwise, use a wheel scroll animation, also reusing one if possible.
+      if (mState != WHEEL_SCROLL) {
+        CancelAnimation();
+        SetState(WHEEL_SCROLL);
+
+        nsPoint initialPosition =
+            CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+        StartAnimation(do_AddRef(new WheelScrollAnimation(
+            *this, initialPosition, aEvent.mDeltaType)));
+      }
+      // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and
+      // then to appunits/second.
+
+      nsPoint deltaInAppUnits;
+      nsPoint velocity;
+      if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+        deltaInAppUnits = CSSPoint::ToAppUnits(delta / Metrics().GetZoom());
+        velocity =
+            CSSPoint::ToAppUnits(ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                                  mY.GetVelocity() * 1000.0f) /
+                                 Metrics().GetZoom());
+      }
+
+      WheelScrollAnimation* animation = mAnimation->AsWheelScrollAnimation();
+      animation->UpdateDelta(aEvent.mTimeStamp, deltaInAppUnits,
+                             nsSize(velocity.x, velocity.y));
+      break;
+    }
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+void AsyncPanZoomController::NotifyMozMouseScrollEvent(
+    const nsString& aString) const {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  controller->NotifyMozMouseScrollEvent(GetScrollId(), aString);
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMayBegin(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-maybegin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  StartTouch(aEvent.mLocalPanStartPoint, aEvent.mTimeStamp);
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  GetCurrentPanGestureBlock()->GetOverscrollHandoffChain()->CancelAnimations();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanCancelled(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-cancelled in state %s\n", this,
+                  ToString(mState).c_str());
+
+  mX.CancelGesture();
+  mY.CancelGesture();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanBegin(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-begin in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == SMOOTHMSD_SCROLL) {
+    // SMOOTHMSD_SCROLL scrolls are cancelled by pan gestures.
+    CancelAnimation();
+  }
+
+  StartTouch(aEvent.mLocalPanStartPoint, aEvent.mTimeStamp);
+
+  if (!UsingStatefulAxisLock()) {
+    SetState(PANNING);
+  } else {
+    float dx = aEvent.mPanDisplacement.x, dy = aEvent.mPanDisplacement.y;
+
+    if (dx != 0.0f || dy != 0.0f) {
+      double angle = atan2(dy, dx);  // range [-pi, pi]
+      angle = fabs(angle);           // range [0, pi]
+      HandlePanning(angle);
+    } else {
+      SetState(PANNING);
+    }
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::Yes);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+std::tuple<ParentLayerPoint, ScreenPoint>
+AsyncPanZoomController::GetDisplacementsForPanGesture(
+    const PanGestureInput& aEvent) {
+  // Note that there is a multiplier that applies onto the "physical" pan
+  // displacement (how much the user's fingers moved) that produces the
+  // "logical" pan displacement (how much the page should move). For some of the
+  // code below it makes more sense to use the physical displacement rather than
+  // the logical displacement, and vice-versa.
+  ScreenPoint physicalPanDisplacement = aEvent.mPanDisplacement;
+  ParentLayerPoint logicalPanDisplacement =
+      aEvent.UserMultipliedLocalPanDisplacement();
+  if (aEvent.mDeltaType == PanGestureInput::PANDELTA_PAGE) {
+    // Pan events with page units are used by Gtk, so this replicates Gtk:
+    // https://gitlab.gnome.org/GNOME/gtk/blob/c734c7e9188b56f56c3a504abee05fa40c5475ac/gtk/gtkrange.c#L3065-3073
+    CSSSize pageScrollSize;
+    CSSToParentLayerScale zoom;
+    {
+      // Grab the lock to access the frame metrics.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      pageScrollSize = mScrollMetadata.GetPageScrollAmount() /
+                       Metrics().GetDevPixelsPerCSSPixel();
+      zoom = Metrics().GetZoom();
+    }
+    // scrollUnit* is in units of "ParentLayer pixels per page proportion"...
+    auto scrollUnitWidth = std::min(std::pow(pageScrollSize.width, 2.0 / 3.0),
+                                    pageScrollSize.width / 2.0) *
+                           zoom.scale;
+    auto scrollUnitHeight = std::min(std::pow(pageScrollSize.height, 2.0 / 3.0),
+                                     pageScrollSize.height / 2.0) *
+                            zoom.scale;
+    // ... and pan displacements are in units of "page proportion count"
+    // here, so the products of them and scrollUnit* are in ParentLayer pixels
+    ParentLayerPoint physicalPanDisplacementPL(
+        physicalPanDisplacement.x * scrollUnitWidth,
+        physicalPanDisplacement.y * scrollUnitHeight);
+    physicalPanDisplacement = ToScreenCoordinates(physicalPanDisplacementPL,
+                                                  aEvent.mLocalPanStartPoint);
+    logicalPanDisplacement.x *= scrollUnitWidth;
+    logicalPanDisplacement.y *= scrollUnitHeight;
+
+    // Accelerate (decelerate) any pans by raising it to a user configurable
+    // power (apz.touch_acceleration_factor_x, apz.touch_acceleration_factor_y)
+    //
+    // Confine input for pow() to greater than or equal to 0 to avoid domain
+    // errors with non-integer exponents
+    if (mX.GetVelocity() != 0) {
+      float absVelocity = std::abs(mX.GetVelocity());
+      logicalPanDisplacement.x *=
+          std::pow(absVelocity,
+                   StaticPrefs::apz_touch_acceleration_factor_x()) /
+          absVelocity;
+    }
+
+    if (mY.GetVelocity() != 0) {
+      float absVelocity = std::abs(mY.GetVelocity());
+      logicalPanDisplacement.y *=
+          std::pow(absVelocity,
+                   StaticPrefs::apz_touch_acceleration_factor_y()) /
+          absVelocity;
+    }
+  }
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  AdjustDeltaForAllowedScrollDirections(
+      logicalPanDisplacement,
+      GetCurrentPanGestureBlock()->GetAllowedScrollDirections());
+
+  if (GetAxisLockMode() == DOMINANT_AXIS) {
+    // Given a pan gesture and both directions have a delta, implement
+    // dominant axis scrolling and only use the delta for the larger
+    // axis.
+    if (logicalPanDisplacement.y != 0 && logicalPanDisplacement.x != 0) {
+      if (fabs(logicalPanDisplacement.y) >= fabs(logicalPanDisplacement.x)) {
+        logicalPanDisplacement.x = 0;
+        physicalPanDisplacement.x = 0;
+      } else {
+        logicalPanDisplacement.y = 0;
+        physicalPanDisplacement.y = 0;
+      }
+    }
+  }
+
+  return {logicalPanDisplacement, physicalPanDisplacement};
+}
+
+nsEventStatus AsyncPanZoomController::OnPan(
+    const PanGestureInput& aEvent, FingersOnTouchpad aFingersOnTouchpad) {
+  APZC_LOG_DETAIL("got a pan-pan in state %s\n", this,
+                  ToString(GetState()).c_str());
+
+  if (GetState() == SMOOTHMSD_SCROLL) {
+    if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+      // When a SMOOTHMSD_SCROLL scroll is being processed on a frame, mouse
+      // wheel and trackpad momentum scroll position updates will not cancel the
+      // SMOOTHMSD_SCROLL scroll animations, enabling scripts that depend on
+      // them to be responsive without forcing the user to wait for the momentum
+      // scrolling to completely stop.
+      return nsEventStatus_eConsumeNoDefault;
+    }
+
+    // SMOOTHMSD_SCROLL scrolls are cancelled by pan gestures.
+    CancelAnimation();
+  }
+
+  if (GetState() == NOTHING) {
+    // This event block was interrupted by something else. If the user's fingers
+    // are still on on the touchpad we want to resume scrolling, otherwise we
+    // ignore the rest of the scroll gesture.
+    if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+      return nsEventStatus_eConsumeNoDefault;
+    }
+    // Resume / restart the pan.
+    // PanBegin will call back into this function with mState == PANNING.
+    return OnPanBegin(aEvent);
+  }
+
+  auto [logicalPanDisplacement, physicalPanDisplacement] =
+      GetDisplacementsForPanGesture(aEvent);
+
+  {
+    // Grab the lock to protect the animation from being canceled on the updater
+    // thread.
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    MOZ_ASSERT_IF(GetState() == OVERSCROLL_ANIMATION, mAnimation);
+
+    if (GetState() == OVERSCROLL_ANIMATION && mAnimation &&
+        aFingersOnTouchpad == FingersOnTouchpad::No) {
+      // If there is an on-going overscroll animation, we tell the animation
+      // whether the displacements should be handled by the animation or not.
+      MOZ_ASSERT(mAnimation->AsOverscrollAnimation());
+      if (RefPtr<OverscrollAnimation> overscrollAnimation =
+              mAnimation->AsOverscrollAnimation()) {
+        overscrollAnimation->HandlePanMomentum(logicalPanDisplacement);
+        // And then as a result of the above call, if the animation is currently
+        // affecting on the axis, drop the displacement value on the axis so
+        // that we stop further oversrolling on the axis.
+        if (overscrollAnimation->IsManagingXAxis()) {
+          logicalPanDisplacement.x = 0;
+          physicalPanDisplacement.x = 0;
+        }
+        if (overscrollAnimation->IsManagingYAxis()) {
+          logicalPanDisplacement.y = 0;
+          physicalPanDisplacement.y = 0;
+        }
+      }
+    }
+  }
+
+  HandlePanningUpdate(physicalPanDisplacement);
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  ScreenPoint panDistance(fabs(physicalPanDisplacement.x),
+                          fabs(physicalPanDisplacement.y));
+  OverscrollHandoffState handoffState(
+      *GetCurrentPanGestureBlock()->GetOverscrollHandoffChain(), panDistance,
+      ScrollSource::Touchpad);
+
+  // Create fake "touch" positions that will result in the desired scroll
+  // motion. Note that the pan displacement describes the change in scroll
+  // position: positive displacement values mean that the scroll position
+  // increases. However, an increase in scroll position means that the scrolled
+  // contents are moved to the left / upwards. Since our simulated "touches"
+  // determine the motion of the scrolled contents, not of the scroll position,
+  // they need to move in the opposite direction of the pan displacement.
+  ParentLayerPoint startPoint = aEvent.mLocalPanStartPoint;
+  ParentLayerPoint endPoint =
+      aEvent.mLocalPanStartPoint - logicalPanDisplacement;
+  if (logicalPanDisplacement != ParentLayerPoint()) {
+    // Don't expect a composite to be triggered if the displacement is zero
+    RecordScrollPayload(aEvent.mTimeStamp);
+  }
+
+  const ParentLayerPoint velocity = GetVelocityVector();
+  bool consumed = CallDispatchScroll(startPoint, endPoint, handoffState);
+
+  const ParentLayerPoint visualDisplacement = ToParentLayerCoordinates(
+      handoffState.mTotalMovement, aEvent.mPanStartPoint);
+  // We need to update the axis velocity in order to get a useful display port
+  // size and position. We need to do so even if this is a momentum pan (i.e.
+  // aFingersOnTouchpad == No); in that case the "with touch" part is not
+  // really appropriate, so we may want to rethink this at some point.
+  // Note that we have to make all simulated positions relative to
+  // Axis::GetPos(), because the current position is an invented position, and
+  // because resetting the position to the mouse position (e.g.
+  // aEvent.mLocalStartPoint) would mess up velocity calculation. (This is
+  // the only caller of UpdateWithTouchAtDevicePoint() for pan events, so
+  // there is no risk of other calls resetting the position.)
+  // Also note that if there is an on-going overscroll animation in the axis,
+  // we shouldn't call UpdateWithTouchAtDevicePoint because the call changes
+  // the velocity which should be managed by the overscroll animation.
+  // Finally, note that we do this *after* CallDispatchScroll(), so that the
+  // position we use reflects the actual amount of movement that occurred
+  // (in particular, if we're in overscroll, if reflects the amount of movement
+  // *after* applying resistance). This is important because we want the axis
+  // velocity to track the visual movement speed of the page.
+  if (visualDisplacement.x != 0) {
+    mX.UpdateWithTouchAtDevicePoint(mX.GetPos() - visualDisplacement.x,
+                                    aEvent.mTimeStamp);
+  }
+  if (visualDisplacement.y != 0) {
+    mY.UpdateWithTouchAtDevicePoint(mY.GetPos() - visualDisplacement.y,
+                                    aEvent.mTimeStamp);
+  }
+
+  if (aFingersOnTouchpad == FingersOnTouchpad::No) {
+    if (IsOverscrolled() && GetState() != OVERSCROLL_ANIMATION) {
+      StartOverscrollAnimation(velocity, GetOverscrollSideBits());
+    } else if (!consumed) {
+      // If there is unconsumed scroll and we're in the momentum part of the
+      // pan gesture, terminate the momentum scroll. This prevents momentum
+      // scroll events from unexpectedly causing scrolling later if somehow
+      // the APZC becomes scrollable again in this direction (e.g. if the user
+      // uses some other input method to scroll in the opposite direction).
+      SetState(NOTHING);
+    }
+  }
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanEnd(const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-end in state %s\n", this,
+                  ToString(mState).c_str());
+
+  // This can happen if the OS sends a second pan-end event after the first one
+  // has already started an overscroll animation or entered a fling state.
+  // This has been observed on some Wayland versions.
+  PanZoomState currentState = GetState();
+  if (currentState == OVERSCROLL_ANIMATION || currentState == NOTHING ||
+      currentState == FLING) {
+    return nsEventStatus_eIgnore;
+  }
+
+  if (aEvent.mPanDisplacement != ScreenPoint{}) {
+    // Call into OnPan in order to process the delta included in this event.
+    OnPan(aEvent, FingersOnTouchpad::Yes);
+  }
+
+  // Do not unlock the axis lock at the end of a pan gesture. The axis lock
+  // should extend into the momentum scroll.
+  EndTouch(aEvent.mTimeStamp, Axis::ClearAxisLock::No);
+
+  // Use HandleEndOfPan for fling on platforms that don't
+  // emit momentum events (Gtk).
+  if (aEvent.mSimulateMomentum) {
+    return HandleEndOfPan();
+  }
+
+  MOZ_ASSERT(GetCurrentPanGestureBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentPanGestureBlock()->GetOverscrollHandoffChain();
+
+  // Call SnapBackOverscrolledApzcForMomentum regardless whether this APZC is
+  // overscrolled or not since overscroll animations for ancestor APZCs in this
+  // overscroll handoff chain might have been cancelled by the current pan
+  // gesture block.
+  overscrollHandoffChain->SnapBackOverscrolledApzcForMomentum(
+      this, GetVelocityVector());
+  // If this APZC is overscrolled, the above SnapBackOverscrolledApzcForMomentum
+  // triggers an overscroll animation. When we're finished with the overscroll
+  // animation, the state will be reset and a TransformEnd will be sent to the
+  // main thread.
+  currentState = GetState();
+  if (currentState != OVERSCROLL_ANIMATION) {
+    // Do not send a state change notification to the content controller here.
+    // Instead queue a delayed task to dispatch the notification if no
+    // momentum pan or scroll snap follows the pan-end.
+    RefPtr<GeckoContentController> controller = GetGeckoContentController();
+    if (controller) {
+      SetDelayedTransformEnd(true);
+      controller->PostDelayedTask(
+          NewRunnableMethod<PanZoomState>(
+              "layers::AsyncPanZoomController::"
+              "DoDelayedTransformEndNotification",
+              this, &AsyncPanZoomController::DoDelayedTransformEndNotification,
+              currentState),
+          StaticPrefs::apz_scrollend_event_content_delay_ms());
+      SetStateNoContentControllerDispatch(NOTHING);
+    } else {
+      SetState(NOTHING);
+    }
+  }
+
+  // Drop any velocity on axes where we don't have room to scroll anyways
+  // (in this APZC, or an APZC further in the handoff chain).
+  // This ensures that we don't enlarge the display port unnecessarily.
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (!overscrollHandoffChain->CanScrollInDirection(
+            this, ScrollDirection::eHorizontal)) {
+      mX.SetVelocity(0);
+    }
+    if (!overscrollHandoffChain->CanScrollInDirection(
+            this, ScrollDirection::eVertical)) {
+      mY.SetVelocity(0);
+    }
+  }
+
+  RequestContentRepaint();
+  ScrollSnapToDestination();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMomentumStart(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-momentumstart in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == SMOOTHMSD_SCROLL || mState == OVERSCROLL_ANIMATION) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  if (IsDelayedTransformEndSet()) {
+    // Do not send another TransformBegin notification if we have not
+    // delivered a corresponding TransformEnd. Also ensure that any
+    // queued transform-end due to a pan-end is not sent. Instead rely
+    // on the transform-end sent due to the momentum pan.
+    SetDelayedTransformEnd(false);
+    SetStateNoContentControllerDispatch(PAN_MOMENTUM);
+  } else {
+    SetState(PAN_MOMENTUM);
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::No);
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanMomentumEnd(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-momentumend in state %s\n", this,
+                  ToString(mState).c_str());
+
+  if (mState == OVERSCROLL_ANIMATION) {
+    return nsEventStatus_eConsumeNoDefault;
+  }
+
+  // Call into OnPan in order to process any delta included in this event.
+  OnPan(aEvent, FingersOnTouchpad::No);
+
+  // We need to reset the velocity to zero. We don't really have a "touch"
+  // here because the touch has already ended long before the momentum
+  // animation started, but I guess it doesn't really matter for now.
+  mX.CancelGesture();
+  mY.CancelGesture();
+  SetState(NOTHING);
+
+  RequestContentRepaint();
+
+  return nsEventStatus_eConsumeNoDefault;
+}
+
+nsEventStatus AsyncPanZoomController::OnPanInterrupted(
+    const PanGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a pan-interrupted in state %s\n", this,
+                  ToString(mState).c_str());
+
+  CancelAnimation();
+
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnLongPress(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a long-press in state %s\n", this,
+                  ToString(mState).c_str());
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (Maybe<LayoutDevicePoint> geckoScreenPoint =
+            ConvertToGecko(aEvent.mPoint)) {
+      TouchBlockState* touch = GetCurrentTouchBlock();
+      if (!touch) {
+        APZC_LOG(
+            "%p dropping long-press because some non-touch block interrupted "
+            "it\n",
+            this);
+        return nsEventStatus_eIgnore;
+      }
+      if (touch->IsDuringFastFling()) {
+        APZC_LOG("%p dropping long-press because of fast fling\n", this);
+        return nsEventStatus_eIgnore;
+      }
+      uint64_t blockId = GetInputQueue()->InjectNewTouchBlock(this);
+      controller->HandleTap(TapType::eLongTap, *geckoScreenPoint,
+                            aEvent.modifiers, GetGuid(), blockId, Nothing());
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnLongPressUp(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a long-tap-up in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eLongTapUp, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::GenerateSingleTap(
+    TapType aType, const ScreenIntPoint& aPoint,
+    mozilla::Modifiers aModifiers) {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (Maybe<LayoutDevicePoint> geckoScreenPoint = ConvertToGecko(aPoint)) {
+      TouchBlockState* touch = GetCurrentTouchBlock();
+      // |touch| may be null in the case where this function is
+      // invoked by GestureEventListener on a timeout. In that case we already
+      // verified that the single tap is allowed so we let it through.
+      // XXX there is a bug here that in such a case the touch block that
+      // generated this tap will not get its mSingleTapOccurred flag set.
+      // See https://bugzilla.mozilla.org/show_bug.cgi?id=1256344#c6
+      if (touch) {
+        if (touch->IsDuringFastFling()) {
+          APZC_LOG(
+              "%p dropping single-tap because it was during a fast-fling\n",
+              this);
+          return nsEventStatus_eIgnore;
+        }
+
+        // The below `single-tap-occurred` flag is only used to tell whether the
+        // touch block caused a `click` event or not, thus for long-tap events,
+        // it's not necessary.
+        if (aType != TapType::eLongTapUp) {
+          touch->SetSingleTapOccurred();
+        }
+      }
+      // Because this may be being running as part of
+      // APZCTreeManager::ReceiveInputEvent, calling controller->HandleTap
+      // directly might mean that content receives the single tap message before
+      // the corresponding touch-up. To avoid that we schedule the singletap
+      // message to run on the next spin of the event loop. See bug 965381 for
+      // the issue this was causing.
+      APZC_LOG("posting runnable for HandleTap from GenerateSingleTap");
+      RefPtr<Runnable> runnable =
+          NewRunnableMethod<TapType, LayoutDevicePoint, mozilla::Modifiers,
+                            ScrollableLayerGuid, uint64_t,
+                            Maybe<DoubleTapToZoomMetrics>>(
+              "layers::GeckoContentController::HandleTap", controller,
+              &GeckoContentController::HandleTap, aType, *geckoScreenPoint,
+              aModifiers, GetGuid(), touch ? touch->GetBlockId() : 0,
+              Nothing());
+
+      controller->PostDelayedTask(runnable.forget(), 0);
+      return nsEventStatus_eConsumeNoDefault;
+    }
+  }
+  return nsEventStatus_eIgnore;
+}
+
+void AsyncPanZoomController::OnTouchEndOrCancel() {
+  if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
+    MOZ_ASSERT(GetCurrentTouchBlock());
+    controller->NotifyAPZStateChange(
+        GetGuid(), APZStateChange::eEndTouch,
+        GetCurrentTouchBlock()->SingleTapOccurred(),
+        Some(GetCurrentTouchBlock()->GetBlockId()));
+  }
+}
+
+nsEventStatus AsyncPanZoomController::OnSingleTapUp(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a single-tap-up in state %s\n", this,
+                  ToString(mState).c_str());
+  // If mZoomConstraints.mAllowDoubleTapZoom is true we wait for a call to
+  // OnSingleTapConfirmed before sending event to content
+  MOZ_ASSERT(GetCurrentTouchBlock());
+  if (!(ZoomConstraintsAllowDoubleTapZoom() &&
+        GetCurrentTouchBlock()->TouchActionAllowsDoubleTapZoom())) {
+    return GenerateSingleTap(TapType::eSingleTap, aEvent.mPoint,
+                             aEvent.modifiers);
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnSingleTapConfirmed(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a single-tap-confirmed in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eSingleTap, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::OnDoubleTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a double-tap in state %s\n", this,
+                  ToString(mState).c_str());
+
+  MOZ_ASSERT(IsRootForLayersId(),
+             "This function should be called for the root content APZC or "
+             "OOPIF root APZC");
+
+  CSSToCSSMatrix4x4 transformToRootContentApzc;
+  RefPtr<AsyncPanZoomController> rootContentApzc;
+  if (IsRootContent()) {
+    rootContentApzc = RefPtr{this};
+  } else {
+    if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+      rootContentApzc = treeManagerLocal->FindZoomableApzc(this);
+      if (rootContentApzc) {
+        MOZ_ASSERT(rootContentApzc->GetLayersId() != GetLayersId());
+        MOZ_ASSERT(this == treeManagerLocal->FindRootApzcFor(GetLayersId()));
+        transformToRootContentApzc =
+            treeManagerLocal->GetOopifToRootContentTransform(this);
+      }
+    }
+  }
+
+  if (!rootContentApzc) {
+    return nsEventStatus_eIgnore;
+  }
+
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    if (rootContentApzc->ZoomConstraintsAllowDoubleTapZoom() &&
+        (!GetCurrentTouchBlock() ||
+         GetCurrentTouchBlock()->TouchActionAllowsDoubleTapZoom())) {
+      if (Maybe<LayoutDevicePoint> geckoScreenPoint =
+              ConvertToGecko(aEvent.mPoint)) {
+        controller->HandleTap(
+            TapType::eDoubleTap, *geckoScreenPoint, aEvent.modifiers, GetGuid(),
+            GetCurrentTouchBlock() ? GetCurrentTouchBlock()->GetBlockId() : 0,
+            Some(DoubleTapToZoomMetrics{rootContentApzc->GetVisualViewport(),
+                                        rootContentApzc->GetScrollableRect(),
+                                        transformToRootContentApzc}));
+      }
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus AsyncPanZoomController::OnSecondTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a second-tap in state %s\n", this,
+                  ToString(mState).c_str());
+  return GenerateSingleTap(TapType::eSecondTap, aEvent.mPoint,
+                           aEvent.modifiers);
+}
+
+nsEventStatus AsyncPanZoomController::OnCancelTap(
+    const TapGestureInput& aEvent) {
+  APZC_LOG_DETAIL("got a cancel-tap in state %s\n", this,
+                  ToString(mState).c_str());
+  // XXX: Implement this.
+  return nsEventStatus_eIgnore;
+}
+
+ScreenToParentLayerMatrix4x4 AsyncPanZoomController::GetTransformToThis()
+    const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    return treeManagerLocal->GetScreenToApzcTransform(this);
+  }
+  return ScreenToParentLayerMatrix4x4();
+}
+
+ScreenPoint AsyncPanZoomController::ToScreenCoordinates(
+    const ParentLayerPoint& aVector, const ParentLayerPoint& aAnchor) const {
+  return TransformVector(GetTransformToThis().Inverse(), aVector, aAnchor);
+}
+
+// TODO: figure out a good way to check the w-coordinate is positive and return
+// the result
+ParentLayerPoint AsyncPanZoomController::ToParentLayerCoordinates(
+    const ScreenPoint& aVector, const ScreenPoint& aAnchor) const {
+  return TransformVector(GetTransformToThis(), aVector, aAnchor);
+}
+
+ParentLayerPoint AsyncPanZoomController::ToParentLayerCoordinates(
+    const ScreenPoint& aVector, const ExternalPoint& aAnchor) const {
+  return ToParentLayerCoordinates(
+      aVector,
+      ViewAs<ScreenPixel>(aAnchor, PixelCastJustification::ExternalIsScreen));
+}
+
+ExternalPoint AsyncPanZoomController::ToExternalPoint(
+    const ExternalPoint& aScreenOffset, const ScreenPoint& aScreenPoint) {
+  return aScreenOffset +
+         ViewAs<ExternalPixel>(aScreenPoint,
+                               PixelCastJustification::ExternalIsScreen);
+}
+
+ScreenPoint AsyncPanZoomController::PanVector(const ExternalPoint& aPos) const {
+  return ScreenPoint(fabs(aPos.x - mStartTouch.x),
+                     fabs(aPos.y - mStartTouch.y));
+}
+
+bool AsyncPanZoomController::Contains(const ScreenIntPoint& aPoint) const {
+  ScreenToParentLayerMatrix4x4 transformToThis = GetTransformToThis();
+  Maybe<ParentLayerIntPoint> point = UntransformBy(transformToThis, aPoint);
+  if (!point) {
+    return false;
+  }
+
+  ParentLayerIntRect cb;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    GetFrameMetrics().GetCompositionBounds().ToIntRect(&cb);
+  }
+  return cb.Contains(*point);
+}
+
+bool AsyncPanZoomController::IsInOverscrollGutter(
+    const ScreenPoint& aHitTestPoint) const {
+  if (!IsPhysicallyOverscrolled()) {
+    return false;
+  }
+
+  Maybe<ParentLayerPoint> apzcPoint =
+      UntransformBy(GetTransformToThis(), aHitTestPoint);
+  if (!apzcPoint) return false;
+  return IsInOverscrollGutter(*apzcPoint);
+}
+
+bool AsyncPanZoomController::IsInOverscrollGutter(
+    const ParentLayerPoint& aHitTestPoint) const {
+  ParentLayerRect compositionBounds;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    compositionBounds = GetFrameMetrics().GetCompositionBounds();
+  }
+  if (!compositionBounds.Contains(aHitTestPoint)) {
+    // Point is outside of scrollable element's bounds altogether.
+    return false;
+  }
+  auto overscrollTransform = GetOverscrollTransform(eForEventHandling);
+  ParentLayerPoint overscrollUntransformed =
+      overscrollTransform.Inverse().TransformPoint(aHitTestPoint);
+
+  if (compositionBounds.Contains(overscrollUntransformed)) {
+    // Point is over scrollable content.
+    return false;
+  }
+
+  // Point is in gutter.
+  return true;
+}
+
+bool AsyncPanZoomController::IsOverscrolled() const {
+  return mOverscrollEffect->IsOverscrolled();
+}
+
+bool AsyncPanZoomController::IsPhysicallyOverscrolled() const {
+  // As an optimization, avoid calling Apply/UnapplyAsyncTestAttributes
+  // unless we're in a test environment where we need it.
+  if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+    return mX.IsOverscrolled() || mY.IsOverscrolled();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.IsOverscrolled() || mY.IsOverscrolled();
+}
+
+bool AsyncPanZoomController::IsInInvalidOverscroll() const {
+  return mX.IsInInvalidOverscroll() || mY.IsInInvalidOverscroll();
+}
+
+ParentLayerPoint AsyncPanZoomController::PanStart() const {
+  return ParentLayerPoint(mX.PanStart(), mY.PanStart());
+}
+
+const ParentLayerPoint AsyncPanZoomController::GetVelocityVector() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return ParentLayerPoint(mX.GetVelocity(), mY.GetVelocity());
+}
+
+void AsyncPanZoomController::SetVelocityVector(
+    const ParentLayerPoint& aVelocityVector) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.SetVelocity(aVelocityVector.x);
+  mY.SetVelocity(aVelocityVector.y);
+}
+
+void AsyncPanZoomController::HandlePanningWithTouchAction(double aAngle) {
+  // Handling of cross sliding will need to be added in this method after
+  // touch-action released enabled by default.
+  MOZ_ASSERT(GetCurrentTouchBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentInputBlock()->GetOverscrollHandoffChain();
+  bool canScrollHorizontal =
+      !mX.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eHorizontal);
+  bool canScrollVertical =
+      !mY.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eVertical);
+  if (GetCurrentTouchBlock()->TouchActionAllowsPanningXY()) {
+    if (canScrollHorizontal && canScrollVertical) {
+      if (apz::IsCloseToHorizontal(aAngle,
+                                   StaticPrefs::apz_axis_lock_lock_angle())) {
+        mY.SetAxisLocked(true);
+        SetState(PANNING_LOCKED_X);
+      } else if (apz::IsCloseToVertical(
+                     aAngle, StaticPrefs::apz_axis_lock_lock_angle())) {
+        mX.SetAxisLocked(true);
+        SetState(PANNING_LOCKED_Y);
+      } else {
+        SetState(PANNING);
+      }
+    } else if (canScrollHorizontal || canScrollVertical) {
+      SetState(PANNING);
+    } else {
+      SetState(NOTHING);
+    }
+  } else if (GetCurrentTouchBlock()->TouchActionAllowsPanningX()) {
+    // Using bigger angle for panning to keep behavior consistent
+    // with IE.
+    if (apz::IsCloseToHorizontal(
+            aAngle, StaticPrefs::apz_axis_lock_direct_pan_angle())) {
+      mY.SetAxisLocked(true);
+      SetState(PANNING_LOCKED_X);
+      mPanDirRestricted = true;
+    } else {
+      // Don't treat these touches as pan/zoom movements since 'touch-action'
+      // value requires it.
+      SetState(NOTHING);
+    }
+  } else if (GetCurrentTouchBlock()->TouchActionAllowsPanningY()) {
+    if (apz::IsCloseToVertical(aAngle,
+                               StaticPrefs::apz_axis_lock_direct_pan_angle())) {
+      mX.SetAxisLocked(true);
+      SetState(PANNING_LOCKED_Y);
+      mPanDirRestricted = true;
+    } else {
+      SetState(NOTHING);
+    }
+  } else {
+    SetState(NOTHING);
+  }
+  if (!IsInPanningState()) {
+    // If we didn't enter a panning state because touch-action disallowed it,
+    // make sure to clear any leftover velocity from the pre-threshold
+    // touchmoves.
+    mX.SetVelocity(0);
+    mY.SetVelocity(0);
+  }
+}
+
+void AsyncPanZoomController::HandlePanning(double aAngle) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  MOZ_ASSERT(GetCurrentInputBlock());
+  RefPtr<const OverscrollHandoffChain> overscrollHandoffChain =
+      GetCurrentInputBlock()->GetOverscrollHandoffChain();
+  bool canScrollHorizontal =
+      !mX.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eHorizontal);
+  bool canScrollVertical =
+      !mY.IsAxisLocked() && overscrollHandoffChain->CanScrollInDirection(
+                                this, ScrollDirection::eVertical);
+
+  MOZ_ASSERT(UsingStatefulAxisLock());
+
+  if (!canScrollHorizontal || !canScrollVertical) {
+    SetState(PANNING);
+  } else if (apz::IsCloseToHorizontal(
+                 aAngle, StaticPrefs::apz_axis_lock_lock_angle())) {
+    mY.SetAxisLocked(true);
+    if (canScrollHorizontal) {
+      SetState(PANNING_LOCKED_X);
+    }
+  } else if (apz::IsCloseToVertical(aAngle,
+                                    StaticPrefs::apz_axis_lock_lock_angle())) {
+    mX.SetAxisLocked(true);
+    if (canScrollVertical) {
+      SetState(PANNING_LOCKED_Y);
+    }
+  } else {
+    SetState(PANNING);
+  }
+}
+
+void AsyncPanZoomController::HandlePanningUpdate(
+    const ScreenPoint& aPanDistance) {
+  // If we're axis-locked, check if the user is trying to break the lock
+  if (GetAxisLockMode() == STICKY && !mPanDirRestricted) {
+    ParentLayerPoint vector =
+        ToParentLayerCoordinates(aPanDistance, mStartTouch);
+
+    float angle = atan2f(vector.y, vector.x);  // range [-pi, pi]
+    angle = fabsf(angle);                      // range [0, pi]
+
+    float breakThreshold =
+        StaticPrefs::apz_axis_lock_breakout_threshold() * GetDPI();
+
+    if (fabs(aPanDistance.x) > breakThreshold ||
+        fabs(aPanDistance.y) > breakThreshold) {
+      switch (mState) {
+        case PANNING_LOCKED_X:
+          if (!apz::IsCloseToHorizontal(
+                  angle, StaticPrefs::apz_axis_lock_breakout_angle())) {
+            mY.SetAxisLocked(false);
+            // If we are within the lock angle from the Y axis, lock
+            // onto the Y axis.
+            if (apz::IsCloseToVertical(
+                    angle, StaticPrefs::apz_axis_lock_lock_angle())) {
+              mX.SetAxisLocked(true);
+              SetState(PANNING_LOCKED_Y);
+            } else {
+              SetState(PANNING);
+            }
+          }
+          break;
+
+        case PANNING_LOCKED_Y:
+          if (!apz::IsCloseToVertical(
+                  angle, StaticPrefs::apz_axis_lock_breakout_angle())) {
+            mX.SetAxisLocked(false);
+            // If we are within the lock angle from the X axis, lock
+            // onto the X axis.
+            if (apz::IsCloseToHorizontal(
+                    angle, StaticPrefs::apz_axis_lock_lock_angle())) {
+              mY.SetAxisLocked(true);
+              SetState(PANNING_LOCKED_X);
+            } else {
+              SetState(PANNING);
+            }
+          }
+          break;
+
+        case PANNING:
+          HandlePanning(angle);
+          break;
+
+        default:
+          break;
+      }
+    }
+  }
+}
+
+void AsyncPanZoomController::HandlePinchLocking(
+    const PinchGestureInput& aEvent) {
+  // Focus change and span distance calculated from an event buffer
+  // Used to handle pinch locking irrespective of touch screen sensitivity
+  // Note: both values fall back to the same value as
+  //       their un-buffered counterparts if there is only one (the latest)
+  //       event in the buffer. ie: when the touch screen is dispatching
+  //       events slower than the lifetime of the buffer
+  ParentLayerCoord bufferedSpanDistance;
+  ParentLayerPoint focusPoint, bufferedFocusChange;
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    focusPoint = mPinchEventBuffer.back().mLocalFocusPoint -
+                 Metrics().GetCompositionBounds().TopLeft();
+    ParentLayerPoint bufferedLastZoomFocus =
+        (mPinchEventBuffer.size() > 1)
+            ? mPinchEventBuffer.front().mLocalFocusPoint -
+                  Metrics().GetCompositionBounds().TopLeft()
+            : mLastZoomFocus;
+
+    bufferedFocusChange = bufferedLastZoomFocus - focusPoint;
+    bufferedSpanDistance = fabsf(mPinchEventBuffer.front().mPreviousSpan -
+                                 mPinchEventBuffer.back().mCurrentSpan);
+  }
+
+  // Convert to screen coordinates
+  ScreenCoord spanDistance =
+      ToScreenCoordinates(ParentLayerPoint(0, bufferedSpanDistance), focusPoint)
+          .Length();
+  ScreenPoint focusChange =
+      ToScreenCoordinates(bufferedFocusChange, focusPoint);
+
+  if (mPinchLocked) {
+    if (GetPinchLockMode() == PINCH_STICKY) {
+      ScreenCoord spanBreakoutThreshold =
+          StaticPrefs::apz_pinch_lock_span_breakout_threshold() * GetDPI();
+      mPinchLocked = !(spanDistance > spanBreakoutThreshold);
+    }
+  } else {
+    if (GetPinchLockMode() != PINCH_FREE) {
+      ScreenCoord spanLockThreshold =
+          StaticPrefs::apz_pinch_lock_span_lock_threshold() * GetDPI();
+      ScreenCoord scrollLockThreshold =
+          StaticPrefs::apz_pinch_lock_scroll_lock_threshold() * GetDPI();
+
+      if (spanDistance < spanLockThreshold &&
+          focusChange.Length() > scrollLockThreshold) {
+        mPinchLocked = true;
+
+        // We are transitioning to a two-finger pan that could trigger
+        // a fling at its end, so start tracking velocity.
+        StartTouch(aEvent.mLocalFocusPoint, aEvent.mTimeStamp);
+      }
+    }
+  }
+}
+
+nsEventStatus AsyncPanZoomController::StartPanning(
+    const ExternalPoint& aStartPoint, const TimeStamp& aEventTime) {
+  ParentLayerPoint vector =
+      ToParentLayerCoordinates(PanVector(aStartPoint), mStartTouch);
+  double angle = atan2(vector.y, vector.x);  // range [-pi, pi]
+  angle = fabs(angle);                       // range [0, pi]
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  HandlePanningWithTouchAction(angle);
+
+  if (IsInPanningState()) {
+    mTouchStartRestingTimeBeforePan = aEventTime - mTouchStartTime;
+    mMinimumVelocityDuringPan = Nothing();
+
+    if (RefPtr<GeckoContentController> controller =
+            GetGeckoContentController()) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eStartPanning);
+    }
+    return nsEventStatus_eConsumeNoDefault;
+  }
+  // Don't consume an event that didn't trigger a panning.
+  return nsEventStatus_eIgnore;
+}
+
+void AsyncPanZoomController::UpdateWithTouchAtDevicePoint(
+    const MultiTouchInput& aEvent) {
+  const SingleTouchData& touchData = aEvent.mTouches[0];
+  // Take historical touch data into account in order to improve the accuracy
+  // of the velocity estimate. On many Android devices, the touch screen samples
+  // at a higher rate than vsync (e.g. 100Hz vs 60Hz), and the historical data
+  // lets us take advantage of those high-rate samples.
+  for (const auto& historicalData : touchData.mHistoricalData) {
+    ParentLayerPoint historicalPoint = historicalData.mLocalScreenPoint;
+    mX.UpdateWithTouchAtDevicePoint(historicalPoint.x,
+                                    historicalData.mTimeStamp);
+    mY.UpdateWithTouchAtDevicePoint(historicalPoint.y,
+                                    historicalData.mTimeStamp);
+  }
+  ParentLayerPoint point = touchData.mLocalScreenPoint;
+  mX.UpdateWithTouchAtDevicePoint(point.x, aEvent.mTimeStamp);
+  mY.UpdateWithTouchAtDevicePoint(point.y, aEvent.mTimeStamp);
+}
+
+Maybe<CompositionPayload> AsyncPanZoomController::NotifyScrollSampling() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mSampledState.front().TakeScrollPayload();
+}
+
+bool AsyncPanZoomController::AttemptScroll(
+    ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
+    OverscrollHandoffState& aOverscrollHandoffState) {
+  // "start - end" rather than "end - start" because e.g. moving your finger
+  // down (*positive* direction along y axis) causes the vertical scroll offset
+  // to *decrease* as the page follows your finger.
+  ParentLayerPoint displacement = aStartPoint - aEndPoint;
+
+  ParentLayerPoint overscroll;  // will be used outside monitor block
+
+  // If the direction of panning is reversed within the same input block,
+  // a later event in the block could potentially scroll an APZC earlier
+  // in the handoff chain, than an earlier event in the block (because
+  // the earlier APZC was scrolled to its extent in the original direction).
+  // We want to disallow this.
+  bool scrollThisApzc = false;
+  if (InputBlockState* block = GetCurrentInputBlock()) {
+    scrollThisApzc =
+        !block->GetScrolledApzc() || block->IsDownchainOfScrolledApzc(this);
+  }
+
+  ParentLayerPoint adjustedDisplacement;
+  if (scrollThisApzc) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    bool respectDisregardedDirections =
+        ScrollSourceRespectsDisregardedDirections(
+            aOverscrollHandoffState.mScrollSource);
+    bool forcesVerticalOverscroll = respectDisregardedDirections &&
+                                    mScrollMetadata.GetDisregardedDirection() ==
+                                        Some(ScrollDirection::eVertical);
+    bool forcesHorizontalOverscroll =
+        respectDisregardedDirections &&
+        mScrollMetadata.GetDisregardedDirection() ==
+            Some(ScrollDirection::eHorizontal);
+
+    bool yChanged =
+        mY.AdjustDisplacement(displacement.y, adjustedDisplacement.y,
+                              overscroll.y, forcesVerticalOverscroll);
+    bool xChanged =
+        mX.AdjustDisplacement(displacement.x, adjustedDisplacement.x,
+                              overscroll.x, forcesHorizontalOverscroll);
+    if (xChanged || yChanged) {
+      ScheduleComposite();
+    }
+
+    if (!IsZero(adjustedDisplacement) &&
+        Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+      ScrollBy(adjustedDisplacement / Metrics().GetZoom());
+      if (InputBlockState* block = GetCurrentInputBlock()) {
+        bool displacementIsUserVisible = true;
+
+        {  // Release the APZC lock before calling ToScreenCoordinates which
+          // acquires the APZ tree lock. Note that this just unlocks the mutex
+          // once, so if we're locking it multiple times on the callstack then
+          // this will be insufficient.
+          RecursiveMutexAutoUnlock unlock(mRecursiveMutex);
+
+          ScreenIntPoint screenDisplacement = RoundedToInt(
+              ToScreenCoordinates(adjustedDisplacement, aStartPoint));
+          // If the displacement we just applied rounds to zero in screen space,
+          // then it's probably not going to be visible to the user. In that
+          // case let's not mark this APZC as scrolled, so that even if the
+          // immediate handoff pref is disabled, we'll allow doing the handoff
+          // to the next APZC.
+          if (screenDisplacement == ScreenIntPoint()) {
+            displacementIsUserVisible = false;
+          }
+        }
+        if (displacementIsUserVisible) {
+          block->SetScrolledApzc(this);
+        }
+      }
+      // Note that in the case of instant scrolling, the last snap target ids
+      // will be set after AttemptScroll call so that we can clobber them
+      // unconditionally here.
+      mLastSnapTargetIds = ScrollSnapTargetIds{};
+      ScheduleCompositeAndMaybeRepaint();
+    }
+
+    // Adjust the start point to reflect the consumed portion of the scroll.
+    aStartPoint = aEndPoint + overscroll;
+  } else {
+    overscroll = displacement;
+  }
+
+  // Accumulate the amount of actual scrolling that occurred into the handoff
+  // state. Note that ToScreenCoordinates() needs to be called outside the
+  // mutex.
+  if (!IsZero(adjustedDisplacement)) {
+    aOverscrollHandoffState.mTotalMovement +=
+        ToScreenCoordinates(adjustedDisplacement, aEndPoint);
+  }
+
+  // If we consumed the entire displacement as a normal scroll, great.
+  if (IsZero(overscroll)) {
+    return true;
+  }
+
+  if (AllowScrollHandoffInCurrentBlock()) {
+    // If there is overscroll, first try to hand it off to an APZC later
+    // in the handoff chain to consume (either as a normal scroll or as
+    // overscroll).
+    // Note: "+ overscroll" rather than "- overscroll" because "overscroll"
+    // is what's left of "displacement", and "displacement" is "start - end".
+    ++aOverscrollHandoffState.mChainIndex;
+    bool consumed =
+        CallDispatchScroll(aStartPoint, aEndPoint, aOverscrollHandoffState);
+    if (consumed) {
+      return true;
+    }
+
+    overscroll = aStartPoint - aEndPoint;
+    MOZ_ASSERT(!IsZero(overscroll));
+  }
+
+  // If there is no APZC later in the handoff chain that accepted the
+  // overscroll, try to accept it ourselves. We only accept it if we
+  // are pannable.
+  if (ScrollSourceAllowsOverscroll(aOverscrollHandoffState.mScrollSource)) {
+    APZC_LOG("%p taking overscroll during panning\n", this);
+
+    ParentLayerPoint prevVisualOverscroll = GetOverscrollAmount();
+
+    OverscrollForPanning(overscroll, aOverscrollHandoffState.mPanDistance);
+
+    // Accumulate the amount of change to the overscroll that occurred into the
+    // handoff state. Note that the input amount, |overscroll|, is turned into
+    // some smaller visual overscroll amount (queried via GetOverscrollAmount())
+    // by applying resistance (Axis::ApplyResistance()), and it's the latter we
+    // want to count towards OverscrollHandoffState::mTotalMovement.
+    ParentLayerPoint visualOverscrollChange =
+        GetOverscrollAmount() - prevVisualOverscroll;
+    if (!IsZero(visualOverscrollChange)) {
+      aOverscrollHandoffState.mTotalMovement +=
+          ToScreenCoordinates(visualOverscrollChange, aEndPoint);
+    }
+  }
+
+  aStartPoint = aEndPoint + overscroll;
+
+  return IsZero(overscroll);
+}
+
+void AsyncPanZoomController::OverscrollForPanning(
+    ParentLayerPoint& aOverscroll, const ScreenPoint& aPanDistance) {
+  // Only allow entering overscroll along an axis if the pan distance along
+  // that axis is greater than the pan distance along the other axis by a
+  // configurable factor. If we are already overscrolled, don't check this.
+  if (!IsOverscrolled()) {
+    if (aPanDistance.x <
+        StaticPrefs::apz_overscroll_min_pan_distance_ratio() * aPanDistance.y) {
+      aOverscroll.x = 0;
+    }
+    if (aPanDistance.y <
+        StaticPrefs::apz_overscroll_min_pan_distance_ratio() * aPanDistance.x) {
+      aOverscroll.y = 0;
+    }
+  }
+
+  OverscrollBy(aOverscroll);
+}
+
+ScrollDirections AsyncPanZoomController::GetOverscrollableDirections() const {
+  ScrollDirections result;
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  // If the target has the disregarded direction, it means it's single line
+  // text control, thus we don't want to overscroll in both directions.
+  if (mScrollMetadata.GetDisregardedDirection()) {
+    return result;
+  }
+
+  if (mX.CanScroll() && mX.OverscrollBehaviorAllowsOverscrollEffect()) {
+    result += ScrollDirection::eHorizontal;
+  }
+
+  if (mY.CanScroll() && mY.OverscrollBehaviorAllowsOverscrollEffect()) {
+    result += ScrollDirection::eVertical;
+  }
+
+  return result;
+}
+
+void AsyncPanZoomController::OverscrollBy(ParentLayerPoint& aOverscroll) {
+  if (!StaticPrefs::apz_overscroll_enabled()) {
+    return;
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // Do not go into overscroll in a direction in which we have no room to
+  // scroll to begin with.
+  ScrollDirections overscrollableDirections = GetOverscrollableDirections();
+  if (IsZero(aOverscroll.x)) {
+    overscrollableDirections -= ScrollDirection::eHorizontal;
+  }
+  if (IsZero(aOverscroll.y)) {
+    overscrollableDirections -= ScrollDirection::eVertical;
+  }
+
+  mOverscrollEffect->ConsumeOverscroll(aOverscroll, overscrollableDirections);
+}
+
+RefPtr<const OverscrollHandoffChain>
+AsyncPanZoomController::BuildOverscrollHandoffChain() {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    return treeManagerLocal->BuildOverscrollHandoffChain(this);
+  }
+
+  // This APZC IsDestroyed(). To avoid callers having to special-case this
+  // scenario, just build a 1-element chain containing ourselves.
+  OverscrollHandoffChain* result = new OverscrollHandoffChain;
+  result->Add(this);
+  return result;
+}
+
+ParentLayerPoint AsyncPanZoomController::AttemptFling(
+    const FlingHandoffState& aHandoffState) {
+  // The PLPPI computation acquires the tree lock, so it needs to be performed
+  // on the controller thread, and before the APZC lock is acquired.
+  APZThreadUtils::AssertOnControllerThread();
+  float PLPPI = ComputePLPPI(PanStart(), aHandoffState.mVelocity);
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (!IsPannable()) {
+    return aHandoffState.mVelocity;
+  }
+
+  // We may have a pre-existing velocity for whatever reason (for example,
+  // a previously handed off fling). We don't want to clobber that.
+  APZC_LOG("%p accepting fling with velocity %s\n", this,
+           ToString(aHandoffState.mVelocity).c_str());
+  ParentLayerPoint residualVelocity = aHandoffState.mVelocity;
+  if (mX.CanScroll()) {
+    mX.SetVelocity(mX.GetVelocity() + aHandoffState.mVelocity.x);
+    residualVelocity.x = 0;
+  }
+  if (mY.CanScroll()) {
+    mY.SetVelocity(mY.GetVelocity() + aHandoffState.mVelocity.y);
+    residualVelocity.y = 0;
+  }
+
+  // If we're not scrollable in at least one of the directions in which we
+  // were handed velocity, don't start a fling animation.
+  // The |IsFinite()| condition should only fail when running some tests
+  // that generate events faster than the clock resolution.
+  ParentLayerPoint velocity = GetVelocityVector();
+  if (!velocity.IsFinite() ||
+      velocity.Length() <= StaticPrefs::apz_fling_min_velocity_threshold()) {
+    // Relieve overscroll now if needed, since we will not transition to a fling
+    // animation and then an overscroll animation, and relieve it then.
+    aHandoffState.mChain->SnapBackOverscrolledApzc(this);
+    return residualVelocity;
+  }
+
+  // If there's a scroll snap point near the predicted fling destination,
+  // scroll there using a smooth scroll animation. Otherwise, start a
+  // fling animation.
+  ScrollSnapToDestination();
+  if (mState != SMOOTHMSD_SCROLL) {
+    SetState(FLING);
+    RefPtr<AsyncPanZoomAnimation> fling =
+        GetPlatformSpecificState()->CreateFlingAnimation(*this, aHandoffState,
+                                                         PLPPI);
+    StartAnimation(fling.forget());
+  }
+
+  return residualVelocity;
+}
+
+float AsyncPanZoomController::ComputePLPPI(ParentLayerPoint aPoint,
+                                           ParentLayerPoint aDirection) const {
+  // Avoid division-by-zero.
+  if (aDirection == ParentLayerPoint()) {
+    return GetDPI();
+  }
+
+  // Convert |aDirection| into a unit vector.
+  aDirection = aDirection / aDirection.Length();
+
+  // Place the vector at |aPoint| and convert to screen coordinates.
+  // The length of the resulting vector is the number of Screen coordinates
+  // that equal 1 ParentLayer coordinate in the given direction.
+  float screenPerParent = ToScreenCoordinates(aDirection, aPoint).Length();
+
+  // Finally, factor in the DPI scale.
+  return GetDPI() / screenPerParent;
+}
+
+Maybe<CSSPoint> AsyncPanZoomController::GetCurrentAnimationDestination(
+    const RecursiveMutexAutoLock& aProofOfLock) const {
+  if (mState == WHEEL_SCROLL) {
+    return Some(mAnimation->AsWheelScrollAnimation()->GetDestination());
+  }
+  if (mState == SMOOTH_SCROLL) {
+    return Some(mAnimation->AsSmoothScrollAnimation()->GetDestination());
+  }
+  if (mState == SMOOTHMSD_SCROLL) {
+    return Some(mAnimation->AsSmoothMsdScrollAnimation()->GetDestination());
+  }
+  if (mState == KEYBOARD_SCROLL) {
+    return Some(mAnimation->AsSmoothScrollAnimation()->GetDestination());
+  }
+
+  return Nothing();
+}
+
+ParentLayerPoint
+AsyncPanZoomController::AdjustHandoffVelocityForOverscrollBehavior(
+    ParentLayerPoint& aHandoffVelocity) const {
+  ParentLayerPoint residualVelocity;
+  ScrollDirections handoffDirections = GetAllowedHandoffDirections();
+  if (!handoffDirections.contains(ScrollDirection::eHorizontal)) {
+    residualVelocity.x = aHandoffVelocity.x;
+    aHandoffVelocity.x = 0;
+  }
+  if (!handoffDirections.contains(ScrollDirection::eVertical)) {
+    residualVelocity.y = aHandoffVelocity.y;
+    aHandoffVelocity.y = 0;
+  }
+  return residualVelocity;
+}
+
+bool AsyncPanZoomController::OverscrollBehaviorAllowsSwipe() const {
+  // Swipe navigation is a "non-local" overscroll behavior like handoff.
+  return GetAllowedHandoffDirections().contains(ScrollDirection::eHorizontal);
+}
+
+void AsyncPanZoomController::HandleFlingOverscroll(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits,
+    const RefPtr<const OverscrollHandoffChain>& aOverscrollHandoffChain,
+    const RefPtr<const AsyncPanZoomController>& aScrolledApzc) {
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  if (treeManagerLocal) {
+    const FlingHandoffState handoffState{
+        aVelocity, aOverscrollHandoffChain, Nothing(),
+        0,         true /* handoff */,      aScrolledApzc};
+    ParentLayerPoint residualVelocity =
+        treeManagerLocal->DispatchFling(this, handoffState);
+    FLING_LOG("APZC %p left with residual velocity %s\n", this,
+              ToString(residualVelocity).c_str());
+    if (!IsZero(residualVelocity) && IsPannable() &&
+        StaticPrefs::apz_overscroll_enabled()) {
+      // Obey overscroll-behavior.
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      if (!mX.OverscrollBehaviorAllowsOverscrollEffect()) {
+        residualVelocity.x = 0;
+      }
+      if (!mY.OverscrollBehaviorAllowsOverscrollEffect()) {
+        residualVelocity.y = 0;
+      }
+
+      if (!IsZero(residualVelocity)) {
+        mOverscrollEffect->RelieveOverscroll(residualVelocity,
+                                             aOverscrollSideBits);
+      }
+    }
+  }
+}
+
+void AsyncPanZoomController::HandleSmoothScrollOverscroll(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits) {
+  // We must call BuildOverscrollHandoffChain from this deferred callback
+  // function in order to avoid a deadlock when acquiring the tree lock.
+  HandleFlingOverscroll(aVelocity, aOverscrollSideBits,
+                        BuildOverscrollHandoffChain(), nullptr);
+}
+
+ParentLayerPoint AsyncPanZoomController::ConvertDestinationToDelta(
+    CSSPoint& aDestination) const {
+  ParentLayerPoint startPoint, endPoint;
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    startPoint = aDestination * Metrics().GetZoom();
+    endPoint = Metrics().GetVisualScrollOffset() * Metrics().GetZoom();
+  }
+
+  return startPoint - endPoint;
+}
+
+void AsyncPanZoomController::SmoothScrollTo(
+    CSSSnapDestination&& aDestination,
+    ScrollTriggeredByScript aTriggeredByScript, const ScrollOrigin& aOrigin) {
+  // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s and then
+  // to appunits/second.
+  nsPoint destination = CSSPoint::ToAppUnits(aDestination.mPosition);
+  nsSize velocity;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    velocity = CSSSize::ToAppUnits(ParentLayerSize(mX.GetVelocity() * 1000.0f,
+                                                   mY.GetVelocity() * 1000.0f) /
+                                   Metrics().GetZoom());
+  }
+
+  if (mState == SMOOTH_SCROLL && mAnimation) {
+    RefPtr<SmoothScrollAnimation> animation(
+        mAnimation->AsSmoothScrollAnimation());
+    if (animation->GetScrollOrigin() == aOrigin) {
+      APZC_LOG("%p updating destination on existing animation\n", this);
+      animation->UpdateDestinationAndSnapTargets(
+          GetFrameTime().Time(), destination, velocity,
+          std::move(aDestination.mTargetIds), aTriggeredByScript);
+      return;
+    }
+  }
+
+  CancelAnimation();
+
+  // If no scroll is required, we should exit early to avoid triggering
+  // a scrollend event when no scrolling occurred.
+  if (ConvertDestinationToDelta(aDestination.mPosition) == ParentLayerPoint()) {
+    return;
+  }
+
+  SetState(SMOOTH_SCROLL);
+  nsPoint initialPosition =
+      CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset());
+  RefPtr<SmoothScrollAnimation> animation =
+      new SmoothScrollAnimation(*this, initialPosition, aOrigin);
+  animation->UpdateDestinationAndSnapTargets(
+      GetFrameTime().Time(), destination, velocity,
+      std::move(aDestination.mTargetIds), aTriggeredByScript);
+  StartAnimation(animation.forget());
+}
+
+void AsyncPanZoomController::SmoothMsdScrollTo(
+    CSSSnapDestination&& aDestination,
+    ScrollTriggeredByScript aTriggeredByScript) {
+  if (mState == SMOOTHMSD_SCROLL && mAnimation) {
+    APZC_LOG("%p updating destination on existing animation\n", this);
+    RefPtr<SmoothMsdScrollAnimation> animation(
+        static_cast<SmoothMsdScrollAnimation*>(mAnimation.get()));
+    animation->SetDestination(aDestination.mPosition,
+                              std::move(aDestination.mTargetIds),
+                              aTriggeredByScript);
+    return;
+  }
+
+  // If no scroll is required, we should exit early to avoid triggering
+  // a scrollend event when no scrolling occurred.
+  if (ConvertDestinationToDelta(aDestination.mPosition) == ParentLayerPoint()) {
+    return;
+  }
+  CancelAnimation();
+  SetState(SMOOTHMSD_SCROLL);
+  // Convert velocity from ParentLayerPoints/ms to ParentLayerPoints/s.
+  CSSPoint initialVelocity;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    initialVelocity = ParentLayerPoint(mX.GetVelocity() * 1000.0f,
+                                       mY.GetVelocity() * 1000.0f) /
+                      Metrics().GetZoom();
+  }
+
+  StartAnimation(do_AddRef(new SmoothMsdScrollAnimation(
+      *this, Metrics().GetVisualScrollOffset(), initialVelocity,
+      aDestination.mPosition,
+      StaticPrefs::layout_css_scroll_behavior_spring_constant(),
+      StaticPrefs::layout_css_scroll_behavior_damping_ratio(),
+      std::move(aDestination.mTargetIds), aTriggeredByScript)));
+}
+
+void AsyncPanZoomController::StartOverscrollAnimation(
+    const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits) {
+  MOZ_ASSERT(mState != OVERSCROLL_ANIMATION);
+
+  SetState(OVERSCROLL_ANIMATION);
+
+  ParentLayerPoint velocity = aVelocity;
+  AdjustDeltaForAllowedScrollDirections(velocity,
+                                        GetOverscrollableDirections());
+  StartAnimation(
+      do_AddRef(new OverscrollAnimation(*this, velocity, aOverscrollSideBits)));
+}
+
+bool AsyncPanZoomController::CallDispatchScroll(
+    ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
+    OverscrollHandoffState& aOverscrollHandoffState) {
+  // Make a local copy of the tree manager pointer and check if it's not
+  // null before calling DispatchScroll(). This is necessary because
+  // Destroy(), which nulls out mTreeManager, could be called concurrently.
+  APZCTreeManager* treeManagerLocal = GetApzcTreeManager();
+  if (!treeManagerLocal) {
+    return false;
+  }
+
+  // Obey overscroll-behavior.
+  ParentLayerPoint endPoint = aEndPoint;
+  if (aOverscrollHandoffState.mChainIndex > 0) {
+    ScrollDirections handoffDirections = GetAllowedHandoffDirections();
+    if (!handoffDirections.contains(ScrollDirection::eHorizontal)) {
+      endPoint.x = aStartPoint.x;
+    }
+    if (!handoffDirections.contains(ScrollDirection::eVertical)) {
+      endPoint.y = aStartPoint.y;
+    }
+    if (aStartPoint == endPoint) {
+      // Handoff not allowed in either direction - don't even bother.
+      return false;
+    }
+  }
+
+  return treeManagerLocal->DispatchScroll(this, aStartPoint, endPoint,
+                                          aOverscrollHandoffState);
+}
+
+void AsyncPanZoomController::RecordScrollPayload(const TimeStamp& aTimeStamp) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (!mScrollPayload) {
+    mScrollPayload = Some(
+        CompositionPayload{CompositionPayloadType::eAPZScroll, aTimeStamp});
+  }
+}
+
+void AsyncPanZoomController::StartTouch(const ParentLayerPoint& aPoint,
+                                        TimeStamp aTimestamp) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.StartTouch(aPoint.x, aTimestamp);
+  mY.StartTouch(aPoint.y, aTimestamp);
+}
+
+void AsyncPanZoomController::EndTouch(TimeStamp aTimestamp,
+                                      Axis::ClearAxisLock aClearAxisLock) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.EndTouch(aTimestamp, aClearAxisLock);
+  mY.EndTouch(aTimestamp, aClearAxisLock);
+}
+
+void AsyncPanZoomController::TrackTouch(const MultiTouchInput& aEvent) {
+  ExternalPoint extPoint = GetFirstExternalTouchPoint(aEvent);
+  ScreenPoint panVector = PanVector(extPoint);
+  HandlePanningUpdate(panVector);
+
+  ParentLayerPoint prevTouchPoint(mX.GetPos(), mY.GetPos());
+  ParentLayerPoint touchPoint = GetFirstTouchPoint(aEvent);
+
+  UpdateWithTouchAtDevicePoint(aEvent);
+
+  auto velocity = GetVelocityVector().Length();
+  if (mMinimumVelocityDuringPan) {
+    mMinimumVelocityDuringPan =
+        Some(std::min(*mMinimumVelocityDuringPan, velocity));
+  } else {
+    mMinimumVelocityDuringPan = Some(velocity);
+  }
+
+  if (prevTouchPoint != touchPoint) {
+    MOZ_ASSERT(GetCurrentTouchBlock());
+    OverscrollHandoffState handoffState(
+        *GetCurrentTouchBlock()->GetOverscrollHandoffChain(), panVector,
+        ScrollSource::Touchscreen);
+    RecordScrollPayload(aEvent.mTimeStamp);
+    CallDispatchScroll(prevTouchPoint, touchPoint, handoffState);
+  }
+}
+
+ParentLayerPoint AsyncPanZoomController::GetFirstTouchPoint(
+    const MultiTouchInput& aEvent) {
+  return ((SingleTouchData&)aEvent.mTouches[0]).mLocalScreenPoint;
+}
+
+ExternalPoint AsyncPanZoomController::GetFirstExternalTouchPoint(
+    const MultiTouchInput& aEvent) {
+  return ToExternalPoint(aEvent.mScreenOffset,
+                         ((SingleTouchData&)aEvent.mTouches[0]).mScreenPoint);
+}
+
+ParentLayerPoint AsyncPanZoomController::GetOverscrollAmount() const {
+  if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+    return GetOverscrollAmountInternal();
+  }
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return GetOverscrollAmountInternal();
+}
+
+ParentLayerPoint AsyncPanZoomController::GetOverscrollAmountInternal() const {
+  return {mX.GetOverscroll(), mY.GetOverscroll()};
+}
+
+SideBits AsyncPanZoomController::GetOverscrollSideBits() const {
+  return apz::GetOverscrollSideBits({mX.GetOverscroll(), mY.GetOverscroll()});
+}
+
+void AsyncPanZoomController::RestoreOverscrollAmount(
+    const ParentLayerPoint& aOverscroll) {
+  mX.RestoreOverscroll(aOverscroll.x);
+  mY.RestoreOverscroll(aOverscroll.y);
+}
+
+void AsyncPanZoomController::StartAnimation(
+    already_AddRefed<AsyncPanZoomAnimation> aAnimation) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mAnimation = aAnimation;
+  mLastSampleTime = GetFrameTime();
+  ScheduleComposite();
+}
+
+void AsyncPanZoomController::CancelAnimation(CancelAnimationFlags aFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  APZC_LOG_DETAIL("running CancelAnimation(0x%x) in state %s\n", this, aFlags,
+                  ToString(mState).c_str());
+
+  if ((aFlags & ExcludeWheel) && mState == WHEEL_SCROLL) {
+    return;
+  }
+
+  if (mAnimation) {
+    mAnimation->Cancel(aFlags);
+  }
+
+  SetState(NOTHING);
+  mLastSnapTargetIds = ScrollSnapTargetIds{};
+  mAnimation = nullptr;
+  // Since there is no animation in progress now the axes should
+  // have no velocity either. If we are dropping the velocity from a non-zero
+  // value we should trigger a repaint as the displayport margins are dependent
+  // on the velocity and the last repaint request might not have good margins
+  // any more.
+  bool repaint = !IsZero(GetVelocityVector());
+  mX.SetVelocity(0);
+  mY.SetVelocity(0);
+  mX.SetAxisLocked(false);
+  mY.SetAxisLocked(false);
+  // Setting the state to nothing and cancelling the animation can
+  // preempt normal mechanisms for relieving overscroll, so we need to clear
+  // overscroll here.
+  if (!(aFlags & ExcludeOverscroll) && IsOverscrolled()) {
+    ClearOverscroll();
+    repaint = true;
+  }
+  // Similar to relieving overscroll, we also need to snap to any snap points
+  // if appropriate.
+  if (aFlags & CancelAnimationFlags::ScrollSnap) {
+    ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+  }
+  if (repaint) {
+    RequestContentRepaint();
+    ScheduleComposite();
+  }
+}
+
+void AsyncPanZoomController::ClearOverscroll() {
+  mOverscrollEffect->ClearOverscroll();
+}
+
+void AsyncPanZoomController::ClearPhysicalOverscroll() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mX.ClearOverscroll();
+  mY.ClearOverscroll();
+}
+
+void AsyncPanZoomController::SetCompositorController(
+    CompositorController* aCompositorController) {
+  mCompositorController = aCompositorController;
+}
+
+void AsyncPanZoomController::SetVisualScrollOffset(const CSSPoint& aOffset) {
+  Metrics().SetVisualScrollOffset(aOffset);
+  Metrics().RecalculateLayoutViewportOffset();
+}
+
+void AsyncPanZoomController::ClampAndSetVisualScrollOffset(
+    const CSSPoint& aOffset) {
+  Metrics().ClampAndSetVisualScrollOffset(aOffset);
+  Metrics().RecalculateLayoutViewportOffset();
+}
+
+void AsyncPanZoomController::ScrollBy(const CSSPoint& aOffset) {
+  SetVisualScrollOffset(Metrics().GetVisualScrollOffset() + aOffset);
+}
+
+void AsyncPanZoomController::ScrollByAndClamp(const CSSPoint& aOffset) {
+  ClampAndSetVisualScrollOffset(Metrics().GetVisualScrollOffset() + aOffset);
+}
+
+void AsyncPanZoomController::ScaleWithFocus(float aScale,
+                                            const CSSPoint& aFocus) {
+  Metrics().ZoomBy(aScale);
+  // We want to adjust the scroll offset such that the CSS point represented by
+  // aFocus remains at the same position on the screen before and after the
+  // change in zoom. The below code accomplishes this; see
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=923431#c6 for an in-depth
+  // explanation of how.
+  SetVisualScrollOffset((Metrics().GetVisualScrollOffset() + aFocus) -
+                        (aFocus / aScale));
+}
+
+/*static*/
+gfx::IntSize AsyncPanZoomController::GetDisplayportAlignmentMultiplier(
+    const ScreenSize& aBaseSize) {
+  gfx::IntSize multiplier(1, 1);
+  float baseWidth = aBaseSize.width;
+  while (baseWidth > 500) {
+    baseWidth /= 2;
+    multiplier.width *= 2;
+    if (multiplier.width >= 8) {
+      break;
+    }
+  }
+  float baseHeight = aBaseSize.height;
+  while (baseHeight > 500) {
+    baseHeight /= 2;
+    multiplier.height *= 2;
+    if (multiplier.height >= 8) {
+      break;
+    }
+  }
+  return multiplier;
+}
+
+/**
+ * Enlarges the displayport along both axes based on the velocity.
+ */
+static CSSSize CalculateDisplayPortSize(
+    const CSSSize& aCompositionSize, const CSSPoint& aVelocity,
+    AsyncPanZoomController::ZoomInProgress aZoomInProgress,
+    const CSSToScreenScale2D& aDpPerCSS) {
+  bool xIsStationarySpeed =
+      fabsf(aVelocity.x) < StaticPrefs::apz_min_skate_speed();
+  bool yIsStationarySpeed =
+      fabsf(aVelocity.y) < StaticPrefs::apz_min_skate_speed();
+  float xMultiplier = xIsStationarySpeed
+                          ? StaticPrefs::apz_x_stationary_size_multiplier()
+                          : StaticPrefs::apz_x_skate_size_multiplier();
+  float yMultiplier = yIsStationarySpeed
+                          ? StaticPrefs::apz_y_stationary_size_multiplier()
+                          : StaticPrefs::apz_y_skate_size_multiplier();
+
+  if (IsHighMemSystem() && !xIsStationarySpeed) {
+    xMultiplier += StaticPrefs::apz_x_skate_highmem_adjust();
+  }
+
+  if (IsHighMemSystem() && !yIsStationarySpeed) {
+    yMultiplier += StaticPrefs::apz_y_skate_highmem_adjust();
+  }
+
+  if (aZoomInProgress == AsyncPanZoomController::ZoomInProgress::Yes) {
+    // If a zoom is in progress, we will be making content visible on the
+    // x and y axes in equal proportion, because the zoom operation scales
+    // equally on the x and y axes. The default multipliers computed above are
+    // biased towards the y-axis since that's where most scrolling occurs, but
+    // in the case of zooming, we should really use equal multipliers on both
+    // axes. This does that while preserving the total displayport area
+    // quantity (aCompositionSize.Area() * xMultiplier * yMultiplier).
+    // Note that normally changing the shape of the displayport is expensive
+    // and should be avoided, but if a zoom is in progress the displayport
+    // is likely going to be fully repainted anyway due to changes in resolution
+    // so there should be no marginal cost to also changing the shape of it.
+    float areaMultiplier = xMultiplier * yMultiplier;
+    xMultiplier = sqrt(areaMultiplier);
+    yMultiplier = xMultiplier;
+  }
+
+  // Scale down the margin multipliers by the alignment multiplier because
+  // the alignment code will expand the displayport outward to the multiplied
+  // alignment. This is not necessary for correctness, but for performance;
+  // if we don't do this the displayport can end up much larger. The math here
+  // is actually just scaling the part of the multipler that is > 1, so that
+  // we never end up with xMultiplier or yMultiplier being less than 1 (that
+  // would result in a guaranteed checkerboarding situation). Note that the
+  // calculation doesn't cancel exactly the increased margin from applying
+  // the alignment multiplier, but this is simple and should provide
+  // reasonable behaviour in most cases.
+  gfx::IntSize alignmentMultipler =
+      AsyncPanZoomController::GetDisplayportAlignmentMultiplier(
+          aCompositionSize * aDpPerCSS);
+  if (xMultiplier > 1) {
+    xMultiplier = ((xMultiplier - 1) / alignmentMultipler.width) + 1;
+  }
+  if (yMultiplier > 1) {
+    yMultiplier = ((yMultiplier - 1) / alignmentMultipler.height) + 1;
+  }
+
+  return aCompositionSize * CSSSize(xMultiplier, yMultiplier);
+}
+
+/**
+ * Ensures that the displayport is at least as large as the visible area
+ * inflated by the danger zone. If this is not the case then the
+ * "AboutToCheckerboard" function in TiledContentClient.cpp will return true
+ * even in the stable state.
+ */
+static CSSSize ExpandDisplayPortToDangerZone(
+    const CSSSize& aDisplayPortSize, const FrameMetrics& aFrameMetrics) {
+  CSSSize dangerZone(0.0f, 0.0f);
+  if (aFrameMetrics.DisplayportPixelsPerCSSPixel().xScale != 0 &&
+      aFrameMetrics.DisplayportPixelsPerCSSPixel().yScale != 0) {
+    dangerZone = ScreenSize(StaticPrefs::apz_danger_zone_x(),
+                            StaticPrefs::apz_danger_zone_y()) /
+                 aFrameMetrics.DisplayportPixelsPerCSSPixel();
+  }
+  const CSSSize compositionSize =
+      aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels();
+
+  const float xSize = std::max(aDisplayPortSize.width,
+                               compositionSize.width + (2 * dangerZone.width));
+
+  const float ySize =
+      std::max(aDisplayPortSize.height,
+               compositionSize.height + (2 * dangerZone.height));
+
+  return CSSSize(xSize, ySize);
+}
+
+/**
+ * Attempts to redistribute any area in the displayport that would get clipped
+ * by the scrollable rect, or be inaccessible due to disabled scrolling, to the
+ * other axis, while maintaining total displayport area.
+ */
+static void RedistributeDisplayPortExcess(CSSSize& aDisplayPortSize,
+                                          const CSSRect& aScrollableRect) {
+  // As aDisplayPortSize.height * aDisplayPortSize.width does not change,
+  // we are just scaling by the ratio and its inverse.
+  if (aDisplayPortSize.height > aScrollableRect.Height()) {
+    aDisplayPortSize.width *=
+        (aDisplayPortSize.height / aScrollableRect.Height());
+    aDisplayPortSize.height = aScrollableRect.Height();
+  } else if (aDisplayPortSize.width > aScrollableRect.Width()) {
+    aDisplayPortSize.height *=
+        (aDisplayPortSize.width / aScrollableRect.Width());
+    aDisplayPortSize.width = aScrollableRect.Width();
+  }
+}
+
+/* static */
+const ScreenMargin AsyncPanZoomController::CalculatePendingDisplayPort(
+    const FrameMetrics& aFrameMetrics, const ParentLayerPoint& aVelocity,
+    ZoomInProgress aZoomInProgress) {
+  if (aFrameMetrics.IsScrollInfoLayer()) {
+    // Don't compute margins. Since we can't asynchronously scroll this frame,
+    // we don't want to paint anything more than the composition bounds.
+    return ScreenMargin();
+  }
+
+  CSSSize compositionSize =
+      aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels();
+  CSSPoint velocity;
+  if (aFrameMetrics.GetZoom() != CSSToParentLayerScale(0)) {
+    velocity = aVelocity / aFrameMetrics.GetZoom();  // avoid division by zero
+  }
+  CSSRect scrollableRect = aFrameMetrics.GetExpandedScrollableRect();
+
+  // Calculate the displayport size based on how fast we're moving along each
+  // axis.
+  CSSSize displayPortSize =
+      CalculateDisplayPortSize(compositionSize, velocity, aZoomInProgress,
+                               aFrameMetrics.DisplayportPixelsPerCSSPixel());
+
+  displayPortSize =
+      ExpandDisplayPortToDangerZone(displayPortSize, aFrameMetrics);
+
+  if (StaticPrefs::apz_enlarge_displayport_when_clipped()) {
+    RedistributeDisplayPortExcess(displayPortSize, scrollableRect);
+  }
+
+  // We calculate a "displayport" here which is relative to the scroll offset.
+  // Note that the scroll offset we have here in the APZ code may not be the
+  // same as the base rect that gets used on the layout side when the
+  // displayport margins are actually applied, so it is important to only
+  // consider the displayport as margins relative to a scroll offset rather than
+  // relative to something more unchanging like the scrollable rect origin.
+
+  // Center the displayport based on its expansion over the composition size.
+  CSSRect displayPort((compositionSize.width - displayPortSize.width) / 2.0f,
+                      (compositionSize.height - displayPortSize.height) / 2.0f,
+                      displayPortSize.width, displayPortSize.height);
+
+  // Offset the displayport, depending on how fast we're moving and the
+  // estimated time it takes to paint, to try to minimise checkerboarding.
+  float paintFactor = kDefaultEstimatedPaintDurationMs;
+  displayPort.MoveBy(velocity * paintFactor * StaticPrefs::apz_velocity_bias());
+
+  APZC_LOGV_FM(aFrameMetrics,
+               "Calculated displayport as %s from velocity %s zooming %d paint "
+               "time %f metrics",
+               ToString(displayPort).c_str(), ToString(aVelocity).c_str(),
+               (int)aZoomInProgress, paintFactor);
+
+  CSSMargin cssMargins;
+  cssMargins.left = -displayPort.X();
+  cssMargins.top = -displayPort.Y();
+  cssMargins.right =
+      displayPort.Width() - compositionSize.width - cssMargins.left;
+  cssMargins.bottom =
+      displayPort.Height() - compositionSize.height - cssMargins.top;
+
+  return cssMargins * aFrameMetrics.DisplayportPixelsPerCSSPixel();
+}
+
+void AsyncPanZoomController::ScheduleComposite() {
+  if (mCompositorController) {
+    mCompositorController->ScheduleRenderOnCompositorThread(
+        wr::RenderReasons::APZ);
+  }
+}
+
+void AsyncPanZoomController::ScheduleCompositeAndMaybeRepaint() {
+  ScheduleComposite();
+  RequestContentRepaint();
+}
+
+void AsyncPanZoomController::FlushRepaintForOverscrollHandoff() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  RequestContentRepaint();
+}
+
+void AsyncPanZoomController::FlushRepaintForNewInputBlock() {
+  APZC_LOG("%p flushing repaint for new input block\n", this);
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  RequestContentRepaint();
+}
+
+bool AsyncPanZoomController::SnapBackIfOverscrolled() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (SnapBackIfOverscrolledForMomentum(ParentLayerPoint(0, 0))) {
+    return true;
+  }
+  // If we don't kick off an overscroll animation, we still need to snap to any
+  // nearby snap points, assuming we haven't already done so when we started
+  // this fling
+  if (mState != FLING) {
+    ScrollSnap(ScrollSnapFlags::IntendedEndPosition);
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::SnapBackIfOverscrolledForMomentum(
+    const ParentLayerPoint& aVelocity) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // It's possible that we're already in the middle of an overscroll
+  // animation - if so, don't start a new one.
+  if (IsOverscrolled() && mState != OVERSCROLL_ANIMATION) {
+    APZC_LOG("%p is overscrolled, starting snap-back\n", this);
+    mOverscrollEffect->RelieveOverscroll(aVelocity, GetOverscrollSideBits());
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::IsFlingingFast() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  if (mState == FLING && GetVelocityVector().Length() >
+                             StaticPrefs::apz_fling_stop_on_tap_threshold()) {
+    APZC_LOG("%p is moving fast\n", this);
+    return true;
+  }
+  return false;
+}
+
+bool AsyncPanZoomController::IsPannable() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mX.CanScroll() || mY.CanScroll();
+}
+
+bool AsyncPanZoomController::IsScrollInfoLayer() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return Metrics().IsScrollInfoLayer();
+}
+
+int32_t AsyncPanZoomController::GetLastTouchIdentifier() const {
+  RefPtr<GestureEventListener> listener = GetGestureEventListener();
+  return listener ? listener->GetLastTouchIdentifier() : -1;
+}
+
+void AsyncPanZoomController::RequestContentRepaint(
+    RepaintUpdateType aUpdateType) {
+  // Reinvoke this method on the repaint thread if it's not there already. It's
+  // important to do this before the call to CalculatePendingDisplayPort, so
+  // that CalculatePendingDisplayPort uses the most recent available version of
+  // Metrics(). just before the paint request is dispatched to content.
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  if (!controller->IsRepaintThread()) {
+    // Even though we want to do the actual repaint request on the repaint
+    // thread, we want to update the expected gecko metrics synchronously.
+    // Otherwise we introduce a race condition where we might read from the
+    // expected gecko metrics on the controller thread before or after it gets
+    // updated on the repaint thread, when in fact we always want the updated
+    // version when reading.
+    {  // scope lock
+      RecursiveMutexAutoLock lock(mRecursiveMutex);
+      mExpectedGeckoMetrics.UpdateFrom(Metrics());
+    }
+
+    // use the local variable to resolve the function overload.
+    auto func =
+        static_cast<void (AsyncPanZoomController::*)(RepaintUpdateType)>(
+            &AsyncPanZoomController::RequestContentRepaint);
+    controller->DispatchToRepaintThread(NewRunnableMethod<RepaintUpdateType>(
+        "layers::AsyncPanZoomController::RequestContentRepaint", this, func,
+        aUpdateType));
+    return;
+  }
+
+  MOZ_ASSERT(controller->IsRepaintThread());
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ParentLayerPoint velocity = GetVelocityVector();
+  ScreenMargin displayportMargins = CalculatePendingDisplayPort(
+      Metrics(), velocity,
+      (mState == PINCHING || mState == ANIMATING_ZOOM) ? ZoomInProgress::Yes
+                                                       : ZoomInProgress::No);
+  Metrics().SetPaintRequestTime(TimeStamp::Now());
+  RequestContentRepaint(velocity, displayportMargins, aUpdateType);
+}
+
+static CSSRect GetDisplayPortRect(const FrameMetrics& aFrameMetrics,
+                                  const ScreenMargin& aDisplayportMargins) {
+  // This computation is based on what happens in CalculatePendingDisplayPort.
+  // If that changes then this might need to change too.
+  // Note that the display port rect APZ computes is relative to the visual
+  // scroll offset. It's adjusted to be relative to the layout scroll offset
+  // when the main thread processes a repaint request (in
+  // APZCCallbackHelper::AdjustDisplayPortForScrollDelta()) and ultimately
+  // applied (in DisplayPortUtils::GetDisplayPort()) in this adjusted form.
+  CSSRect baseRect(aFrameMetrics.GetVisualScrollOffset(),
+                   aFrameMetrics.CalculateBoundedCompositedSizeInCssPixels());
+  baseRect.Inflate(aDisplayportMargins /
+                   aFrameMetrics.DisplayportPixelsPerCSSPixel());
+  return baseRect;
+}
+
+void AsyncPanZoomController::RequestContentRepaint(
+    const ParentLayerPoint& aVelocity, const ScreenMargin& aDisplayportMargins,
+    RepaintUpdateType aUpdateType) {
+  mRecursiveMutex.AssertCurrentThreadIn();
+
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (!controller) {
+    return;
+  }
+  MOZ_ASSERT(controller->IsRepaintThread());
+
+  APZScrollAnimationType animationType = APZScrollAnimationType::No;
+  if (mAnimation) {
+    animationType = mAnimation->WasTriggeredByScript()
+                        ? APZScrollAnimationType::TriggeredByScript
+                        : APZScrollAnimationType::TriggeredByUserInput;
+  }
+  RepaintRequest request(Metrics(), aDisplayportMargins, aUpdateType,
+                         animationType, mScrollGeneration, mLastSnapTargetIds,
+                         IsInScrollingGesture());
+
+  if (request.IsRootContent() && request.GetZoom() != mLastNotifiedZoom &&
+      mState != PINCHING && mState != ANIMATING_ZOOM) {
+    controller->NotifyScaleGestureComplete(
+        GetGuid(),
+        (request.GetZoom() / request.GetDevPixelsPerCSSPixel()).scale);
+    mLastNotifiedZoom = request.GetZoom();
+  }
+
+  // If we're trying to paint what we already think is painted, discard this
+  // request since it's a pointless paint.
+  if (request.GetDisplayPortMargins().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetDisplayPortMargins(), EPSILON) &&
+      request.GetVisualScrollOffset().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetVisualScrollOffset(), EPSILON) &&
+      request.GetPresShellResolution() ==
+          mLastPaintRequestMetrics.GetPresShellResolution() &&
+      request.GetZoom() == mLastPaintRequestMetrics.GetZoom() &&
+      request.GetLayoutViewport().WithinEpsilonOf(
+          mLastPaintRequestMetrics.GetLayoutViewport(), EPSILON) &&
+      request.GetScrollGeneration() ==
+          mLastPaintRequestMetrics.GetScrollGeneration() &&
+      request.GetScrollUpdateType() ==
+          mLastPaintRequestMetrics.GetScrollUpdateType() &&
+      request.GetScrollAnimationType() ==
+          mLastPaintRequestMetrics.GetScrollAnimationType() &&
+      request.GetLastSnapTargetIds() ==
+          mLastPaintRequestMetrics.GetLastSnapTargetIds()) {
+    return;
+  }
+
+  APZC_LOGV("%p requesting content repaint %s", this,
+            ToString(request).c_str());
+  {  // scope lock
+    MutexAutoLock lock(mCheckerboardEventLock);
+    if (mCheckerboardEvent && mCheckerboardEvent->IsRecordingTrace()) {
+      std::stringstream info;
+      info << " velocity " << aVelocity;
+      std::string str = info.str();
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::RequestedDisplayPort,
+          GetDisplayPortRect(Metrics(), aDisplayportMargins), str);
+    }
+  }
+
+  controller->RequestContentRepaint(request);
+  mExpectedGeckoMetrics.UpdateFrom(Metrics());
+  mLastPaintRequestMetrics = request;
+
+  // We're holding the APZC lock here, so redispatch this so we can get
+  // the tree lock without the APZC lock.
+  controller->DispatchToRepaintThread(
+      NewRunnableMethod<AsyncPanZoomController*>(
+          "layers::APZCTreeManager::SendSubtreeTransformsToChromeMainThread",
+          GetApzcTreeManager(),
+          &APZCTreeManager::SendSubtreeTransformsToChromeMainThread, this));
+}
+
+bool AsyncPanZoomController::UpdateAnimation(
+    const RecursiveMutexAutoLock& aProofOfLock, const SampleTime& aSampleTime,
+    nsTArray<RefPtr<Runnable>>* aOutDeferredTasks) {
+  AssertOnSamplerThread();
+
+  // This function may get called multiple with the same sample time, if we
+  // composite multiple times at the same timestamp.
+  // However we only want to do one animation step per composition so we need
+  // to deduplicate these calls first.
+  // Even if there's no animation, if we have a scroll offset change pending due
+  // to the frame delay, we need to keep compositing.
+  if (mLastSampleTime == aSampleTime) {
+    APZC_LOG_DETAIL(
+        "UpdateAnimation short-circuit, animation=%p, pending frame-delayed "
+        "offset=%d\n",
+        this, mAnimation.get(), HavePendingFrameDelayedOffset());
+    return !!mAnimation || HavePendingFrameDelayedOffset();
+  }
+
+  // We're at a new timestamp, so advance to the next sample in the deque, if
+  // there is one. That one will be used for all the code that reads the
+  // eForCompositing transforms in this vsync interval.
+  AdvanceToNextSample();
+
+  // And then create a new sample, which will be used in the *next* vsync
+  // interval. We do the sample at this point and not later in order to try
+  // and enforce one frame delay between computing the async transform and
+  // compositing it to the screen. This one-frame delay gives code running on
+  // the main thread a chance to try and respond to the scroll position change,
+  // so that e.g. a main-thread animation can stay in sync with user-driven
+  // scrolling or a compositor animation.
+  bool needComposite = SampleCompositedAsyncTransform(aProofOfLock);
+  APZC_LOG_DETAIL("UpdateAnimation needComposite=%d mAnimation=%p\n", this,
+                  needComposite, mAnimation.get());
+
+  TimeDuration sampleTimeDelta = aSampleTime - mLastSampleTime;
+  mLastSampleTime = aSampleTime;
+
+  if (needComposite || mAnimation) {
+    // Bump the scroll generation before we call RequestContentRepaint below
+    // so that the RequestContentRepaint call will surely use the new
+    // generation.
+    if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+      mScrollGeneration = treeManagerLocal->NewAPZScrollGeneration();
+    }
+  }
+
+  if (mAnimation) {
+    bool continueAnimation = mAnimation->Sample(Metrics(), sampleTimeDelta);
+    bool wantsRepaints = mAnimation->WantsRepaints();
+    *aOutDeferredTasks = mAnimation->TakeDeferredTasks();
+    if (!continueAnimation) {
+      SetState(NOTHING);
+      if (mAnimation->AsSmoothMsdScrollAnimation()) {
+        {
+          RecursiveMutexAutoLock lock(mRecursiveMutex);
+          mLastSnapTargetIds =
+              mAnimation->AsSmoothMsdScrollAnimation()->TakeSnapTargetIds();
+        }
+      } else if (mAnimation->AsSmoothScrollAnimation()) {
+        RecursiveMutexAutoLock lock(mRecursiveMutex);
+        mLastSnapTargetIds =
+            mAnimation->AsSmoothScrollAnimation()->TakeSnapTargetIds();
+      }
+      mAnimation = nullptr;
+    }
+    // Request a repaint at the end of the animation in case something such as a
+    // call to NotifyLayersUpdated was invoked during the animation and Gecko's
+    // current state is some intermediate point of the animation.
+    if (!continueAnimation || wantsRepaints) {
+      RequestContentRepaint();
+    }
+    needComposite = true;
+  }
+  return needComposite;
+}
+
+AsyncTransformComponentMatrix AsyncPanZoomController::GetOverscrollTransform(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  if (aMode == eForCompositing && mScrollMetadata.IsApzForceDisabled()) {
+    return AsyncTransformComponentMatrix();
+  }
+
+  if (!IsPhysicallyOverscrolled()) {
+    return AsyncTransformComponentMatrix();
+  }
+
+  // The overscroll effect is a simple translation by the overscroll offset.
+  ParentLayerPoint overscrollOffset(-mX.GetOverscroll(), -mY.GetOverscroll());
+  return AsyncTransformComponentMatrix().PostTranslate(overscrollOffset.x,
+                                                       overscrollOffset.y, 0);
+}
+
+bool AsyncPanZoomController::AdvanceAnimations(const SampleTime& aSampleTime) {
+  AssertOnSamplerThread();
+
+  // Don't send any state-change notifications until the end of the function,
+  // because we may go through some intermediate states while we finish
+  // animations and start new ones.
+  StateChangeNotificationBlocker blocker(this);
+
+  // The eventual return value of this function. The compositor needs to know
+  // whether or not to advance by a frame as soon as it can. For example, if a
+  // fling is happening, it has to keep compositing so that the animation is
+  // smooth. If an animation frame is requested, it is the compositor's
+  // responsibility to schedule a composite.
+  bool requestAnimationFrame = false;
+  nsTArray<RefPtr<Runnable>> deferredTasks;
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    {  // scope lock
+      CSSRect visibleRect = GetVisibleRect(lock);
+      MutexAutoLock lock2(mCheckerboardEventLock);
+      // Update RendertraceProperty before UpdateAnimation() call, since
+      // the UpdateAnimation() updates effective ScrollOffset for next frame
+      // if APZFrameDelay is enabled.
+      if (mCheckerboardEvent) {
+        mCheckerboardEvent->UpdateRendertraceProperty(
+            CheckerboardEvent::UserVisible, visibleRect);
+      }
+    }
+
+    requestAnimationFrame = UpdateAnimation(lock, aSampleTime, &deferredTasks);
+  }
+  // Execute any deferred tasks queued up by mAnimation's Sample() (called by
+  // UpdateAnimation()). This needs to be done after the monitor is released
+  // since the tasks are allowed to call APZCTreeManager methods which can grab
+  // the tree lock.
+  for (uint32_t i = 0; i < deferredTasks.Length(); ++i) {
+    APZThreadUtils::RunOnControllerThread(std::move(deferredTasks[i]));
+  }
+
+  // If any of the deferred tasks starts a new animation, it will request a
+  // new composite directly, so we can just return requestAnimationFrame here.
+  return requestAnimationFrame;
+}
+
+ParentLayerPoint AsyncPanZoomController::GetCurrentAsyncScrollOffset(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  return GetEffectiveScrollOffset(aMode, lock) * GetEffectiveZoom(aMode, lock);
+}
+
+CSSRect AsyncPanZoomController::GetCurrentAsyncVisualViewport(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  return CSSRect(
+      GetEffectiveScrollOffset(aMode, lock),
+      FrameMetrics::CalculateCompositedSizeInCssPixels(
+          Metrics().GetCompositionBounds(), GetEffectiveZoom(aMode, lock)));
+}
+
+AsyncTransform AsyncPanZoomController::GetCurrentAsyncTransform(
+    AsyncTransformConsumer aMode, AsyncTransformComponents aComponents,
+    std::size_t aSampleIndex) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+
+  CSSToParentLayerScale effectiveZoom;
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    effectiveZoom = GetEffectiveZoom(aMode, lock, aSampleIndex);
+  } else {
+    effectiveZoom =
+        Metrics().LayersPixelsPerCSSPixel() * LayerToParentLayerScale(1.0f);
+  }
+
+  LayerToParentLayerScale compositedAsyncZoom =
+      effectiveZoom / Metrics().LayersPixelsPerCSSPixel();
+
+  ParentLayerPoint translation;
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    // There is no "lastPaintVisualOffset" to subtract here; the visual offset
+    // is entirely async.
+
+    CSSPoint currentVisualOffset =
+        GetEffectiveScrollOffset(aMode, lock, aSampleIndex) -
+        GetEffectiveLayoutViewport(aMode, lock, aSampleIndex).TopLeft();
+
+    translation += currentVisualOffset * effectiveZoom;
+  }
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    CSSPoint lastPaintLayoutOffset;
+    if (mLastContentPaintMetrics.IsScrollable()) {
+      lastPaintLayoutOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+    }
+
+    CSSPoint currentLayoutOffset =
+        GetEffectiveLayoutViewport(aMode, lock, aSampleIndex).TopLeft();
+
+    translation +=
+        (currentLayoutOffset - lastPaintLayoutOffset) * effectiveZoom;
+  }
+
+  return AsyncTransform(compositedAsyncZoom, -translation);
+}
+
+AsyncTransformComponentMatrix
+AsyncPanZoomController::GetAsyncTransformForInputTransformation(
+    AsyncTransformComponents aComponents, LayersId aForLayersId) const {
+  AsyncTransformComponentMatrix result;
+  // If we are the root, and |aForLayersId| is different from our LayersId,
+  // |aForLayersId| must be in a remote subdocument.
+  if (IsRootContent() && aForLayersId != GetLayersId()) {
+    result =
+        ViewAs<AsyncTransformComponentMatrix>(GetPaintedResolutionTransform());
+  }
+  // Order of transforms: the painted resolution (if any) applies first, and
+  // any async transform on top of that.
+  result = result * AsyncTransformComponentMatrix(GetCurrentAsyncTransform(
+                        eForEventHandling, aComponents));
+  // The overscroll transform is considered part of the layout component of
+  // the async transform, because it should not apply to fixed content.
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    result = result * GetOverscrollTransform(eForEventHandling);
+  }
+  return result;
+}
+
+Matrix4x4 AsyncPanZoomController::GetPaintedResolutionTransform() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  MOZ_ASSERT(IsRootContent());
+  float resolution = mLastContentPaintMetrics.GetPresShellResolution();
+  return Matrix4x4::Scaling(resolution, resolution, 1.f);
+}
+
+LayoutDeviceToParentLayerScale AsyncPanZoomController::GetCurrentPinchZoomScale(
+    AsyncTransformConsumer aMode) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, lock);
+  CSSToParentLayerScale scale = GetEffectiveZoom(aMode, lock);
+  return scale / Metrics().GetDevPixelsPerCSSPixel();
+}
+
+AutoTArray<wr::SampledScrollOffset, 2>
+AsyncPanZoomController::GetSampledScrollOffsets() const {
+  AssertOnSamplerThread();
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const AsyncTransformComponents asyncTransformComponents =
+      GetZoomAnimationId()
+          ? AsyncTransformComponents{AsyncTransformComponent::eLayout}
+          : LayoutAndVisual;
+
+  // If layerTranslation includes only the layout component of the async
+  // transform then it has not been scaled by the async zoom, so we want to
+  // divide it by the resolution. If layerTranslation includes the visual
+  // component, then we should use the pinch zoom scale, which includes the
+  // async zoom. However, we only use LayoutAndVisual for non-zoomable APZCs,
+  // so it makes no difference.
+  LayoutDeviceToParentLayerScale resolution =
+      GetCumulativeResolution() * LayerToParentLayerScale(1.0f);
+
+  AutoTArray<wr::SampledScrollOffset, 2> sampledOffsets;
+
+  for (std::deque<SampledAPZCState>::size_type index = 0;
+       index < mSampledState.size(); index++) {
+    ParentLayerPoint layerTranslation =
+        GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing,
+                                 asyncTransformComponents, index)
+            .mTranslation;
+
+    // Include the overscroll transform here in scroll offsets transform
+    // to ensure that we do not overscroll fixed content.
+    layerTranslation =
+        GetOverscrollTransform(AsyncPanZoomController::eForCompositing)
+            .TransformPoint(layerTranslation);
+    // The positive translation means the painted content is supposed to
+    // move down (or to the right), and that corresponds to a reduction in
+    // the scroll offset. Since we are effectively giving WR the async
+    // scroll delta here, we want to negate the translation.
+    LayoutDevicePoint asyncScrollDelta = -layerTranslation / resolution;
+    sampledOffsets.AppendElement(wr::SampledScrollOffset{
+        wr::ToLayoutVector2D(asyncScrollDelta),
+        wr::ToWrAPZScrollGeneration(mSampledState[index].Generation())});
+  }
+
+  return sampledOffsets;
+}
+
+bool AsyncPanZoomController::SuppressAsyncScrollOffset() const {
+  return mScrollMetadata.IsApzForceDisabled() ||
+         (Metrics().IsMinimalDisplayPort() &&
+          StaticPrefs::apz_prefer_jank_minimal_displayports());
+}
+
+CSSRect AsyncPanZoomController::GetEffectiveLayoutViewport(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetLayoutViewport();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetLayoutViewport();
+  }
+  return Metrics().GetLayoutViewport();
+}
+
+CSSPoint AsyncPanZoomController::GetEffectiveScrollOffset(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetVisualScrollOffset();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetVisualScrollOffset();
+  }
+  return Metrics().GetVisualScrollOffset();
+}
+
+CSSToParentLayerScale AsyncPanZoomController::GetEffectiveZoom(
+    AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+    std::size_t aSampleIndex) const {
+  if (aMode == eForCompositing && SuppressAsyncScrollOffset()) {
+    return mLastContentPaintMetrics.GetZoom();
+  }
+  if (aMode == eForCompositing) {
+    return mSampledState[aSampleIndex].GetZoom();
+  }
+  return Metrics().GetZoom();
+}
+
+void AsyncPanZoomController::AdvanceToNextSample() {
+  AssertOnSamplerThread();
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  // Always keep at least one state in mSampledState.
+  if (mSampledState.size() > 1) {
+    mSampledState.pop_front();
+  }
+}
+
+bool AsyncPanZoomController::HavePendingFrameDelayedOffset() const {
+  AssertOnSamplerThread();
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  const bool nextFrameWillChange =
+      mSampledState.size() >= 2 && mSampledState[0] != mSampledState[1];
+  const bool frameAfterThatWillChange =
+      mSampledState.back() != SampledAPZCState(Metrics());
+  return nextFrameWillChange || frameAfterThatWillChange;
+}
+
+bool AsyncPanZoomController::SampleCompositedAsyncTransform(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  MOZ_ASSERT(mSampledState.size() <= 2);
+  bool sampleChanged = (mSampledState.back() != SampledAPZCState(Metrics()));
+  mSampledState.emplace_back(Metrics(), std::move(mScrollPayload),
+                             mScrollGeneration);
+  return sampleChanged;
+}
+
+void AsyncPanZoomController::ResampleCompositedAsyncTransform(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  // This only gets called during testing situations, so the fact that this
+  // drops the scroll payload from mSampledState.front() is not really a
+  // problem.
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    mScrollGeneration = treeManagerLocal->NewAPZScrollGeneration();
+  }
+  mSampledState.front() = SampledAPZCState(Metrics(), {}, mScrollGeneration);
+}
+
+void AsyncPanZoomController::ApplyAsyncTestAttributes(
+    const RecursiveMutexAutoLock& aProofOfLock) {
+  if (mTestAttributeAppliers == 0) {
+    if (mTestAsyncScrollOffset != CSSPoint() ||
+        mTestAsyncZoom != LayerToParentLayerScale()) {
+      // TODO Currently we update Metrics() and resample, which will cause
+      // the very latest user input to get immediately captured in the sample,
+      // and may defeat our attempt at "frame delay" (i.e. delaying the user
+      // input from affecting composition by one frame).
+      // Instead, maybe we should just apply the mTest* stuff directly to
+      // mSampledState.front(). We can even save/restore that SampledAPZCState
+      // instance in the AutoApplyAsyncTestAttributes instead of Metrics().
+      Metrics().ZoomBy(mTestAsyncZoom.scale);
+      CSSPoint asyncScrollPosition = Metrics().GetVisualScrollOffset();
+      CSSPoint requestedPoint =
+          asyncScrollPosition + this->mTestAsyncScrollOffset;
+      CSSPoint clampedPoint =
+          Metrics().CalculateScrollRange().ClampPoint(requestedPoint);
+      CSSPoint difference = mTestAsyncScrollOffset - clampedPoint;
+
+      ScrollByAndClamp(mTestAsyncScrollOffset);
+
+      if (StaticPrefs::apz_overscroll_test_async_scroll_offset_enabled()) {
+        ParentLayerPoint overscroll = difference * Metrics().GetZoom();
+        OverscrollBy(overscroll);
+      }
+      ResampleCompositedAsyncTransform(aProofOfLock);
+    }
+  }
+  ++mTestAttributeAppliers;
+}
+
+void AsyncPanZoomController::UnapplyAsyncTestAttributes(
+    const RecursiveMutexAutoLock& aProofOfLock,
+    const FrameMetrics& aPrevFrameMetrics,
+    const ParentLayerPoint& aPrevOverscroll) {
+  MOZ_ASSERT(mTestAttributeAppliers >= 1);
+  --mTestAttributeAppliers;
+  if (mTestAttributeAppliers == 0) {
+    if (mTestAsyncScrollOffset != CSSPoint() ||
+        mTestAsyncZoom != LayerToParentLayerScale()) {
+      Metrics() = aPrevFrameMetrics;
+      RestoreOverscrollAmount(aPrevOverscroll);
+      ResampleCompositedAsyncTransform(aProofOfLock);
+    }
+  }
+}
+
+Matrix4x4 AsyncPanZoomController::GetTransformToLastDispatchedPaint(
+    const AsyncTransformComponents& aComponents, LayersId aForLayersId) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSPoint componentOffset;
+
+  // The computation of the componentOffset should roughly be the negation
+  // of the translation in GetCurrentAsyncTransform() with the expected
+  // gecko metrics substituted for the effective scroll offsets.
+  if (aComponents.contains(AsyncTransformComponent::eVisual)) {
+    componentOffset += mExpectedGeckoMetrics.GetLayoutScrollOffset() -
+                       mExpectedGeckoMetrics.GetVisualScrollOffset();
+  }
+
+  if (aComponents.contains(AsyncTransformComponent::eLayout)) {
+    CSSPoint lastPaintLayoutOffset;
+
+    if (mLastContentPaintMetrics.IsScrollable()) {
+      lastPaintLayoutOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+    }
+
+    componentOffset +=
+        lastPaintLayoutOffset - mExpectedGeckoMetrics.GetLayoutScrollOffset();
+  }
+
+  LayerPoint scrollChange = componentOffset *
+                            mLastContentPaintMetrics.GetDevPixelsPerCSSPixel() *
+                            mLastContentPaintMetrics.GetCumulativeResolution();
+
+  // We're interested in the async zoom change. Factor out the content scale
+  // that may change when dragging the window to a monitor with a different
+  // content scale.
+  LayoutDeviceToParentLayerScale lastContentZoom =
+      mLastContentPaintMetrics.GetZoom() /
+      mLastContentPaintMetrics.GetDevPixelsPerCSSPixel();
+  LayoutDeviceToParentLayerScale lastDispatchedZoom =
+      mExpectedGeckoMetrics.GetZoom() /
+      mExpectedGeckoMetrics.GetDevPixelsPerCSSPixel();
+  float zoomChange = 1.0;
+  if (aComponents.contains(AsyncTransformComponent::eVisual) &&
+      lastDispatchedZoom != LayoutDeviceToParentLayerScale(0)) {
+    zoomChange = lastContentZoom.scale / lastDispatchedZoom.scale;
+  }
+  Matrix4x4 result;
+  // If we are the root, and |aForLayersId| is different from our LayersId,
+  // |aForLayersId| must be in a remote subdocument.
+  if (IsRootContent() && aForLayersId != GetLayersId()) {
+    result = GetPaintedResolutionTransform();
+  }
+  // Order of transforms: the painted resolution (if any) applies first, and
+  // any async transform on top of that.
+  return result * Matrix4x4::Translation(scrollChange.x, scrollChange.y, 0)
+                      .PostScale(zoomChange, zoomChange, 1);
+}
+
+CSSRect AsyncPanZoomController::GetVisibleRect(
+    const RecursiveMutexAutoLock& aProofOfLock) const {
+  AutoApplyAsyncTestAttributes testAttributeApplier(this, aProofOfLock);
+  CSSPoint currentScrollOffset = GetEffectiveScrollOffset(
+      AsyncPanZoomController::eForCompositing, aProofOfLock);
+  CSSRect visible = CSSRect(currentScrollOffset,
+                            Metrics().CalculateCompositedSizeInCssPixels());
+  return visible;
+}
+
+static CSSRect GetPaintedRect(const FrameMetrics& aFrameMetrics) {
+  CSSRect displayPort = aFrameMetrics.GetDisplayPort();
+  if (displayPort.IsEmpty()) {
+    // Fallback to use the viewport if the diplayport hasn't been set.
+    // This situation often happens non-scrollable iframe's root scroller in
+    // Fission.
+    return aFrameMetrics.GetVisualViewport();
+  }
+
+  return displayPort + aFrameMetrics.GetLayoutScrollOffset();
+}
+
+uint32_t AsyncPanZoomController::GetCheckerboardMagnitude(
+    const ParentLayerRect& aClippedCompositionBounds) const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  CSSRect painted = GetPaintedRect(mLastContentPaintMetrics);
+  painted.Inflate(CSSMargin::FromAppUnits(
+      nsMargin(1, 1, 1, 1)));  // fuzz for rounding error
+
+  CSSRect visible = GetVisibleRect(lock);  // relative to scrolled frame origin
+  if (visible.IsEmpty() || painted.Contains(visible)) {
+    // early-exit if we're definitely not checkerboarding
+    return 0;
+  }
+
+  // aClippedCompositionBounds and Metrics().GetCompositionBounds() are both
+  // relative to the layer tree origin.
+  // The "*RelativeToItself*" variables are relative to the comp bounds origin
+  ParentLayerRect visiblePartOfCompBoundsRelativeToItself =
+      aClippedCompositionBounds - Metrics().GetCompositionBounds().TopLeft();
+  CSSRect visiblePartOfCompBoundsRelativeToItselfInCssSpace;
+  if (Metrics().GetZoom() != CSSToParentLayerScale(0)) {
+    visiblePartOfCompBoundsRelativeToItselfInCssSpace =
+        (visiblePartOfCompBoundsRelativeToItself / Metrics().GetZoom());
+  }
+
+  // This one is relative to the scrolled frame origin, same as `visible`
+  CSSRect visiblePartOfCompBoundsInCssSpace =
+      visiblePartOfCompBoundsRelativeToItselfInCssSpace + visible.TopLeft();
+
+  visible = visible.Intersect(visiblePartOfCompBoundsInCssSpace);
+
+  CSSIntRegion checkerboard;
+  // Round so as to minimize checkerboarding; if we're only showing fractional
+  // pixels of checkerboarding it's not really worth counting
+  checkerboard.Sub(RoundedIn(visible), RoundedOut(painted));
+  uint32_t area = checkerboard.Area();
+  if (area) {
+    APZC_LOG_FM(Metrics(),
+                "%p is currently checkerboarding (painted %s visible %s)", this,
+                ToString(painted).c_str(), ToString(visible).c_str());
+  }
+  return area;
+}
+
+void AsyncPanZoomController::ReportCheckerboard(
+    const SampleTime& aSampleTime,
+    const ParentLayerRect& aClippedCompositionBounds) {
+  if (mLastCheckerboardReport == aSampleTime) {
+    // This function will get called multiple times for each APZC on a single
+    // composite (once for each layer it is attached to). Only report the
+    // checkerboard once per composite though.
+    return;
+  }
+  mLastCheckerboardReport = aSampleTime;
+
+  bool recordTrace = StaticPrefs::apz_record_checkerboarding();
+  bool forTelemetry = Telemetry::CanRecordBase();
+  uint32_t magnitude = GetCheckerboardMagnitude(aClippedCompositionBounds);
+
+  // IsInTransformingState() acquires the APZC lock and thus needs to
+  // be called before acquiring mCheckerboardEventLock.
+  bool inTransformingState = IsInTransformingState();
+
+  MutexAutoLock lock(mCheckerboardEventLock);
+  if (!mCheckerboardEvent && (recordTrace || forTelemetry)) {
+    mCheckerboardEvent = MakeUnique<CheckerboardEvent>(recordTrace);
+  }
+  mPotentialCheckerboardTracker.InTransform(inTransformingState,
+                                            recordTrace || forTelemetry);
+  if (magnitude) {
+    mPotentialCheckerboardTracker.CheckerboardSeen();
+  }
+  UpdateCheckerboardEvent(lock, magnitude);
+}
+
+void AsyncPanZoomController::UpdateCheckerboardEvent(
+    const MutexAutoLock& aProofOfLock, uint32_t aMagnitude) {
+  if (mCheckerboardEvent && mCheckerboardEvent->RecordFrameInfo(aMagnitude)) {
+    // This checkerboard event is done. Report some metrics to telemetry.
+    mozilla::glean::gfx_checkerboard::severity.AccumulateSamples(
+        {mCheckerboardEvent->GetSeverity()});
+    mozilla::glean::gfx_checkerboard::peak_pixel_count.AccumulateSamples(
+        {mCheckerboardEvent->GetPeak()});
+    mozilla::glean::gfx_checkerboard::duration.AccumulateRawDuration(
+        mCheckerboardEvent->GetDuration());
+
+    // mCheckerboardEvent only gets created if we are supposed to record
+    // telemetry so we always pass true for aRecordTelemetry.
+    mPotentialCheckerboardTracker.CheckerboardDone(
+        /* aRecordTelemetry = */ true);
+
+    if (StaticPrefs::apz_record_checkerboarding()) {
+      // if the pref is enabled, also send it to the storage class. it may be
+      // chosen for public display on about:checkerboard, the hall of fame for
+      // checkerboard events.
+      uint32_t severity = mCheckerboardEvent->GetSeverity();
+      std::string log = mCheckerboardEvent->GetLog();
+      CheckerboardEventStorage::Report(severity, log);
+    }
+    mCheckerboardEvent = nullptr;
+  }
+}
+
+void AsyncPanZoomController::FlushActiveCheckerboardReport() {
+  MutexAutoLock lock(mCheckerboardEventLock);
+  // Pretend like we got a frame with 0 pixels checkerboarded. This will
+  // terminate the checkerboard event and flush it out
+  UpdateCheckerboardEvent(lock, 0);
+}
+
+void AsyncPanZoomController::NotifyLayersUpdated(
+    const ScrollMetadata& aScrollMetadata, bool aIsFirstPaint,
+    bool aThisLayerTreeUpdated) {
+  AssertOnUpdaterThread();
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  bool isDefault = mScrollMetadata.IsDefault();
+
+  const FrameMetrics& aLayerMetrics = aScrollMetadata.GetMetrics();
+
+  if ((aScrollMetadata == mLastContentPaintMetadata) && !isDefault) {
+    // No new information here, skip it.
+    APZC_LOGV("%p NotifyLayersUpdated short-circuit\n", this);
+    return;
+  }
+
+  // If the Metrics scroll offset is different from the last scroll offset
+  // that the main-thread sent us, then we know that the user has been doing
+  // something that triggers a scroll. This check is the APZ equivalent of the
+  // check on the main-thread at
+  // https://hg.mozilla.org/mozilla-central/file/97a52326b06a/layout/generic/nsGfxScrollFrame.cpp#l4050
+  // There is code below (the use site of userScrolled) that prevents a
+  // restored- scroll-position update from overwriting a user scroll, again
+  // equivalent to how the main thread code does the same thing.
+  // XXX Suspicious comparison between layout and visual scroll offsets.
+  // This may not do the right thing when we're zoomed in.
+  CSSPoint lastScrollOffset = mLastContentPaintMetrics.GetLayoutScrollOffset();
+  bool userScrolled =
+      !FuzzyEqualsCoordinate(Metrics().GetVisualScrollOffset().x,
+                             lastScrollOffset.x) ||
+      !FuzzyEqualsCoordinate(Metrics().GetVisualScrollOffset().y,
+                             lastScrollOffset.y);
+
+  if (aScrollMetadata.DidContentGetPainted()) {
+    mLastContentPaintMetadata = aScrollMetadata;
+  }
+
+  mScrollMetadata.SetScrollParentId(aScrollMetadata.GetScrollParentId());
+  APZC_LOGV_FM(aLayerMetrics,
+               "%p got a NotifyLayersUpdated with aIsFirstPaint=%d, "
+               "aThisLayerTreeUpdated=%d",
+               this, aIsFirstPaint, aThisLayerTreeUpdated);
+
+  {  // scope lock
+    MutexAutoLock lock(mCheckerboardEventLock);
+    if (mCheckerboardEvent && mCheckerboardEvent->IsRecordingTrace()) {
+      std::string str;
+      if (aThisLayerTreeUpdated) {
+        if (!aLayerMetrics.GetPaintRequestTime().IsNull()) {
+          // Note that we might get the paint request time as non-null, but with
+          // aThisLayerTreeUpdated false. That can happen if we get a layer
+          // transaction from a different process right after we get the layer
+          // transaction with aThisLayerTreeUpdated == true. In this case we
+          // want to ignore the paint request time because it was already dumped
+          // in the previous layer transaction.
+          TimeDuration paintTime =
+              TimeStamp::Now() - aLayerMetrics.GetPaintRequestTime();
+          std::stringstream info;
+          info << " painttime " << paintTime.ToMilliseconds();
+          str = info.str();
+        } else {
+          // This might be indicative of a wasted paint particularly if it
+          // happens during a checkerboard event.
+          str = " (this layertree updated)";
+        }
+      }
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::Page, aLayerMetrics.GetScrollableRect());
+      mCheckerboardEvent->UpdateRendertraceProperty(
+          CheckerboardEvent::PaintedDisplayPort, GetPaintedRect(aLayerMetrics),
+          str);
+    }
+  }
+
+  // The main thread may send us a visual scroll offset update. This is
+  // different from a layout viewport offset update in that the layout viewport
+  // offset is limited to the layout scroll range, while the visual viewport
+  // offset is not.
+  // However, there are some conditions in which the layout update will clobber
+  // the visual update, and we want to ignore the visual update in those cases.
+  // This variable tracks that.
+  bool ignoreVisualUpdate = false;
+
+  // TODO if we're in a drag and scrollOffsetUpdated is set then we want to
+  // ignore it
+
+  bool needContentRepaint = false;
+  RepaintUpdateType contentRepaintType = RepaintUpdateType::eNone;
+  bool viewportSizeUpdated = false;
+  bool needToReclampScroll = false;
+
+  if ((aIsFirstPaint && aThisLayerTreeUpdated) || isDefault ||
+      Metrics().IsRootContent() != aLayerMetrics.IsRootContent()) {
+    if (Metrics().IsRootContent() && !aLayerMetrics.IsRootContent()) {
+      // We only support zooming on root content APZCs
+      SetZoomAnimationId(Nothing());
+    }
+
+    // Initialize our internal state to something sane when the content
+    // that was just painted is something we knew nothing about previously
+    CancelAnimation();
+
+    // Keep our existing scroll generation, if there are scroll updates. In this
+    // case we'll update our scroll generation when processing the scroll update
+    // array below. If there are no scroll updates, take the generation from the
+    // incoming metrics. Bug 1662019 will simplify this later.
+    ScrollGeneration oldScrollGeneration = Metrics().GetScrollGeneration();
+    mScrollMetadata = aScrollMetadata;
+    if (!aScrollMetadata.GetScrollUpdates().IsEmpty()) {
+      Metrics().SetScrollGeneration(oldScrollGeneration);
+    }
+
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+
+    for (auto& sampledState : mSampledState) {
+      sampledState.UpdateScrollProperties(Metrics());
+      sampledState.UpdateZoomProperties(Metrics());
+    }
+
+    if (aLayerMetrics.HasNonZeroDisplayPortMargins()) {
+      // A non-zero display port margin here indicates a displayport has
+      // been set by a previous APZC for the content at this guid. The
+      // scrollable rect may have changed since then, making the margins
+      // wrong, so we need to calculate a new display port.
+      // It is important that we request a repaint here only when we need to
+      // otherwise we will end up setting a display port on every frame that
+      // gets a view id.
+      APZC_LOG("%p detected non-empty margins which probably need updating\n",
+               this);
+      needContentRepaint = true;
+    }
+  } else {
+    // If we're not taking the aLayerMetrics wholesale we still need to pull
+    // in some things into our local Metrics() because these things are
+    // determined by Gecko and our copy in Metrics() may be stale.
+
+    if (Metrics().GetLayoutViewport().Size() !=
+        aLayerMetrics.GetLayoutViewport().Size()) {
+      CSSRect layoutViewport = Metrics().GetLayoutViewport();
+      // The offset will be updated if necessary via
+      // RecalculateLayoutViewportOffset().
+      layoutViewport.SizeTo(aLayerMetrics.GetLayoutViewport().Size());
+      Metrics().SetLayoutViewport(layoutViewport);
+
+      needContentRepaint = true;
+      viewportSizeUpdated = true;
+    }
+
+    // TODO: Rely entirely on |aScrollMetadata.IsResolutionUpdated()| to
+    //       determine which branch to take, and drop the other conditions.
+    CSSToParentLayerScale oldZoom = Metrics().GetZoom();
+    if (FuzzyEqualsAdditive(
+            Metrics().GetCompositionBoundsWidthIgnoringScrollbars(),
+            aLayerMetrics.GetCompositionBoundsWidthIgnoringScrollbars()) &&
+        Metrics().GetDevPixelsPerCSSPixel() ==
+            aLayerMetrics.GetDevPixelsPerCSSPixel() &&
+        !viewportSizeUpdated && !aScrollMetadata.IsResolutionUpdated()) {
+      // Any change to the pres shell resolution was requested by APZ and is
+      // already included in our zoom; however, other components of the
+      // cumulative resolution (a parent document's pres-shell resolution, or
+      // the css-driven resolution) may have changed, and we need to update
+      // our zoom to reflect that. Note that we can't just take
+      // aLayerMetrics.mZoom because the APZ may have additional async zoom
+      // since the repaint request.
+      float totalResolutionChange = 1.0;
+
+      if (Metrics().GetCumulativeResolution() != LayoutDeviceToLayerScale(0)) {
+        totalResolutionChange = aLayerMetrics.GetCumulativeResolution().scale /
+                                Metrics().GetCumulativeResolution().scale;
+      }
+
+      float presShellResolutionChange = aLayerMetrics.GetPresShellResolution() /
+                                        Metrics().GetPresShellResolution();
+      if (presShellResolutionChange != 1.0f) {
+        needContentRepaint = true;
+      }
+      Metrics().ZoomBy(totalResolutionChange / presShellResolutionChange);
+      for (auto& sampledState : mSampledState) {
+        sampledState.ZoomBy(totalResolutionChange / presShellResolutionChange);
+      }
+    } else {
+      // Take the new zoom as either device scale or composition width or
+      // viewport size got changed (e.g. due to orientation change, or content
+      // changing the meta-viewport tag), or the main thread originated a
+      // resolution change for another reason (e.g. Ctrl+0 was pressed to
+      // reset the zoom).
+      Metrics().SetZoom(aLayerMetrics.GetZoom());
+      for (auto& sampledState : mSampledState) {
+        sampledState.UpdateZoomProperties(aLayerMetrics);
+      }
+      Metrics().SetDevPixelsPerCSSPixel(
+          aLayerMetrics.GetDevPixelsPerCSSPixel());
+    }
+
+    if (Metrics().GetZoom() != oldZoom) {
+      // If the zoom changed, the scroll range in CSS pixels may have changed
+      // even if the composition bounds didn't.
+      needToReclampScroll = true;
+    }
+
+    mExpectedGeckoMetrics.UpdateZoomFrom(aLayerMetrics);
+
+    if (!Metrics().GetScrollableRect().IsEqualEdges(
+            aLayerMetrics.GetScrollableRect())) {
+      Metrics().SetScrollableRect(aLayerMetrics.GetScrollableRect());
+      needContentRepaint = true;
+      needToReclampScroll = true;
+    }
+    if (!Metrics().GetCompositionBounds().IsEqualEdges(
+            aLayerMetrics.GetCompositionBounds())) {
+      Metrics().SetCompositionBounds(aLayerMetrics.GetCompositionBounds());
+      needToReclampScroll = true;
+    }
+    Metrics().SetCompositionBoundsWidthIgnoringScrollbars(
+        aLayerMetrics.GetCompositionBoundsWidthIgnoringScrollbars());
+
+    if (Metrics().IsRootContent() &&
+        Metrics().GetCompositionSizeWithoutDynamicToolbar() !=
+            aLayerMetrics.GetCompositionSizeWithoutDynamicToolbar()) {
+      Metrics().SetCompositionSizeWithoutDynamicToolbar(
+          aLayerMetrics.GetCompositionSizeWithoutDynamicToolbar());
+      needToReclampScroll = true;
+    }
+    Metrics().SetBoundingCompositionSize(
+        aLayerMetrics.GetBoundingCompositionSize());
+    Metrics().SetPresShellResolution(aLayerMetrics.GetPresShellResolution());
+    Metrics().SetCumulativeResolution(aLayerMetrics.GetCumulativeResolution());
+    Metrics().SetTransformToAncestorScale(
+        aLayerMetrics.GetTransformToAncestorScale());
+    mScrollMetadata.SetHasScrollgrab(aScrollMetadata.GetHasScrollgrab());
+    mScrollMetadata.SetLineScrollAmount(aScrollMetadata.GetLineScrollAmount());
+    mScrollMetadata.SetPageScrollAmount(aScrollMetadata.GetPageScrollAmount());
+    mScrollMetadata.SetSnapInfo(ScrollSnapInfo(aScrollMetadata.GetSnapInfo()));
+    mScrollMetadata.SetIsLayersIdRoot(aScrollMetadata.IsLayersIdRoot());
+    mScrollMetadata.SetIsAutoDirRootContentRTL(
+        aScrollMetadata.IsAutoDirRootContentRTL());
+    Metrics().SetIsScrollInfoLayer(aLayerMetrics.IsScrollInfoLayer());
+    Metrics().SetHasNonZeroDisplayPortMargins(
+        aLayerMetrics.HasNonZeroDisplayPortMargins());
+    Metrics().SetMinimalDisplayPort(aLayerMetrics.IsMinimalDisplayPort());
+    mScrollMetadata.SetForceDisableApz(aScrollMetadata.IsApzForceDisabled());
+    mScrollMetadata.SetIsRDMTouchSimulationActive(
+        aScrollMetadata.GetIsRDMTouchSimulationActive());
+    mScrollMetadata.SetForceMousewheelAutodir(
+        aScrollMetadata.ForceMousewheelAutodir());
+    mScrollMetadata.SetForceMousewheelAutodirHonourRoot(
+        aScrollMetadata.ForceMousewheelAutodirHonourRoot());
+    mScrollMetadata.SetIsPaginatedPresentation(
+        aScrollMetadata.IsPaginatedPresentation());
+    mScrollMetadata.SetDisregardedDirection(
+        aScrollMetadata.GetDisregardedDirection());
+    mScrollMetadata.SetOverscrollBehavior(
+        aScrollMetadata.GetOverscrollBehavior());
+  }
+
+  if (needToReclampScroll) {
+    // Whenever scrollable rect or composition bounds has changed, we need to
+    // re-clamp the scroll offset since it may be out of bounds. Also note that
+    // we need to re-clamp before updating new scroll offsets from content since
+    // we will use the last scroll offset to reflect the new offsets.
+    ClampAndSetVisualScrollOffset(Metrics().GetVisualScrollOffset());
+    for (auto& sampledState : mSampledState) {
+      sampledState.ClampVisualScrollOffset(Metrics());
+    }
+  }
+
+  bool instantScrollMayTriggerTransform = false;
+  bool scrollOffsetUpdated = false;
+  bool smoothScrollRequested = false;
+  bool didCancelAnimation = false;
+  Maybe<CSSPoint> cumulativeRelativeDelta;
+  for (const auto& scrollUpdate : aScrollMetadata.GetScrollUpdates()) {
+    APZC_LOG("%p processing scroll update %s\n", this,
+             ToString(scrollUpdate).c_str());
+    if (!(Metrics().GetScrollGeneration() < scrollUpdate.GetGeneration())) {
+      // This is stale, let's ignore it
+      APZC_LOG("%p scrollupdate generation stale, dropping\n", this);
+      continue;
+    }
+    Metrics().SetScrollGeneration(scrollUpdate.GetGeneration());
+
+    MOZ_ASSERT(scrollUpdate.GetOrigin() != ScrollOrigin::Apz);
+    if (userScrolled &&
+        !nsLayoutUtils::CanScrollOriginClobberApz(scrollUpdate.GetOrigin())) {
+      APZC_LOG("%p scrollupdate cannot clobber APZ userScrolled\n", this);
+      continue;
+    }
+    // XXX: if we get here, |scrollUpdate| is clobbering APZ, so we may want
+    // to reset |userScrolled| back to false so that subsequent scrollUpdates
+    // in this loop don't get dropped by the check above. Need to add a test
+    // that exercises this scenario, as we don't currently have one.
+
+    if (scrollUpdate.GetMode() == ScrollMode::Smooth ||
+        scrollUpdate.GetMode() == ScrollMode::SmoothMsd) {
+      smoothScrollRequested = true;
+
+      // Requests to animate the visual scroll position override requests to
+      // simply update the visual scroll offset to a particular point. Since
+      // we have an animation request, we set ignoreVisualUpdate to true to
+      // indicate we don't need to apply the visual scroll update in
+      // aLayerMetrics.
+      ignoreVisualUpdate = true;
+
+      // For relative updates we want to add the relative offset to any existing
+      // destination, or the current visual offset if there is no existing
+      // destination.
+      CSSPoint base = GetCurrentAnimationDestination(lock).valueOr(
+          Metrics().GetVisualScrollOffset());
+
+      CSSPoint destination;
+      if (scrollUpdate.GetType() == ScrollUpdateType::Relative) {
+        CSSPoint delta =
+            scrollUpdate.GetDestination() - scrollUpdate.GetSource();
+        APZC_LOG("%p relative smooth scrolling from %s by %s\n", this,
+                 ToString(base).c_str(), ToString(delta).c_str());
+        destination = Metrics().CalculateScrollRange().ClampPoint(base + delta);
+      } else if (scrollUpdate.GetType() == ScrollUpdateType::PureRelative) {
+        CSSPoint delta = scrollUpdate.GetDelta();
+        APZC_LOG("%p pure-relative smooth scrolling from %s by %s\n", this,
+                 ToString(base).c_str(), ToString(delta).c_str());
+        destination = Metrics().CalculateScrollRange().ClampPoint(base + delta);
+      } else {
+        APZC_LOG("%p smooth scrolling to %s\n", this,
+                 ToString(scrollUpdate.GetDestination()).c_str());
+        destination = scrollUpdate.GetDestination();
+      }
+
+      if (scrollUpdate.GetMode() == ScrollMode::SmoothMsd) {
+        SmoothMsdScrollTo(
+            CSSSnapDestination{destination, scrollUpdate.GetSnapTargetIds()},
+            scrollUpdate.GetScrollTriggeredByScript());
+      } else {
+        MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Smooth);
+        SmoothScrollTo(
+            CSSSnapDestination{destination, scrollUpdate.GetSnapTargetIds()},
+            scrollUpdate.GetScrollTriggeredByScript(),
+            scrollUpdate.GetOrigin());
+      }
+      continue;
+    }
+
+    MOZ_ASSERT(scrollUpdate.GetMode() == ScrollMode::Instant ||
+               scrollUpdate.GetMode() == ScrollMode::Normal);
+
+    instantScrollMayTriggerTransform =
+        scrollUpdate.GetMode() == ScrollMode::Instant &&
+        scrollUpdate.GetScrollTriggeredByScript() ==
+            ScrollTriggeredByScript::No;
+
+    // If the layout update is of a higher priority than the visual update, then
+    // we don't want to apply the visual update.
+    // If the layout update is of a clobbering type (or a smooth scroll request,
+    // which is handled above) then it takes precedence over an eRestore visual
+    // update. But we also allow the possibility for the main thread to ask us
+    // to scroll both the layout and visual viewports to distinct (but
+    // compatible) locations (via e.g. both updates being of a non-clobbering/
+    // eRestore type).
+    if (nsLayoutUtils::CanScrollOriginClobberApz(scrollUpdate.GetOrigin()) &&
+        aLayerMetrics.GetVisualScrollUpdateType() !=
+            FrameMetrics::eMainThread) {
+      ignoreVisualUpdate = true;
+    }
+
+    Maybe<CSSPoint> relativeDelta;
+    if (scrollUpdate.GetType() == ScrollUpdateType::Relative) {
+      APZC_LOG(
+          "%p relative updating scroll offset from %s by %s\n", this,
+          ToString(Metrics().GetVisualScrollOffset()).c_str(),
+          ToString(scrollUpdate.GetDestination() - scrollUpdate.GetSource())
+              .c_str());
+
+      scrollOffsetUpdated = true;
+
+      // It's possible that the main thread has ignored an APZ scroll offset
+      // update for the pending relative scroll that we have just received.
+      // When this happens, we need to send a new scroll offset update with
+      // the combined scroll offset or else the main thread may have an
+      // incorrect scroll offset for a period of time.
+      if (Metrics().HasPendingScroll(aLayerMetrics)) {
+        needContentRepaint = true;
+        contentRepaintType = RepaintUpdateType::eUserAction;
+      }
+
+      relativeDelta =
+          Some(Metrics().ApplyRelativeScrollUpdateFrom(scrollUpdate));
+      Metrics().RecalculateLayoutViewportOffset();
+    } else if (scrollUpdate.GetType() == ScrollUpdateType::PureRelative) {
+      APZC_LOG("%p pure-relative updating scroll offset from %s by %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDelta()).c_str());
+
+      scrollOffsetUpdated = true;
+
+      // Always need a repaint request with a repaint type for pure relative
+      // scrolls because apz is doing the scroll at the main thread's request.
+      // The main thread has not updated it's scroll offset yet, it is depending
+      // on apz to tell it where to scroll.
+      needContentRepaint = true;
+      contentRepaintType = RepaintUpdateType::eVisualUpdate;
+
+      // We have to ignore a visual scroll offset update otherwise it will
+      // clobber the relative scrolling we are about to do. We perform
+      // visualScrollOffset = visualScrollOffset + delta. Then the
+      // visualScrollOffsetUpdated block below will do visualScrollOffset =
+      // aLayerMetrics.GetVisualDestination(). We need visual scroll offset
+      // updates to be incorporated into this scroll update loop to properly fix
+      // this.
+      ignoreVisualUpdate = true;
+
+      relativeDelta =
+          Some(Metrics().ApplyPureRelativeScrollUpdateFrom(scrollUpdate));
+      Metrics().RecalculateLayoutViewportOffset();
+    } else if (scrollUpdate.GetType() == ScrollUpdateType::MergeableAbsolute) {
+      APZC_LOG("%p mergeable updating scroll offset from %s to %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDestination()).c_str());
+      relativeDelta =
+          Some(Metrics().ApplyAbsoluteScrollUpdateFrom(scrollUpdate).second);
+      Metrics().RecalculateLayoutViewportOffset();
+      scrollOffsetUpdated = true;
+    } else {
+      APZC_LOG("%p updating scroll offset from %s to %s\n", this,
+               ToString(Metrics().GetVisualScrollOffset()).c_str(),
+               ToString(scrollUpdate.GetDestination()).c_str());
+      auto [offsetChanged, _] =
+          Metrics().ApplyAbsoluteScrollUpdateFrom(scrollUpdate);
+      Metrics().RecalculateLayoutViewportOffset();
+
+      if (offsetChanged || scrollUpdate.GetMode() != ScrollMode::Instant ||
+          scrollUpdate.GetType() != ScrollUpdateType::Absolute ||
+          scrollUpdate.GetOrigin() != ScrollOrigin::None) {
+        // We get a NewScrollFrame update for newly created scroll frames. Only
+        // if this was not a NewScrollFrame update or the offset changed do we
+        // request repaint. This is important so that we don't request repaint
+        // for every new content and set a full display port on it.
+        scrollOffsetUpdated = true;
+      }
+    }
+
+    if (relativeDelta) {
+      cumulativeRelativeDelta =
+          !cumulativeRelativeDelta
+              ? relativeDelta
+              : Some(*cumulativeRelativeDelta + *relativeDelta);
+    } else {
+      // If the scroll update is not relative, clobber the cumulative delta,
+      // i.e. later updates win.
+      cumulativeRelativeDelta.reset();
+    }
+
+    // If an animation is underway, tell it about the scroll offset update.
+    // Some animations can handle some scroll offset updates and continue
+    // running. Those that can't will return false, and we cancel them.
+    if (ShouldCancelAnimationForScrollUpdate(relativeDelta)) {
+      // Cancel the animation (which might also trigger a repaint request)
+      // after we update the scroll offset above. Otherwise we can be left
+      // in a state where things are out of sync.
+      CancelAnimation();
+      didCancelAnimation = true;
+    }
+  }
+
+  if (scrollOffsetUpdated) {
+    for (auto& sampledState : mSampledState) {
+      if (!didCancelAnimation && cumulativeRelativeDelta.isSome()) {
+        sampledState.UpdateScrollPropertiesWithRelativeDelta(
+            Metrics(), *cumulativeRelativeDelta);
+      } else {
+        sampledState.UpdateScrollProperties(Metrics());
+      }
+    }
+
+    // Because of the scroll generation update, any inflight paint requests
+    // are going to be ignored by layout, and so mExpectedGeckoMetrics becomes
+    // incorrect for the purposes of calculating the LD transform. To correct
+    // this we need to update mExpectedGeckoMetrics to be the last thing we
+    // know was painted by Gecko.
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+
+    // Since the scroll offset has changed, we need to recompute the
+    // displayport margins and send them to layout. Otherwise there might be
+    // scenarios where for example we scroll from the top of a page (where the
+    // top displayport margin is zero) to the bottom of a page, which will
+    // result in a displayport that doesn't extend upwards at all.
+    // Note that even if the CancelAnimation call above requested a repaint
+    // this is fine because we already have repaint request deduplication.
+    needContentRepaint = true;
+    // Since the main-thread scroll offset changed we should trigger a
+    // recomposite to make sure it becomes user-visible.
+    ScheduleComposite();
+
+    // If the scroll offset was updated, we're not in a transforming state,
+    // and we are scrolling by a non-zero delta, we should ensure
+    // TransformBegin and TransformEnd notifications are sent.
+    if (!IsTransformingState(mState) && instantScrollMayTriggerTransform &&
+        cumulativeRelativeDelta && *cumulativeRelativeDelta != CSSPoint() &&
+        !didCancelAnimation) {
+      SendTransformBeginAndEnd();
+    }
+  }
+
+  // If our scroll range changed (for example, because the page dynamically
+  // loaded new content, thereby increasing the size of the scrollable rect),
+  // and we're overscrolled, being overscrolled may no longer be a valid
+  // state (for example, we may no longer be at the edge of our scroll range),
+  // so clear overscroll and discontinue any overscroll animation.
+  // Ideas for improvements here:
+  //    - Instead of collapsing the overscroll gutter, try to "fill it"
+  //      with newly loaded content. This would basically entail checking
+  //      if (GetVisualScrollOffset() + GetOverscrollAmount()) is a valid
+  //      visual scroll offset in our new scroll range, and if so, scrolling
+  //      there.
+  if (needToReclampScroll) {
+    if (IsInInvalidOverscroll()) {
+      if (mState == OVERSCROLL_ANIMATION) {
+        CancelAnimation();
+      } else if (IsOverscrolled()) {
+        ClearOverscroll();
+      }
+    }
+  }
+
+  if (smoothScrollRequested && !scrollOffsetUpdated) {
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+    // Need to acknowledge the request.
+    needContentRepaint = true;
+  }
+
+  // If `isDefault` is true, this APZC is a "new" one (this is the first time
+  // it's getting a NotifyLayersUpdated call). In this case we want to apply the
+  // visual scroll offset from the main thread to our scroll offset.
+  // The main thread may also ask us to scroll the visual viewport to a
+  // particular location. However, in all cases, we want to ignore the visual
+  // offset update if ignoreVisualUpdate is true, because we're clobbering
+  // the visual update with a layout update.
+  bool visualScrollOffsetUpdated =
+      !ignoreVisualUpdate &&
+      (isDefault ||
+       aLayerMetrics.GetVisualScrollUpdateType() != FrameMetrics::eNone);
+
+  if (visualScrollOffsetUpdated) {
+    APZC_LOG("%p updating visual scroll offset from %s to %s (updateType %d)\n",
+             this, ToString(Metrics().GetVisualScrollOffset()).c_str(),
+             ToString(aLayerMetrics.GetVisualDestination()).c_str(),
+             (int)aLayerMetrics.GetVisualScrollUpdateType());
+    bool offsetChanged = Metrics().ClampAndSetVisualScrollOffset(
+        aLayerMetrics.GetVisualDestination());
+
+    // If this is the first time we got metrics for this content (isDefault) and
+    // the update type was none and the offset didn't change then we don't have
+    // to do anything. This is important because we don't want to request
+    // repaint on the initial NotifyLayersUpdated for every content and thus set
+    // a full display port.
+    if (aLayerMetrics.GetVisualScrollUpdateType() == FrameMetrics::eNone &&
+        !offsetChanged) {
+      visualScrollOffsetUpdated = false;
+    }
+  }
+  if (visualScrollOffsetUpdated) {
+    // The rest of this branch largely follows the code in the
+    // |if (scrollOffsetUpdated)| branch above. Eventually it should get
+    // merged into that branch.
+    Metrics().RecalculateLayoutViewportOffset();
+    for (auto& sampledState : mSampledState) {
+      sampledState.UpdateScrollProperties(Metrics());
+    }
+    mExpectedGeckoMetrics.UpdateFrom(aLayerMetrics);
+    if (ShouldCancelAnimationForScrollUpdate(Nothing())) {
+      CancelAnimation();
+    }
+    // The main thread did not actually paint a displayport at the target
+    // visual offset, so we need to ask it to repaint. We need to set the
+    // contentRepaintType to something other than eNone, otherwise the main
+    // thread will short-circuit the repaint request.
+    // Don't do this for eRestore visual updates as a repaint coming from APZ
+    // breaks the scroll offset restoration mechanism.
+    needContentRepaint = true;
+    if (aLayerMetrics.GetVisualScrollUpdateType() ==
+        FrameMetrics::eMainThread) {
+      contentRepaintType = RepaintUpdateType::eVisualUpdate;
+    }
+    ScheduleComposite();
+  }
+
+  if (viewportSizeUpdated) {
+    // While we want to accept the main thread's layout viewport _size_,
+    // its position may be out of date in light of async scrolling, to
+    // adjust it if necessary to make sure it continues to enclose the
+    // visual viewport.
+    // Note: it's important to do this _after_ we've accepted any
+    // updated composition bounds.
+    Metrics().RecalculateLayoutViewportOffset();
+  }
+
+  if (needContentRepaint) {
+    // This repaint request could be driven by a user action if we accept a
+    // relative scroll offset update
+    RequestContentRepaint(contentRepaintType);
+  }
+}
+
+FrameMetrics& AsyncPanZoomController::Metrics() {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata.GetMetrics();
+}
+
+const FrameMetrics& AsyncPanZoomController::Metrics() const {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata.GetMetrics();
+}
+
+GeckoViewMetrics AsyncPanZoomController::GetGeckoViewMetrics() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return GeckoViewMetrics{GetEffectiveScrollOffset(eForCompositing, lock),
+                          GetEffectiveZoom(eForCompositing, lock)};
+}
+
+wr::MinimapData AsyncPanZoomController::GetMinimapData() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  wr::MinimapData result;
+  result.is_root_content = IsRootContent();
+  // We want the minimap to reflect the scroll offset actually composited,
+  // which could be older than the latest one in Metrics() due to the frame
+  // delay.
+  CSSRect visualViewport = GetCurrentAsyncVisualViewport(eForCompositing);
+  result.visual_viewport = wr::ToLayoutRect(visualViewport.ToUnknownRect());
+  CSSRect layoutViewport = GetEffectiveLayoutViewport(eForCompositing, lock);
+  result.layout_viewport = wr::ToLayoutRect(layoutViewport.ToUnknownRect());
+  result.scrollable_rect =
+      wr::ToLayoutRect(Metrics().GetScrollableRect().ToUnknownRect());
+  // The display port is stored relative to the layout viewport origin.
+  // Translate it to be relative to the document origin, like the other rects.
+  CSSRect displayPort = mLastContentPaintMetrics.GetDisplayPort() +
+                        mLastContentPaintMetrics.GetLayoutScrollOffset();
+  result.displayport = wr::ToLayoutRect(displayPort.ToUnknownRect());
+  // Remaining fields (zoom_transform, root_content_scroll_id,
+  // root_content_pipeline_id) will be populated by the caller, since they
+  // require information from other APZCs to compute.
+  return result;
+}
+
+bool AsyncPanZoomController::UpdateRootFrameMetricsIfChanged(
+    GeckoViewMetrics& aMetrics) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  if (!Metrics().IsRootContent()) {
+    return false;
+  }
+
+  GeckoViewMetrics newMetrics = GetGeckoViewMetrics();
+  bool hasChanged = RoundedToInt(aMetrics.mVisualScrollOffset) !=
+                        RoundedToInt(newMetrics.mVisualScrollOffset) ||
+                    aMetrics.mZoom != newMetrics.mZoom;
+
+  if (hasChanged) {
+    aMetrics = newMetrics;
+  }
+
+  return hasChanged;
+}
+
+const FrameMetrics& AsyncPanZoomController::GetFrameMetrics() const {
+  return Metrics();
+}
+
+const ScrollMetadata& AsyncPanZoomController::GetScrollMetadata() const {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  return mScrollMetadata;
+}
+
+void AsyncPanZoomController::AssertOnSamplerThread() const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    treeManagerLocal->AssertOnSamplerThread();
+  }
+}
+
+void AsyncPanZoomController::AssertOnUpdaterThread() const {
+  if (APZCTreeManager* treeManagerLocal = GetApzcTreeManager()) {
+    treeManagerLocal->AssertOnUpdaterThread();
+  }
+}
+
+APZCTreeManager* AsyncPanZoomController::GetApzcTreeManager() const {
+  mRecursiveMutex.AssertNotCurrentThreadIn();
+  return mTreeManager;
+}
+
+void AsyncPanZoomController::ZoomToRect(const ZoomTarget& aZoomTarget,
+                                        const uint32_t aFlags) {
+  CSSRect rect = aZoomTarget.targetRect;
+  if (!rect.IsFinite()) {
+    NS_WARNING("ZoomToRect got called with a non-finite rect; ignoring...");
+    return;
+  }
+
+  if (rect.IsEmpty() && (aFlags & DISABLE_ZOOM_OUT)) {
+    // Double-tap-to-zooming uses an empty rect to mean "zoom out".
+    // If zooming out is disabled, an empty rect is nonsensical
+    // and will produce undesirable scrolling.
+    NS_WARNING(
+        "ZoomToRect got called with an empty rect and zoom out disabled; "
+        "ignoring...");
+    return;
+  }
+
+  SetState(ANIMATING_ZOOM);
+
+  {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+    MOZ_ASSERT(Metrics().IsRootContent());
+
+    const float defaultZoomInAmount =
+        StaticPrefs::apz_doubletapzoom_defaultzoomin();
+
+    ParentLayerRect compositionBounds = Metrics().GetCompositionBounds();
+    CSSRect cssPageRect = Metrics().GetScrollableRect();
+    CSSPoint scrollOffset = Metrics().GetVisualScrollOffset();
+    CSSSize sizeBeforeZoom = Metrics().CalculateCompositedSizeInCssPixels();
+    CSSToParentLayerScale currentZoom = Metrics().GetZoom();
+    CSSToParentLayerScale targetZoom;
+
+    // The minimum zoom to prevent over-zoom-out.
+    // If the zoom factor is lower than this (i.e. we are zoomed more into the
+    // page), then the CSS content rect, in layers pixels, will be smaller than
+    // the composition bounds. If this happens, we can't fill the target
+    // composited area with this frame.
+    const CSSRect cssExpandedPageRect = Metrics().GetExpandedScrollableRect();
+    CSSToParentLayerScale localMinZoom(
+        std::max(compositionBounds.Width() / cssExpandedPageRect.Width(),
+                 compositionBounds.Height() / cssExpandedPageRect.Height()));
+
+    localMinZoom.scale =
+        clamped(localMinZoom.scale, mZoomConstraints.mMinZoom.scale,
+                mZoomConstraints.mMaxZoom.scale);
+
+    localMinZoom = std::max(mZoomConstraints.mMinZoom, localMinZoom);
+    CSSToParentLayerScale localMaxZoom =
+        std::max(localMinZoom, mZoomConstraints.mMaxZoom);
+
+    if (!rect.IsEmpty()) {
+      // Intersect the zoom-to-rect to the CSS rect to make sure it fits.
+      rect = rect.Intersect(cssPageRect);
+      targetZoom = CSSToParentLayerScale(
+          std::min(compositionBounds.Width() / rect.Width(),
+                   compositionBounds.Height() / rect.Height()));
+      if (aFlags & DISABLE_ZOOM_OUT) {
+        targetZoom = std::max(targetZoom, currentZoom);
+      }
+    }
+
+    // 1. If the rect is empty, the content-side logic for handling a double-tap
+    //    requested that we zoom out.
+    // 2. currentZoom is equal to mZoomConstraints.mMaxZoom and user still
+    // double-tapping it
+    // Treat these cases as a request to zoom out as much as possible
+    // unless cantZoomOutBehavior == ZoomIn and currentZoom
+    // is equal to localMinZoom and user still double-tapping it, then try to
+    // zoom in a small amount to provide feedback to the user.
+    bool zoomOut = false;
+    // True if we are already zoomed out and we are asked to either stay there
+    // or zoom out more and cantZoomOutBehavior == ZoomIn.
+    bool zoomInDefaultAmount = false;
+    if (aFlags & DISABLE_ZOOM_OUT) {
+      zoomOut = false;
+    } else {
+      if (rect.IsEmpty()) {
+        if (currentZoom == localMinZoom &&
+            aZoomTarget.cantZoomOutBehavior == CantZoomOutBehavior::ZoomIn &&
+            (defaultZoomInAmount != 1.f)) {
+          zoomInDefaultAmount = true;
+        } else {
+          zoomOut = true;
+        }
+      } else if (currentZoom == localMaxZoom && targetZoom >= localMaxZoom) {
+        zoomOut = true;
+      }
+    }
+
+    // already at min zoom and asked to zoom out further
+    if (!zoomOut && currentZoom == localMinZoom && targetZoom <= localMinZoom &&
+        aZoomTarget.cantZoomOutBehavior == CantZoomOutBehavior::ZoomIn &&
+        (defaultZoomInAmount != 1.f)) {
+      zoomInDefaultAmount = true;
+    }
+    MOZ_ASSERT(!(zoomInDefaultAmount && zoomOut));
+
+    if (zoomInDefaultAmount) {
+      targetZoom =
+          CSSToParentLayerScale(currentZoom.scale * defaultZoomInAmount);
+    }
+
+    if (zoomOut) {
+      targetZoom = localMinZoom;
+    }
+
+    if (aFlags & PAN_INTO_VIEW_ONLY) {
+      targetZoom = currentZoom;
+    } else if (aFlags & ONLY_ZOOM_TO_DEFAULT_SCALE) {
+      CSSToParentLayerScale zoomAtDefaultScale =
+          Metrics().GetDevPixelsPerCSSPixel() *
+          LayoutDeviceToParentLayerScale(1.0);
+      if (targetZoom.scale > zoomAtDefaultScale.scale) {
+        // Only change the zoom if we are less than the default zoom
+        if (currentZoom.scale < zoomAtDefaultScale.scale) {
+          targetZoom = zoomAtDefaultScale;
+        } else {
+          targetZoom = currentZoom;
+        }
+      }
+    }
+
+    targetZoom.scale =
+        clamped(targetZoom.scale, localMinZoom.scale, localMaxZoom.scale);
+
+    FrameMetrics endZoomToMetrics = Metrics();
+    endZoomToMetrics.SetZoom(CSSToParentLayerScale(targetZoom));
+    CSSSize sizeAfterZoom =
+        endZoomToMetrics.CalculateCompositedSizeInCssPixels();
+
+    if (zoomInDefaultAmount || zoomOut) {
+      // For the zoom out case we should always center what was visible
+      // otherwise it feels like we are scrolling as well as zooming out. For
+      // the non-zoomOut case, if we've been provided a pointer location, zoom
+      // around that, otherwise just zoom in to the center of what's currently
+      // visible.
+      if (!zoomOut && aZoomTarget.documentRelativePointerPosition.isSome()) {
+        rect = CSSRect(aZoomTarget.documentRelativePointerPosition->x -
+                           sizeAfterZoom.width / 2,
+                       aZoomTarget.documentRelativePointerPosition->y -
+                           sizeAfterZoom.height / 2,
+                       sizeAfterZoom.Width(), sizeAfterZoom.Height());
+      } else {
+        rect = CSSRect(
+            scrollOffset.x + (sizeBeforeZoom.width - sizeAfterZoom.width) / 2,
+            scrollOffset.y + (sizeBeforeZoom.height - sizeAfterZoom.height) / 2,
+            sizeAfterZoom.Width(), sizeAfterZoom.Height());
+      }
+
+      rect = rect.Intersect(cssPageRect);
+    }
+
+    // Check if we can fit the full elementBoundingRect.
+    if (!aZoomTarget.targetRect.IsEmpty() && !zoomOut &&
+        aZoomTarget.elementBoundingRect.isSome()) {
+      MOZ_ASSERT(aZoomTarget.elementBoundingRect->Contains(rect));
+      CSSRect elementBoundingRect =
+          aZoomTarget.elementBoundingRect->Intersect(cssPageRect);
+      if (elementBoundingRect.width <= sizeAfterZoom.width &&
+          elementBoundingRect.height <= sizeAfterZoom.height) {
+        rect = elementBoundingRect;
+      }
+    }
+
+    // Vertically center the zoomed element in the screen.
+    if (!zoomOut && (sizeAfterZoom.height > rect.Height())) {
+      rect.MoveByY(-(sizeAfterZoom.height - rect.Height()) * 0.5f);
+      if (rect.Y() < 0.0f) {
+        rect.MoveToY(0.0f);
+      }
+    }
+
+    // Horizontally center the zoomed element in the screen.
+    if (!zoomOut && (sizeAfterZoom.width > rect.Width())) {
+      rect.MoveByX(-(sizeAfterZoom.width - rect.Width()) * 0.5f);
+      if (rect.X() < 0.0f) {
+        rect.MoveToX(0.0f);
+      }
+    }
+
+    bool intersectRectAgain = false;
+    // If we can't zoom out enough to show the full rect then shift the rect we
+    // are able to show to center what was visible.
+    // Note that this calculation works no matter the relation of sizeBeforeZoom
+    // to sizeAfterZoom, ie whether we are increasing or decreasing zoom.
+    if (!zoomOut && (sizeAfterZoom.height < rect.Height())) {
+      rect.y =
+          scrollOffset.y + (sizeBeforeZoom.height - sizeAfterZoom.height) / 2;
+      rect.height = sizeAfterZoom.Height();
+
+      intersectRectAgain = true;
+    }
+
+    if (!zoomOut && (sizeAfterZoom.width < rect.Width())) {
+      rect.x =
+          scrollOffset.x + (sizeBeforeZoom.width - sizeAfterZoom.width) / 2;
+      rect.width = sizeAfterZoom.Width();
+
+      intersectRectAgain = true;
+    }
+    if (intersectRectAgain) {
+      rect = rect.Intersect(cssPageRect);
+    }
+
+    // If any of these conditions are met, the page will be overscrolled after
+    // zoomed. Attempting to scroll outside of the valid scroll range will cause
+    // problems.
+    if (rect.Y() + sizeAfterZoom.height > cssPageRect.YMost()) {
+      rect.MoveToY(std::max(cssPageRect.Y(),
+                            cssPageRect.YMost() - sizeAfterZoom.height));
+    }
+    if (rect.Y() < cssPageRect.Y()) {
+      rect.MoveToY(cssPageRect.Y());
+    }
+    if (rect.X() + sizeAfterZoom.width > cssPageRect.XMost()) {
+      rect.MoveToX(
+          std::max(cssPageRect.X(), cssPageRect.XMost() - sizeAfterZoom.width));
+    }
+    if (rect.X() < cssPageRect.X()) {
+      rect.MoveToY(cssPageRect.X());
+    }
+
+    endZoomToMetrics.SetVisualScrollOffset(rect.TopLeft());
+    endZoomToMetrics.RecalculateLayoutViewportOffset();
+
+    StartAnimation(do_AddRef(new ZoomAnimation(
+        *this, Metrics().GetVisualScrollOffset(), Metrics().GetZoom(),
+        endZoomToMetrics.GetVisualScrollOffset(), endZoomToMetrics.GetZoom())));
+
+    RequestContentRepaint(RepaintUpdateType::eUserAction);
+  }
+}
+
+InputBlockState* AsyncPanZoomController::GetCurrentInputBlock() const {
+  return GetInputQueue()->GetCurrentBlock();
+}
+
+TouchBlockState* AsyncPanZoomController::GetCurrentTouchBlock() const {
+  return GetInputQueue()->GetCurrentTouchBlock();
+}
+
+PanGestureBlockState* AsyncPanZoomController::GetCurrentPanGestureBlock()
+    const {
+  return GetInputQueue()->GetCurrentPanGestureBlock();
+}
+
+PinchGestureBlockState* AsyncPanZoomController::GetCurrentPinchGestureBlock()
+    const {
+  return GetInputQueue()->GetCurrentPinchGestureBlock();
+}
+
+void AsyncPanZoomController::ResetTouchInputState() {
+  MultiTouchInput cancel(MultiTouchInput::MULTITOUCH_CANCEL, 0,
+                         TimeStamp::Now(), 0);
+  RefPtr<GestureEventListener> listener = GetGestureEventListener();
+  if (listener) {
+    listener->HandleInputEvent(cancel);
+  }
+  CancelAnimationAndGestureState();
+  // Clear overscroll along the entire handoff chain, in case an APZC
+  // later in the chain is overscrolled.
+  if (TouchBlockState* block = GetCurrentTouchBlock()) {
+    block->GetOverscrollHandoffChain()->ClearOverscroll();
+  }
+}
+
+void AsyncPanZoomController::ResetPanGestureInputState() {
+  // No point sending a PANGESTURE_INTERRUPTED as all it does is
+  // call CancelAnimation(), which we also do here.
+  CancelAnimationAndGestureState();
+  // Clear overscroll along the entire handoff chain, in case an APZC
+  // later in the chain is overscrolled.
+  if (PanGestureBlockState* block = GetCurrentPanGestureBlock()) {
+    block->GetOverscrollHandoffChain()->ClearOverscroll();
+  }
+}
+
+void AsyncPanZoomController::CancelAnimationAndGestureState() {
+  mX.CancelGesture();
+  mY.CancelGesture();
+  CancelAnimation(CancelAnimationFlags::ScrollSnap);
+}
+
+bool AsyncPanZoomController::HasReadyTouchBlock() const {
+  return GetInputQueue()->HasReadyTouchBlock();
+}
+
+bool AsyncPanZoomController::CanHandleScrollOffsetUpdate(PanZoomState aState) {
+  return aState == NOTHING || aState == PAN_MOMENTUM || aState == TOUCHING ||
+         IsPanningState(aState);
+}
+
+bool AsyncPanZoomController::ShouldCancelAnimationForScrollUpdate(
+    const Maybe<CSSPoint>& aRelativeDelta) {
+  // Never call CancelAnimation() for a no-op relative update.
+  if (aRelativeDelta == Some(CSSPoint())) {
+    return false;
+  }
+
+  if (mAnimation) {
+    return !mAnimation->HandleScrollOffsetUpdate(aRelativeDelta);
+  }
+
+  return !CanHandleScrollOffsetUpdate(mState);
+}
+
+AsyncPanZoomController::PanZoomState
+AsyncPanZoomController::SetStateNoContentControllerDispatch(
+    PanZoomState aNewState) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  APZC_LOG_DETAIL("changing from state %s to %s\n", this,
+                  ToString(mState).c_str(), ToString(aNewState).c_str());
+  PanZoomState oldState = mState;
+  mState = aNewState;
+  return oldState;
+}
+
+void AsyncPanZoomController::SetState(PanZoomState aNewState) {
+  // When a state transition to a transforming state is occuring and a delayed
+  // transform end notification exists, send the TransformEnd notification
+  // before the TransformBegin notification is sent for the input state change.
+  if (IsTransformingState(aNewState) && IsDelayedTransformEndSet()) {
+    MOZ_ASSERT(!IsTransformingState(mState));
+    SetDelayedTransformEnd(false);
+    DispatchStateChangeNotification(PANNING, NOTHING);
+  }
+
+  PanZoomState oldState = SetStateNoContentControllerDispatch(aNewState);
+
+  DispatchStateChangeNotification(oldState, aNewState);
+}
+
+auto AsyncPanZoomController::GetState() const -> PanZoomState {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mState;
+}
+
+void AsyncPanZoomController::DispatchStateChangeNotification(
+    PanZoomState aOldState, PanZoomState aNewState) {
+  {  // scope the lock
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    if (mNotificationBlockers > 0) {
+      return;
+    }
+  }
+
+  if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
+    if (!IsTransformingState(aOldState) && IsTransformingState(aNewState)) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eTransformBegin);
+    } else if (IsTransformingState(aOldState) &&
+               !IsTransformingState(aNewState)) {
+      controller->NotifyAPZStateChange(GetGuid(),
+                                       APZStateChange::eTransformEnd);
+    }
+  }
+}
+void AsyncPanZoomController::SendTransformBeginAndEnd() {
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    controller->NotifyAPZStateChange(GetGuid(),
+                                     APZStateChange::eTransformBegin);
+    controller->NotifyAPZStateChange(GetGuid(), APZStateChange::eTransformEnd);
+  }
+}
+
+bool AsyncPanZoomController::IsInTransformingState() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return IsTransformingState(mState);
+}
+
+bool AsyncPanZoomController::IsTransformingState(PanZoomState aState) {
+  return !(aState == NOTHING || aState == TOUCHING);
+}
+
+bool AsyncPanZoomController::IsPanningState(PanZoomState aState) {
+  return (aState == PANNING || aState == PANNING_LOCKED_X ||
+          aState == PANNING_LOCKED_Y);
+}
+
+bool AsyncPanZoomController::IsInPanningState() const {
+  return IsPanningState(mState);
+}
+
+bool AsyncPanZoomController::IsInScrollingGesture() const {
+  return IsPanningState(mState) || mState == SCROLLBAR_DRAG ||
+         mState == TOUCHING || mState == PINCHING;
+}
+
+bool AsyncPanZoomController::IsDelayedTransformEndSet() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mDelayedTransformEnd;
+}
+
+void AsyncPanZoomController::SetDelayedTransformEnd(bool aDelayedTransformEnd) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mDelayedTransformEnd = aDelayedTransformEnd;
+}
+
+void AsyncPanZoomController::UpdateZoomConstraints(
+    const ZoomConstraints& aConstraints) {
+  if ((MOZ_LOG_TEST(sApzCtlLog, LogLevel::Debug) &&
+       (aConstraints != mZoomConstraints)) ||
+      MOZ_LOG_TEST(sApzCtlLog, LogLevel::Verbose)) {
+    APZC_LOG("%p updating zoom constraints to %d %d %f %f\n", this,
+             aConstraints.mAllowZoom, aConstraints.mAllowDoubleTapZoom,
+             aConstraints.mMinZoom.scale, aConstraints.mMaxZoom.scale);
+  }
+
+  if (std::isnan(aConstraints.mMinZoom.scale) ||
+      std::isnan(aConstraints.mMaxZoom.scale)) {
+    NS_WARNING("APZC received zoom constraints with NaN values; dropping...");
+    return;
+  }
+
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSToParentLayerScale min = Metrics().GetDevPixelsPerCSSPixel() *
+                              ViewportMinScale() / ParentLayerToScreenScale(1);
+  CSSToParentLayerScale max = Metrics().GetDevPixelsPerCSSPixel() *
+                              ViewportMaxScale() / ParentLayerToScreenScale(1);
+
+  // inf float values and other bad cases should be sanitized by the code below.
+  mZoomConstraints.mAllowZoom = aConstraints.mAllowZoom;
+  mZoomConstraints.mAllowDoubleTapZoom = aConstraints.mAllowDoubleTapZoom;
+  mZoomConstraints.mMinZoom =
+      (min > aConstraints.mMinZoom ? min : aConstraints.mMinZoom);
+  mZoomConstraints.mMaxZoom =
+      (max > aConstraints.mMaxZoom ? aConstraints.mMaxZoom : max);
+  if (mZoomConstraints.mMaxZoom < mZoomConstraints.mMinZoom) {
+    mZoomConstraints.mMaxZoom = mZoomConstraints.mMinZoom;
+  }
+}
+
+bool AsyncPanZoomController::ZoomConstraintsAllowZoom() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomConstraints.mAllowZoom;
+}
+
+bool AsyncPanZoomController::ZoomConstraintsAllowDoubleTapZoom() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomConstraints.mAllowDoubleTapZoom;
+}
+
+void AsyncPanZoomController::PostDelayedTask(already_AddRefed<Runnable> aTask,
+                                             int aDelayMs) {
+  APZThreadUtils::AssertOnControllerThread();
+  RefPtr<Runnable> task = aTask;
+  RefPtr<GeckoContentController> controller = GetGeckoContentController();
+  if (controller) {
+    controller->PostDelayedTask(task.forget(), aDelayMs);
+  }
+  // If there is no controller, that means this APZC has been destroyed, and
+  // we probably don't need to run the task. It will get destroyed when the
+  // RefPtr goes out of scope.
+}
+
+bool AsyncPanZoomController::Matches(const ScrollableLayerGuid& aGuid) {
+  return aGuid == GetGuid();
+}
+
+bool AsyncPanZoomController::HasTreeManager(
+    const APZCTreeManager* aTreeManager) const {
+  return GetApzcTreeManager() == aTreeManager;
+}
+
+void AsyncPanZoomController::GetGuid(ScrollableLayerGuid* aGuidOut) const {
+  if (aGuidOut) {
+    *aGuidOut = GetGuid();
+  }
+}
+
+ScrollableLayerGuid AsyncPanZoomController::GetGuid() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return ScrollableLayerGuid(mLayersId, Metrics().GetPresShellId(),
+                             Metrics().GetScrollId());
+}
+
+void AsyncPanZoomController::SetTestAsyncScrollOffset(const CSSPoint& aPoint) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mTestAsyncScrollOffset = aPoint;
+  ScheduleComposite();
+}
+
+void AsyncPanZoomController::SetTestAsyncZoom(
+    const LayerToParentLayerScale& aZoom) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mTestAsyncZoom = aZoom;
+  ScheduleComposite();
+}
+
+Maybe<CSSSnapDestination> AsyncPanZoomController::FindSnapPointNear(
+    const CSSPoint& aDestination, ScrollUnit aUnit,
+    ScrollSnapFlags aSnapFlags) {
+  mRecursiveMutex.AssertCurrentThreadIn();
+  APZC_LOG("%p scroll snapping near %s\n", this,
+           ToString(aDestination).c_str());
+  CSSRect scrollRange = Metrics().CalculateScrollRange();
+  if (auto snapDestination = ScrollSnapUtils::GetSnapPointForDestination(
+          mScrollMetadata.GetSnapInfo(), aUnit, aSnapFlags,
+          CSSRect::ToAppUnits(scrollRange),
+          CSSPoint::ToAppUnits(Metrics().GetVisualScrollOffset()),
+          CSSPoint::ToAppUnits(aDestination))) {
+    CSSPoint cssSnapPoint = CSSPoint::FromAppUnits(snapDestination->mPosition);
+    // GetSnapPointForDestination() can produce a destination that's outside
+    // of the scroll frame's scroll range. Clamp it here (this matches the
+    // behaviour of the main-thread code path, which clamps it in
+    // nsGfxScrollFrame::ScrollTo()).
+    return Some(CSSSnapDestination{scrollRange.ClampPoint(cssSnapPoint),
+                                   snapDestination->mTargetIds});
+  }
+  return Nothing();
+}
+
+Maybe<std::pair<MultiTouchInput, MultiTouchInput>>
+AsyncPanZoomController::MaybeSplitTouchMoveEvent(
+    const MultiTouchInput& aOriginalEvent, ScreenCoord aPanThreshold,
+    float aVectorLength, ExternalPoint& aExtPoint) {
+  if (aVectorLength <= aPanThreshold) {
+    return Nothing();
+  }
+
+  auto splitEvent = std::make_pair(aOriginalEvent, aOriginalEvent);
+
+  SingleTouchData& firstTouchData = splitEvent.first.mTouches[0];
+  SingleTouchData& secondTouchData = splitEvent.second.mTouches[0];
+
+  firstTouchData.mHistoricalData.Clear();
+  secondTouchData.mHistoricalData.Clear();
+
+  ExternalPoint destination = aExtPoint;
+  ExternalPoint thresholdPosition;
+
+  const float ratio = aPanThreshold / aVectorLength;
+  thresholdPosition.x = mStartTouch.x + ratio * (destination.x - mStartTouch.x);
+  thresholdPosition.y = mStartTouch.y + ratio * (destination.y - mStartTouch.y);
+
+  TouchSample start{mLastTouch};
+  // To compute the timestamp of the first event (which is at the threshold),
+  // use linear interpolation with the starting point |start| being the last
+  // event that's before the threshold, and the end point |end| being the first
+  // event after the threshold.
+
+  // The initial choice for |start| is the last touch event before
+  // |aOriginalEvent|, and the initial choice for |end| is |aOriginalEvent|.
+
+  // However, the historical data points stored in |aOriginalEvent| may contain
+  // intermediate positions that can serve as tighter bounds for the
+  // interpolation.
+  TouchSample end{destination, aOriginalEvent.mTimeStamp};
+
+  for (const auto& historicalData :
+       aOriginalEvent.mTouches[0].mHistoricalData) {
+    ExternalPoint histExtPoint = ToExternalPoint(aOriginalEvent.mScreenOffset,
+                                                 historicalData.mScreenPoint);
+
+    if (PanVector(histExtPoint).Length() <
+        PanVector(thresholdPosition).Length()) {
+      start = {histExtPoint, historicalData.mTimeStamp};
+    } else {
+      break;
+    }
+  }
+
+  for (const SingleTouchData::HistoricalTouchData& histData :
+       Reversed(aOriginalEvent.mTouches[0].mHistoricalData)) {
+    ExternalPoint histExtPoint =
+        ToExternalPoint(aOriginalEvent.mScreenOffset, histData.mScreenPoint);
+
+    if (PanVector(histExtPoint).Length() >
+        PanVector(thresholdPosition).Length()) {
+      end = {histExtPoint, histData.mTimeStamp};
+    } else {
+      break;
+    }
+  }
+
+  const float totalLength =
+      ScreenPoint(fabs(end.mPosition.x - start.mPosition.x),
+                  fabs(end.mPosition.y - start.mPosition.y))
+          .Length();
+  const float thresholdLength =
+      ScreenPoint(fabs(thresholdPosition.x - start.mPosition.x),
+                  fabs(thresholdPosition.y - start.mPosition.y))
+          .Length();
+  const float splitRatio = thresholdLength / totalLength;
+
+  splitEvent.first.mTimeStamp =
+      start.mTimeStamp +
+      (end.mTimeStamp - start.mTimeStamp).MultDouble(splitRatio);
+
+  for (const auto& historicalData :
+       aOriginalEvent.mTouches[0].mHistoricalData) {
+    if (historicalData.mTimeStamp > splitEvent.first.mTimeStamp) {
+      secondTouchData.mHistoricalData.AppendElement(historicalData);
+    } else {
+      firstTouchData.mHistoricalData.AppendElement(historicalData);
+    }
+  }
+
+  firstTouchData.mScreenPoint = RoundedToInt(
+      ViewAs<ScreenPixel>(thresholdPosition - splitEvent.first.mScreenOffset,
+                          PixelCastJustification::ExternalIsScreen));
+
+  // Recompute firstTouchData.mLocalScreenPoint.
+  splitEvent.first.TransformToLocal(GetTransformToThis());
+
+  // Pass |thresholdPosition| back out to the caller via |aExtPoint|
+  aExtPoint = thresholdPosition;
+
+  return Some(splitEvent);
+}
+
+void AsyncPanZoomController::ScrollSnapNear(const CSSPoint& aDestination,
+                                            ScrollSnapFlags aSnapFlags) {
+  if (Maybe<CSSSnapDestination> snapDestination = FindSnapPointNear(
+          aDestination, ScrollUnit::DEVICE_PIXELS, aSnapFlags)) {
+    if (snapDestination->mPosition != Metrics().GetVisualScrollOffset()) {
+      APZC_LOG("%p smooth scrolling to snap point %s\n", this,
+               ToString(snapDestination->mPosition).c_str());
+      SmoothMsdScrollTo(std::move(*snapDestination),
+                        ScrollTriggeredByScript::No);
+    }
+  }
+}
+
+void AsyncPanZoomController::ScrollSnap(ScrollSnapFlags aSnapFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ScrollSnapNear(Metrics().GetVisualScrollOffset(), aSnapFlags);
+}
+
+void AsyncPanZoomController::ScrollSnapToDestination() {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+
+  float friction = StaticPrefs::apz_fling_friction();
+  ParentLayerPoint velocity(mX.GetVelocity(), mY.GetVelocity());
+  ParentLayerPoint predictedDelta;
+  // "-velocity / log(1.0 - friction)" is the integral of the deceleration
+  // curve modeled for flings in the "Axis" class.
+  if (velocity.x != 0.0f && friction != 0.0f) {
+    predictedDelta.x = -velocity.x / log(1.0 - friction);
+  }
+  if (velocity.y != 0.0f && friction != 0.0f) {
+    predictedDelta.y = -velocity.y / log(1.0 - friction);
+  }
+
+  // If the fling will overscroll, don't scroll snap, because then the user
+  // user would not see any overscroll animation.
+  bool flingWillOverscroll =
+      IsOverscrolled() && ((velocity.x.value * mX.GetOverscroll() >= 0) ||
+                           (velocity.y.value * mY.GetOverscroll() >= 0));
+  if (flingWillOverscroll) {
+    return;
+  }
+
+  CSSPoint startPosition = Metrics().GetVisualScrollOffset();
+  ScrollSnapFlags snapFlags = ScrollSnapFlags::IntendedEndPosition;
+  if (predictedDelta != ParentLayerPoint()) {
+    snapFlags |= ScrollSnapFlags::IntendedDirection;
+  }
+  if (Maybe<CSSSnapDestination> snapDestination =
+          MaybeAdjustDeltaForScrollSnapping(ScrollUnit::DEVICE_PIXELS,
+                                            snapFlags, predictedDelta,
+                                            startPosition)) {
+    APZC_LOG(
+        "%p fling snapping.  friction: %f velocity: %f, %f "
+        "predictedDelta: %f, %f position: %f, %f "
+        "snapDestination: %f, %f\n",
+        this, friction, velocity.x.value, velocity.y.value,
+        predictedDelta.x.value, predictedDelta.y.value,
+        Metrics().GetVisualScrollOffset().x.value,
+        Metrics().GetVisualScrollOffset().y.value, startPosition.x.value,
+        startPosition.y.value);
+
+    // Ensure that any queued transform-end due to a pan-end is not
+    // sent. Instead rely on the transform-end sent due to the
+    // scroll snap animation.
+    SetDelayedTransformEnd(false);
+
+    SmoothMsdScrollTo(std::move(*snapDestination), ScrollTriggeredByScript::No);
+  }
+}
+
+Maybe<CSSSnapDestination>
+AsyncPanZoomController::MaybeAdjustDeltaForScrollSnapping(
+    ScrollUnit aUnit, ScrollSnapFlags aSnapFlags, ParentLayerPoint& aDelta,
+    CSSPoint& aStartPosition) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  CSSToParentLayerScale zoom = Metrics().GetZoom();
+  if (zoom == CSSToParentLayerScale(0)) {
+    return Nothing();
+  }
+  CSSPoint destination = Metrics().CalculateScrollRange().ClampPoint(
+      aStartPosition + (aDelta / zoom));
+
+  if (Maybe<CSSSnapDestination> snapDestination =
+          FindSnapPointNear(destination, aUnit, aSnapFlags)) {
+    aDelta = (snapDestination->mPosition - aStartPosition) * zoom;
+    aStartPosition = snapDestination->mPosition;
+    return snapDestination;
+  }
+  return Nothing();
+}
+
+Maybe<CSSSnapDestination>
+AsyncPanZoomController::MaybeAdjustDeltaForScrollSnappingOnWheelInput(
+    const ScrollWheelInput& aEvent, ParentLayerPoint& aDelta,
+    CSSPoint& aStartPosition) {
+  // Don't scroll snap for pixel scrolls. This matches the main thread
+  // behaviour in EventStateManager::DoScrollText().
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_PIXEL) {
+    return Nothing();
+  }
+
+  // Note that this MaybeAdjustDeltaForScrollSnappingOnWheelInput also gets
+  // called for pan gestures at least on older Mac and Windows. In such cases
+  // `aEvent.mDeltaType` is `SCROLLDELTA_PIXEL` which should be filtered out by
+  // the above `if` block, so we assume all incoming `aEvent` are purely wheel
+  // events, thus we basically use `IntendedDirection` here.
+  // If we want to change the behavior, i.e. we want to do scroll snap for
+  // such cases as well, we need to use `IntendedEndPoint`.
+  ScrollSnapFlags snapFlags = ScrollSnapFlags::IntendedDirection;
+  if (aEvent.mDeltaType == ScrollWheelInput::SCROLLDELTA_PAGE) {
+    // On Windows there are a couple of cases where scroll events happen with
+    // SCROLLDELTA_PAGE, in such case we consider it's a page scroll.
+    snapFlags |= ScrollSnapFlags::IntendedEndPosition;
+  }
+  return MaybeAdjustDeltaForScrollSnapping(
+      ScrollWheelInput::ScrollUnitForDeltaType(aEvent.mDeltaType),
+      ScrollSnapFlags::IntendedDirection, aDelta, aStartPosition);
+}
+
+Maybe<CSSSnapDestination>
+AsyncPanZoomController::MaybeAdjustDestinationForScrollSnapping(
+    const KeyboardInput& aEvent, CSSPoint& aDestination,
+    ScrollSnapFlags aSnapFlags) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  ScrollUnit unit = KeyboardScrollAction::GetScrollUnit(aEvent.mAction.mType);
+
+  if (Maybe<CSSSnapDestination> snapPoint =
+          FindSnapPointNear(aDestination, unit, aSnapFlags)) {
+    aDestination = snapPoint->mPosition;
+    return snapPoint;
+  }
+  return Nothing();
+}
+
+void AsyncPanZoomController::SetZoomAnimationId(
+    const Maybe<uint64_t>& aZoomAnimationId) {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  mZoomAnimationId = aZoomAnimationId;
+}
+
+Maybe<uint64_t> AsyncPanZoomController::GetZoomAnimationId() const {
+  RecursiveMutexAutoLock lock(mRecursiveMutex);
+  return mZoomAnimationId;
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const AsyncPanZoomController::PanZoomState& aState) {
+  switch (aState) {
+    case AsyncPanZoomController::PanZoomState::NOTHING:
+      aOut << "NOTHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::FLING:
+      aOut << "FLING";
+      break;
+    case AsyncPanZoomController::PanZoomState::TOUCHING:
+      aOut << "TOUCHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING:
+      aOut << "PANNING";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING_LOCKED_X:
+      aOut << "PANNING_LOCKED_X";
+      break;
+    case AsyncPanZoomController::PanZoomState::PANNING_LOCKED_Y:
+      aOut << "PANNING_LOCKED_Y";
+      break;
+    case AsyncPanZoomController::PanZoomState::PAN_MOMENTUM:
+      aOut << "PAN_MOMENTUM";
+      break;
+    case AsyncPanZoomController::PanZoomState::PINCHING:
+      aOut << "PINCHING";
+      break;
+    case AsyncPanZoomController::PanZoomState::ANIMATING_ZOOM:
+      aOut << "ANIMATING_ZOOM";
+      break;
+    case AsyncPanZoomController::PanZoomState::OVERSCROLL_ANIMATION:
+      aOut << "OVERSCROLL_ANIMATION";
+      break;
+    case AsyncPanZoomController::PanZoomState::SMOOTH_SCROLL:
+      aOut << "SMOOTH_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::SMOOTHMSD_SCROLL:
+      aOut << "SMOOTHMSD_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::WHEEL_SCROLL:
+      aOut << "WHEEL_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::KEYBOARD_SCROLL:
+      aOut << "KEYBOARD_SCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::AUTOSCROLL:
+      aOut << "AUTOSCROLL";
+      break;
+    case AsyncPanZoomController::PanZoomState::SCROLLBAR_DRAG:
+      aOut << "SCROLLBAR_DRAG";
+      break;
+    default:
+      aOut << "UNKNOWN_STATE";
+      break;
+  }
+  return aOut;
+}
+
+bool operator==(const PointerEventsConsumableFlags& aLhs,
+                const PointerEventsConsumableFlags& aRhs) {
+  return (aLhs.mHasRoom == aRhs.mHasRoom) &&
+         (aLhs.mAllowedByTouchAction == aRhs.mAllowedByTouchAction);
+}
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const PointerEventsConsumableFlags& aFlags) {
+  aOut << std::boolalpha << "{ hasRoom: " << aFlags.mHasRoom
+       << ", allowedByTouchAction: " << aFlags.mAllowedByTouchAction << "}";
+  return aOut;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.h b/gfx/layers/apz/src/AsyncPanZoomController.h
new file mode 100644
index 0000000000..fdc72b1971
--- /dev/null
+++ b/gfx/layers/apz/src/AsyncPanZoomController.h
@@ -0,0 +1,2002 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AsyncPanZoomController_h
+#define mozilla_layers_AsyncPanZoomController_h
+
+#include "mozilla/layers/GeckoContentController.h"
+#include "mozilla/layers/RepaintRequest.h"
+#include "mozilla/layers/SampleTime.h"
+#include "mozilla/layers/ScrollbarData.h"
+#include "mozilla/layers/ZoomConstraints.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/EventForwards.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/RecursiveMutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/UniquePtr.h"
+#include "InputData.h"
+#include "Axis.h"  // for Axis, Side, etc.
+#include "ExpectedGeckoMetrics.h"
+#include "FlingAccelerator.h"
+#include "InputQueue.h"
+#include "APZUtils.h"
+#include "LayersTypes.h"
+#include "mozilla/gfx/Matrix.h"
+#include "nsRegion.h"
+#include "nsTArray.h"
+#include "PotentialCheckerboardDurationTracker.h"
+#include "RecentEventsBuffer.h"  // for RecentEventsBuffer
+#include "SampledAPZCState.h"
+
+#include <iosfwd>
+
+namespace mozilla {
+
+namespace ipc {
+
+class SharedMemoryBasic;
+
+}  // namespace ipc
+
+namespace wr {
+struct MinimapData;
+struct SampledScrollOffset;
+}  // namespace wr
+
+namespace layers {
+
+class AsyncDragMetrics;
+class APZCTreeManager;
+struct ScrollableLayerGuid;
+class CompositorController;
+class GestureEventListener;
+struct AsyncTransform;
+class AsyncPanZoomAnimation;
+class StackScrollerFlingAnimation;
+template <typename FlingPhysics>
+class GenericFlingAnimation;
+class AndroidFlingPhysics;
+class DesktopFlingPhysics;
+class InputBlockState;
+struct FlingHandoffState;
+class TouchBlockState;
+class PanGestureBlockState;
+class OverscrollHandoffChain;
+struct OverscrollHandoffState;
+class StateChangeNotificationBlocker;
+class CheckerboardEvent;
+class OverscrollEffectBase;
+class WidgetOverscrollEffect;
+class GenericOverscrollEffect;
+class AndroidSpecificState;
+struct KeyboardScrollAction;
+struct ZoomTarget;
+
+namespace apz {
+struct AsyncScrollThumbTransformer;
+}
+
+// Base class for grouping platform-specific APZC state variables.
+class PlatformSpecificStateBase {
+ public:
+  virtual ~PlatformSpecificStateBase() = default;
+  virtual AndroidSpecificState* AsAndroidSpecificState() { return nullptr; }
+  // PLPPI = "ParentLayer pixels per (Screen) inch"
+  virtual AsyncPanZoomAnimation* CreateFlingAnimation(
+      AsyncPanZoomController& aApzc, const FlingHandoffState& aHandoffState,
+      float aPLPPI);
+  virtual UniquePtr<VelocityTracker> CreateVelocityTracker(Axis* aAxis);
+
+  static void InitializeGlobalState() {}
+};
+
+/*
+ * Represents a transform from the ParentLayer coordinate space of an APZC
+ * to the ParentLayer coordinate space of its parent APZC.
+ * Each layer along the way contributes to the transform. We track
+ * contributions that are perspective transforms separately, as sometimes
+ * these require special handling.
+ */
+struct AncestorTransform {
+  gfx::Matrix4x4 mTransform;
+  gfx::Matrix4x4 mPerspectiveTransform;
+
+  AncestorTransform() = default;
+
+  AncestorTransform(const gfx::Matrix4x4& aTransform,
+                    bool aTransformIsPerspective) {
+    (aTransformIsPerspective ? mPerspectiveTransform : mTransform) = aTransform;
+  }
+
+  AncestorTransform(const gfx::Matrix4x4& aTransform,
+                    const gfx::Matrix4x4& aPerspectiveTransform)
+      : mTransform(aTransform), mPerspectiveTransform(aPerspectiveTransform) {}
+
+  gfx::Matrix4x4 CombinedTransform() const {
+    return mTransform * mPerspectiveTransform;
+  }
+
+  bool ContainsPerspectiveTransform() const {
+    return !mPerspectiveTransform.IsIdentity();
+  }
+
+  gfx::Matrix4x4 GetPerspectiveTransform() const {
+    return mPerspectiveTransform;
+  }
+
+  friend AncestorTransform operator*(const AncestorTransform& aA,
+                                     const AncestorTransform& aB) {
+    return AncestorTransform{
+        aA.mTransform * aB.mTransform,
+        aA.mPerspectiveTransform * aB.mPerspectiveTransform};
+  }
+};
+
+// Flags returned by AsyncPanZoomController::ArePointerEventsConsumable().
+// See the function for more details.
+struct PointerEventsConsumableFlags {
+  // The APZC has room to pan or zoom in response to the touch event.
+  bool mHasRoom = false;
+
+  // The panning or zooming is allowed by the touch-action property.
+  bool mAllowedByTouchAction = false;
+
+  bool IsConsumable() const { return mHasRoom && mAllowedByTouchAction; }
+  friend bool operator==(const PointerEventsConsumableFlags& aLhs,
+                         const PointerEventsConsumableFlags& aRhs);
+  friend std::ostream& operator<<(std::ostream& aOut,
+                                  const PointerEventsConsumableFlags& aFlags);
+};
+
+/**
+ * Controller for all panning and zooming logic. Any time a user input is
+ * detected and it must be processed in some way to affect what the user sees,
+ * it goes through here. Listens for any input event from InputData and can
+ * optionally handle WidgetGUIEvent-derived touch events, but this must be done
+ * on the main thread. Note that this class completely cross-platform.
+ *
+ * Input events originate on the UI thread of the platform that this runs on,
+ * and are then sent to this class. This class processes the event in some way;
+ * for example, a touch move will usually lead to a panning of content (though
+ * of course there are exceptions, such as if content preventDefaults the event,
+ * or if the target frame is not scrollable). The compositor interacts with this
+ * class by locking it and querying it for the current transform matrix based on
+ * the panning and zooming logic that was invoked on the UI thread.
+ *
+ * Currently, each outer DOM window (i.e. a website in a tab, but not any
+ * subframes) has its own AsyncPanZoomController. In the future, to support
+ * asynchronously scrolled subframes, we want to have one AsyncPanZoomController
+ * per frame.
+ */
+class AsyncPanZoomController {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AsyncPanZoomController)
+
+  typedef mozilla::MonitorAutoLock MonitorAutoLock;
+  typedef mozilla::gfx::Matrix4x4 Matrix4x4;
+  typedef mozilla::layers::RepaintRequest::ScrollOffsetUpdateType
+      RepaintUpdateType;
+
+ public:
+  enum GestureBehavior {
+    // The platform code is responsible for forwarding gesture events here. We
+    // will not attempt to generate gesture events from MultiTouchInputs.
+    DEFAULT_GESTURES,
+    // An instance of GestureEventListener is used to detect gestures. This is
+    // handled completely internally within this class.
+    USE_GESTURE_DETECTOR
+  };
+
+  /**
+   * Gets the DPI from the tree manager.
+   */
+  float GetDPI() const;
+
+  /**
+   * Constant describing the tolerance in distance we use, multiplied by the
+   * device DPI, before we start panning the screen. This is to prevent us from
+   * accidentally processing taps as touch moves, and from very short/accidental
+   * touches moving the screen.
+   * Note: It's an abuse of the 'Coord' class to use it to represent a 2D
+   *       distance, but it's the closest thing we currently have.
+   */
+  ScreenCoord GetTouchStartTolerance() const;
+  /**
+   * Same as GetTouchStartTolerance, but the tolerance for how far the touch
+   * has to move before it starts allowing touchmove events to be dispatched
+   * to content, for non-scrollable content.
+   */
+  ScreenCoord GetTouchMoveTolerance() const;
+  /**
+   * Same as GetTouchStartTolerance, but the tolerance for how close the second
+   * tap has to be to the first tap in order to be counted as part of a
+   * multi-tap gesture (double-tap or one-touch-pinch).
+   */
+  ScreenCoord GetSecondTapTolerance() const;
+
+  AsyncPanZoomController(LayersId aLayersId, APZCTreeManager* aTreeManager,
+                         const RefPtr<InputQueue>& aInputQueue,
+                         GeckoContentController* aController,
+                         GestureBehavior aGestures = DEFAULT_GESTURES);
+
+  // --------------------------------------------------------------------------
+  // These methods must only be called on the gecko thread.
+  //
+
+  /**
+   * Read the various prefs and do any global initialization for all APZC
+   * instances. This must be run on the gecko thread before any APZC instances
+   * are actually used for anything meaningful.
+   */
+  static void InitializeGlobalState();
+
+  // --------------------------------------------------------------------------
+  // These methods must only be called on the controller/UI thread.
+  //
+
+  /**
+   * Kicks an animation to zoom to a rect. This may be either a zoom out or zoom
+   * in. The actual animation is done on the sampler thread after being set
+   * up.
+   */
+  void ZoomToRect(const ZoomTarget& aZoomTarget, const uint32_t aFlags);
+
+  /**
+   * Updates any zoom constraints contained in the <meta name="viewport"> tag.
+   */
+  void UpdateZoomConstraints(const ZoomConstraints& aConstraints);
+
+  /**
+   * Schedules a runnable to run on the controller/UI thread at some time
+   * in the future.
+   */
+  void PostDelayedTask(already_AddRefed<Runnable> aTask, int aDelayMs);
+
+  // --------------------------------------------------------------------------
+  // These methods must only be called on the sampler thread.
+  //
+
+  /**
+   * Advances any animations currently running to the given timestamp.
+   * This may be called multiple times with the same timestamp.
+   *
+   * The return value indicates whether or not any currently running animation
+   * should continue. If true, the compositor should schedule another composite.
+   */
+  bool AdvanceAnimations(const SampleTime& aSampleTime);
+
+  bool UpdateAnimation(const RecursiveMutexAutoLock& aProofOfLock,
+                       const SampleTime& aSampleTime,
+                       nsTArray<RefPtr<Runnable>>* aOutDeferredTasks);
+
+  // --------------------------------------------------------------------------
+  // These methods must only be called on the updater thread.
+  //
+
+  /**
+   * A shadow layer update has arrived. |aScrollMetdata| is the new
+   * ScrollMetadata for the container layer corresponding to this APZC.
+   * |aIsFirstPaint| is a flag passed from the shadow
+   * layers code indicating that the scroll metadata being sent with this call
+   * are the initial metadata and the initial paint of the frame has just
+   * happened.
+   */
+  void NotifyLayersUpdated(const ScrollMetadata& aScrollMetadata,
+                           bool aIsFirstPaint, bool aThisLayerTreeUpdated);
+
+  /**
+   * The platform implementation must set the compositor controller so that we
+   * can request composites.
+   */
+  void SetCompositorController(CompositorController* aCompositorController);
+
+  // --------------------------------------------------------------------------
+  // These methods can be called from any thread.
+  //
+
+  /**
+   * Shut down the controller/UI thread state and prepare to be
+   * deleted (which may happen from any thread).
+   */
+  void Destroy();
+
+  /**
+   * Returns true if Destroy() has already been called on this APZC instance.
+   */
+  bool IsDestroyed() const;
+
+  /**
+   * Returns the transform to take something from the coordinate space of the
+   * last thing we know gecko painted, to the coordinate space of the last thing
+   * we asked gecko to paint. In cases where that last request has not yet been
+   * processed, this is needed to transform input events properly into a space
+   * gecko will understand.
+   *
+   * This is meant to be called in the context of computing a chain of
+   * transforms used for transforming event coordinates (specifically,
+   * APZCTreeManager::GetApzcToGeckoTransform() and
+   * HitTestingTreeNode::GetTransformToGecko()). The caller needs to pass
+   * in |aForLayersId| the LayersId of the content for which the chain will be
+   * used. If this content is in an out-of-process subdocument, the returned
+   * transform includes the painted resolution transform (see bug 1827330 for
+   * details).
+   */
+  Matrix4x4 GetTransformToLastDispatchedPaint(
+      const AsyncTransformComponents& aComponents, LayersId aForLayersId) const;
+
+  /**
+   * Returns the number of CSS pixels of checkerboard according to the metrics
+   * in this APZC. The argument provided by the caller is the composition bounds
+   * of this APZC, additionally clipped by the composition bounds of any
+   * ancestor APZCs, accounting for all the async transforms.
+   */
+  uint32_t GetCheckerboardMagnitude(
+      const ParentLayerRect& aClippedCompositionBounds) const;
+
+  /**
+   * Report the number of CSSPixel-milliseconds of checkerboard to telemetry.
+   * See GetCheckerboardMagnitude for documentation of the
+   * aClippedCompositionBounds argument that needs to be provided by the caller.
+   */
+  void ReportCheckerboard(const SampleTime& aSampleTime,
+                          const ParentLayerRect& aClippedCompositionBounds);
+
+  /**
+   * Flush any active checkerboard report that's in progress. This basically
+   * pretends like any in-progress checkerboard event has terminated, and pushes
+   * out the report to the checkerboard reporting service and telemetry. If the
+   * checkerboard event has not really finished, it will start a new event
+   * on the next composite.
+   */
+  void FlushActiveCheckerboardReport();
+
+  /**
+   * See documentation on corresponding method in APZPublicUtils.h
+   */
+  static gfx::IntSize GetDisplayportAlignmentMultiplier(
+      const ScreenSize& aBaseSize);
+
+  enum class ZoomInProgress {
+    No,
+    Yes,
+  };
+
+  /**
+   * Recalculates the displayport. Ideally, this should paint an area bigger
+   * than the composite-to dimensions so that when you scroll down, you don't
+   * checkerboard immediately. This includes a bunch of logic, including
+   * algorithms to bias painting in the direction of the velocity and other
+   * such things.
+   */
+  static const ScreenMargin CalculatePendingDisplayPort(
+      const FrameMetrics& aFrameMetrics, const ParentLayerPoint& aVelocity,
+      ZoomInProgress aZoomInProgress);
+
+  nsEventStatus HandleDragEvent(const MouseInput& aEvent,
+                                const AsyncDragMetrics& aDragMetrics,
+                                OuterCSSCoord aInitialThumbPos,
+                                const CSSRect& aInitialScrollableRect);
+
+  /**
+   * Handler for events which should not be intercepted by the touch listener.
+   */
+  nsEventStatus HandleInputEvent(
+      const InputData& aEvent,
+      const ScreenToParentLayerMatrix4x4& aTransformToApzc);
+
+  /**
+   * Handler for gesture events.
+   * Currently some gestures are detected in GestureEventListener that calls
+   * APZC back through this handler in order to avoid recursive calls to
+   * APZC::HandleInputEvent() which is supposed to do the work for
+   * ReceiveInputEvent().
+   */
+  nsEventStatus HandleGestureEvent(const InputData& aEvent);
+
+  /**
+   * Start autoscrolling this APZC, anchored at the provided location.
+   */
+  void StartAutoscroll(const ScreenPoint& aAnchorLocation);
+
+  /**
+   * Stop autoscrolling this APZC.
+   */
+  void StopAutoscroll();
+
+  /**
+   * Populates the provided object (if non-null) with the scrollable guid of
+   * this apzc.
+   */
+  void GetGuid(ScrollableLayerGuid* aGuidOut) const;
+
+  /**
+   * Returns the scrollable guid of this apzc.
+   */
+  ScrollableLayerGuid GetGuid() const;
+
+  /**
+   * Returns true if this APZC instance is for the layer identified by the guid.
+   */
+  bool Matches(const ScrollableLayerGuid& aGuid);
+
+  /**
+   * Returns true if the tree manager of this APZC is the same as the one
+   * passed in.
+   */
+  bool HasTreeManager(const APZCTreeManager* aTreeManager) const;
+
+  void StartAnimation(already_AddRefed<AsyncPanZoomAnimation> aAnimation);
+
+  /**
+   * Cancels any currently running animation.
+   * aFlags is a bit-field to provide specifics of how to cancel the animation.
+   * See CancelAnimationFlags.
+   */
+  void CancelAnimation(CancelAnimationFlags aFlags = Default);
+
+  /**
+   * Clear any overscroll on this APZC.
+   */
+  void ClearOverscroll();
+  void ClearPhysicalOverscroll();
+
+  /**
+   * Returns whether this APZC is for an element marked with the 'scrollgrab'
+   * attribute.
+   */
+  bool HasScrollgrab() const { return mScrollMetadata.GetHasScrollgrab(); }
+
+  /**
+   * Returns whether this APZC has scroll snap points.
+   */
+  bool HasScrollSnapping() const {
+    return mScrollMetadata.GetSnapInfo().HasScrollSnapping();
+  }
+
+  /**
+   * Returns whether this APZC has room to be panned (in any direction).
+   */
+  bool IsPannable() const;
+
+  /**
+   * Returns whether this APZC represents a scroll info layer.
+   */
+  bool IsScrollInfoLayer() const;
+
+  /**
+   * Returns true if the APZC has been flung with a velocity greater than the
+   * stop-on-tap fling velocity threshold (which is pref-controlled).
+   */
+  bool IsFlingingFast() const;
+
+  /**
+   * Returns whether this APZC is currently autoscrolling.
+   */
+  bool IsAutoscroll() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mState == AUTOSCROLL;
+  }
+
+  /**
+   * Returns the identifier of the touch in the last touch event processed by
+   * this APZC. This should only be called when the last touch event contained
+   * only one touch.
+   */
+  int32_t GetLastTouchIdentifier() const;
+
+  /**
+   * Returns the matrix that transforms points from global screen space into
+   * this APZC's ParentLayer space.
+   * To respect the lock ordering, mRecursiveMutex must NOT be held when calling
+   * this function (since this function acquires the tree lock).
+   */
+  ScreenToParentLayerMatrix4x4 GetTransformToThis() const;
+
+  /**
+   * Convert the vector |aVector|, rooted at the point |aAnchor|, from
+   * this APZC's ParentLayer coordinates into screen coordinates.
+   * The anchor is necessary because with 3D tranforms, the location of the
+   * vector can affect the result of the transform.
+   * To respect the lock ordering, mRecursiveMutex must NOT be held when calling
+   * this function (since this function acquires the tree lock).
+   */
+  ScreenPoint ToScreenCoordinates(const ParentLayerPoint& aVector,
+                                  const ParentLayerPoint& aAnchor) const;
+
+  /**
+   * Convert the vector |aVector|, rooted at the point |aAnchor|, from
+   * screen coordinates into this APZC's ParentLayer coordinates.
+   * The anchor is necessary because with 3D tranforms, the location of the
+   * vector can affect the result of the transform.
+   * To respect the lock ordering, mRecursiveMutex must NOT be held when calling
+   * this function (since this function acquires the tree lock).
+   */
+  ParentLayerPoint ToParentLayerCoordinates(const ScreenPoint& aVector,
+                                            const ScreenPoint& aAnchor) const;
+
+  /**
+   * Same as above, but uses an ExternalPoint as the anchor.
+   */
+  ParentLayerPoint ToParentLayerCoordinates(const ScreenPoint& aVector,
+                                            const ExternalPoint& aAnchor) const;
+
+  /**
+   * Combines an offset defined as an external point, with a window-relative
+   * offset to give an absolute external point.
+   */
+  static ExternalPoint ToExternalPoint(const ExternalPoint& aScreenOffset,
+                                       const ScreenPoint& aScreenPoint);
+
+  /**
+   * Gets a vector where the head is the given point, and the tail is
+   * the touch start position.
+   */
+  ScreenPoint PanVector(const ExternalPoint& aPos) const;
+
+  // Return whether or not a wheel event will be able to scroll in either
+  // direction.
+  bool CanScroll(const InputData& aEvent) const;
+
+  // Return the directions in which this APZC allows handoff (as governed by
+  // overscroll-behavior).
+  ScrollDirections GetAllowedHandoffDirections() const;
+
+  // Return the directions in which this APZC allows overscrolling.
+  ScrollDirections GetOverscrollableDirections() const;
+
+  // Return whether or not a scroll delta will be able to scroll in either
+  // direction.
+  bool CanScroll(const ParentLayerPoint& aDelta) const;
+
+  // Return whether or not a scroll delta will be able to scroll in either
+  // direction with wheel.
+  bool CanScrollWithWheel(const ParentLayerPoint& aDelta) const;
+
+  // Return whether or not there is room to scroll this APZC
+  // in the given direction.
+  bool CanScroll(ScrollDirection aDirection) const;
+
+  // Return the directions in which this APZC is able to scroll.
+  SideBits ScrollableDirections() const;
+
+  // Return true if there is room to scroll along with moving the dynamic
+  // toolbar.
+  //
+  // NOTE: This function should be used only for the root content APZC.
+  bool CanVerticalScrollWithDynamicToolbar() const;
+
+  // Return true if there is room to scroll downwards.
+  bool CanScrollDownwards() const;
+
+  // Return true if there is room to scroll upwards.
+  bool CanOverscrollUpwards() const;
+
+  /**
+   * Convert a point on the scrollbar from this APZC's ParentLayer coordinates
+   * to OuterCSS coordinates relative to the beginning of the scroll track.
+   * Only the component in the direction of scrolling is returned.
+   */
+  OuterCSSCoord ConvertScrollbarPoint(const ParentLayerPoint& aScrollbarPoint,
+                                      const ScrollbarData& aThumbData) const;
+
+  void NotifyMozMouseScrollEvent(const nsString& aString) const;
+
+  bool OverscrollBehaviorAllowsSwipe() const;
+
+  //|Metrics()| and |Metrics() const| are getter functions that both return
+  // mScrollMetadata.mMetrics
+
+  const FrameMetrics& Metrics() const;
+  FrameMetrics& Metrics();
+
+  /**
+   * Get the GeckoViewMetrics to be sent to Gecko for the current composite.
+   */
+  GeckoViewMetrics GetGeckoViewMetrics() const;
+
+  wr::MinimapData GetMinimapData() const;
+
+  // Helper function to compare root frame metrics and update them
+  // Returns true when the metrics have changed and were updated.
+  bool UpdateRootFrameMetricsIfChanged(GeckoViewMetrics& aMetrics);
+
+  // Returns the cached current frame time.
+  SampleTime GetFrameTime() const;
+
+  bool IsZero(const ParentLayerPoint& aPoint) const;
+  bool IsZero(ParentLayerCoord aCoord) const;
+
+  bool FuzzyGreater(ParentLayerCoord aCoord1, ParentLayerCoord aCoord2) const;
+
+ private:
+  // Get whether the horizontal content of the honoured target of auto-dir
+  // scrolling starts from right to left. If you don't know of auto-dir
+  // scrolling or what a honoured target means,
+  // @see mozilla::WheelDeltaAdjustmentStrategy
+  bool IsContentOfHonouredTargetRightToLeft(bool aHonoursRoot) const;
+
+ protected:
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~AsyncPanZoomController();
+
+  /**
+   * Helper method for touches beginning. Sets everything up for panning and any
+   * multitouch gestures.
+   */
+  nsEventStatus OnTouchStart(const MultiTouchInput& aEvent);
+
+  /**
+   * Helper method for touches moving. Does any transforms needed when panning.
+   */
+  nsEventStatus OnTouchMove(const MultiTouchInput& aEvent);
+
+  /**
+   * Helper method for touches ending. Redraws the screen if necessary and does
+   * any cleanup after a touch has ended.
+   */
+  nsEventStatus OnTouchEnd(const MultiTouchInput& aEvent);
+
+  /**
+   * Helper method for touches being cancelled. Treated roughly the same as a
+   * touch ending (OnTouchEnd()).
+   */
+  nsEventStatus OnTouchCancel(const MultiTouchInput& aEvent);
+
+  /**
+   * Helper method for scales beginning. Distinct from the OnTouch* handlers in
+   * that this implies some outside implementation has determined that the user
+   * is pinching.
+   */
+  nsEventStatus OnScaleBegin(const PinchGestureInput& aEvent);
+
+  /**
+   * Helper method for scaling. As the user moves their fingers when pinching,
+   * this changes the scale of the page.
+   */
+  nsEventStatus OnScale(const PinchGestureInput& aEvent);
+
+  /**
+   * Helper method for scales ending. Redraws the screen if necessary and does
+   * any cleanup after a scale has ended.
+   */
+  nsEventStatus OnScaleEnd(const PinchGestureInput& aEvent);
+
+  /**
+   * Helper methods for handling pan events.
+   */
+  nsEventStatus OnPanMayBegin(const PanGestureInput& aEvent);
+  nsEventStatus OnPanCancelled(const PanGestureInput& aEvent);
+  nsEventStatus OnPanBegin(const PanGestureInput& aEvent);
+  enum class FingersOnTouchpad {
+    Yes,
+    No,
+  };
+  nsEventStatus OnPan(const PanGestureInput& aEvent,
+                      FingersOnTouchpad aFingersOnTouchpad);
+  nsEventStatus OnPanEnd(const PanGestureInput& aEvent);
+  nsEventStatus OnPanMomentumStart(const PanGestureInput& aEvent);
+  nsEventStatus OnPanMomentumEnd(const PanGestureInput& aEvent);
+  nsEventStatus HandleEndOfPan();
+  nsEventStatus OnPanInterrupted(const PanGestureInput& aEvent);
+
+  /**
+   * Helper methods for handling scroll wheel events.
+   */
+  nsEventStatus OnScrollWheel(const ScrollWheelInput& aEvent);
+
+  /**
+   * Gets the scroll wheel delta's values in parent-layer pixels from the
+   * original delta's values of a wheel input.
+   */
+  ParentLayerPoint GetScrollWheelDelta(const ScrollWheelInput& aEvent) const;
+
+  /**
+   * This function is like GetScrollWheelDelta(aEvent).
+   * The difference is the four added parameters provide values as alternatives
+   * to the original wheel input's delta values, so |aEvent|'s delta values are
+   * ignored in this function, we only use some other member variables and
+   * functions of |aEvent|.
+   */
+  ParentLayerPoint GetScrollWheelDelta(const ScrollWheelInput& aEvent,
+                                       double aDeltaX, double aDeltaY,
+                                       double aMultiplierX,
+                                       double aMultiplierY) const;
+
+  /**
+   * This deleted function is used for:
+   * 1. avoiding accidental implicit value type conversions of input delta
+   *    values when callers intend to call the above function;
+   * 2. decoupling the manual relationship between the delta value type and the
+   *    above function. If by any chance the defined delta value type in
+   *    ScrollWheelInput has changed, this will automatically result in build
+   *    time failure, so we can learn of it the first time and accordingly
+   *    redefine those parameters' value types in the above function.
+   */
+  template <typename T>
+  ParentLayerPoint GetScrollWheelDelta(ScrollWheelInput&, T, T, T, T) = delete;
+
+  /**
+   * Helper methods for handling keyboard events.
+   */
+  nsEventStatus OnKeyboard(const KeyboardInput& aEvent);
+
+  CSSPoint GetKeyboardDestination(const KeyboardScrollAction& aAction) const;
+
+  // Returns the corresponding ScrollSnapFlags for the given |aAction|.
+  // See https://drafts.csswg.org/css-scroll-snap/#scroll-types
+  ScrollSnapFlags GetScrollSnapFlagsForKeyboardAction(
+      const KeyboardScrollAction& aAction) const;
+
+  /**
+   * Helper methods for long press gestures.
+   */
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  nsEventStatus OnLongPress(const TapGestureInput& aEvent);
+  nsEventStatus OnLongPressUp(const TapGestureInput& aEvent);
+
+  /**
+   * Helper method for single tap gestures.
+   */
+  nsEventStatus OnSingleTapUp(const TapGestureInput& aEvent);
+
+  /**
+   * Helper method for a single tap confirmed.
+   */
+  nsEventStatus OnSingleTapConfirmed(const TapGestureInput& aEvent);
+
+  /**
+   * Helper method for double taps.
+   */
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  nsEventStatus OnDoubleTap(const TapGestureInput& aEvent);
+
+  /**
+   * Helper method for double taps where the double-tap gesture is disabled.
+   */
+  nsEventStatus OnSecondTap(const TapGestureInput& aEvent);
+
+  /**
+   * Helper method to cancel any gesture currently going to Gecko. Used
+   * primarily when a user taps the screen over some clickable content but then
+   * pans down instead of letting go (i.e. to cancel a previous touch so that a
+   * new one can properly take effect.
+   */
+  nsEventStatus OnCancelTap(const TapGestureInput& aEvent);
+
+  /**
+   * The following five methods modify the scroll offset. For the APZC
+   * representing the RCD-RSF, they also recalculate the offset of the layout
+   * viewport.
+   */
+
+  /**
+   * Scroll the scroll frame to an X,Y offset.
+   */
+  void SetVisualScrollOffset(const CSSPoint& aOffset);
+
+  /**
+   * Scroll the scroll frame to an X,Y offset, clamping the resulting scroll
+   * offset to the scroll range.
+   */
+  void ClampAndSetVisualScrollOffset(const CSSPoint& aOffset);
+
+  /**
+   * Scroll the scroll frame by an X,Y offset.
+   * The resulting scroll offset is not clamped to the scrollable rect;
+   * the caller must ensure it stays within range.
+   */
+  void ScrollBy(const CSSPoint& aOffset);
+
+  /**
+   * Scroll the scroll frame by an X,Y offset, clamping the resulting
+   * scroll offset to the scroll range.
+   */
+  void ScrollByAndClamp(const CSSPoint& aOffset);
+
+  /**
+   * Scales the viewport by an amount (note that it multiplies this scale in to
+   * the current scale, it doesn't set it to |aScale|). Also considers a focus
+   * point so that the page zooms inward/outward from that point.
+   */
+  void ScaleWithFocus(float aScale, const CSSPoint& aFocus);
+
+  /**
+   * Schedules a composite on the compositor thread.
+   */
+  void ScheduleComposite();
+
+  /**
+   * Schedules a composite, and if enough time has elapsed since the last
+   * paint, a paint.
+   */
+  void ScheduleCompositeAndMaybeRepaint();
+
+  /**
+   * Gets the start point of the current touch.
+   * This only makes sense if a touch is currently happening and OnTouchMove()
+   * or the equivalent for pan gestures is being invoked.
+   */
+  ParentLayerPoint PanStart() const;
+
+  /**
+   * Gets a vector of the velocities of each axis.
+   */
+  const ParentLayerPoint GetVelocityVector() const;
+
+  /**
+   * Sets the velocities of each axis.
+   */
+  void SetVelocityVector(const ParentLayerPoint& aVelocityVector);
+
+  /**
+   * Gets the first touch point from a MultiTouchInput.  This gets only
+   * the first one and assumes the rest are either missing or not relevant.
+   */
+  ParentLayerPoint GetFirstTouchPoint(const MultiTouchInput& aEvent);
+
+  /**
+   * Gets the relevant point in the event
+   * (eg. first touch, or pinch focus point) of the given InputData.
+   */
+  ExternalPoint GetExternalPoint(const InputData& aEvent);
+
+  /**
+   * Gets the relevant point in the event, in external screen coordinates.
+   */
+  ExternalPoint GetFirstExternalTouchPoint(const MultiTouchInput& aEvent);
+
+  /**
+   * Gets the amount by which this APZC is overscrolled along both axes.
+   */
+  ParentLayerPoint GetOverscrollAmount() const;
+
+ private:
+  // Internal version of GetOverscrollAmount() which does not set
+  // the test async properties.
+  ParentLayerPoint GetOverscrollAmountInternal() const;
+
+ protected:
+  /**
+   * Returns SideBits where this APZC is overscrolled.
+   */
+  SideBits GetOverscrollSideBits() const;
+
+  /**
+   * Restore the amount by which this APZC is overscrolled along both axes
+   * to the specified amount. This is for test-related use; overscrolling
+   * as a result of user input should happen via OverscrollBy().
+   */
+  void RestoreOverscrollAmount(const ParentLayerPoint& aOverscroll);
+
+  /**
+   * Sets the panning state basing on the pan direction angle and current
+   * touch-action value.
+   */
+  void HandlePanningWithTouchAction(double angle);
+
+  /**
+   * Sets the panning state ignoring the touch action value.
+   */
+  void HandlePanning(double angle);
+
+  /**
+   * Update the panning state and axis locks.
+   */
+  void HandlePanningUpdate(const ScreenPoint& aDelta);
+
+  /**
+   * Set and update the pinch lock
+   */
+  void HandlePinchLocking(const PinchGestureInput& aEvent);
+
+  /**
+   * Sets up anything needed for panning. This takes us out of the "TOUCHING"
+   * state and starts actually panning us. We provide the physical pixel
+   * position of the start point so that the pan gesture is calculated
+   * regardless of if the window/GeckoView moved during the pan.
+   */
+  nsEventStatus StartPanning(const ExternalPoint& aStartPoint,
+                             const TimeStamp& aEventTime);
+
+  /**
+   * Wrapper for Axis::UpdateWithTouchAtDevicePoint(). Calls this function for
+   * both axes and factors in the time delta from the last update.
+   */
+  void UpdateWithTouchAtDevicePoint(const MultiTouchInput& aEvent);
+
+  /**
+   * Does any panning required due to a new touch event.
+   */
+  void TrackTouch(const MultiTouchInput& aEvent);
+
+  /**
+   * Register the start of a touch or pan gesture at the given position and
+   * time.
+   */
+  void StartTouch(const ParentLayerPoint& aPoint, TimeStamp aTimestamp);
+
+  /**
+   * Register the end of a touch or pan gesture at the given time.
+   */
+  void EndTouch(TimeStamp aTimestamp, Axis::ClearAxisLock aClearAxisLock);
+
+  /**
+   * Utility function to send updated FrameMetrics to Gecko so that it can paint
+   * the displayport area. Calls into GeckoContentController to do the actual
+   * work. This call will use the current metrics. If this function is called
+   * from a non-main thread, it will redispatch itself to the main thread, and
+   * use the latest metrics during the redispatch.
+   */
+  void RequestContentRepaint(
+      RepaintUpdateType aUpdateType = RepaintUpdateType::eUserAction);
+
+  /**
+   * Send Metrics() to Gecko to trigger a repaint. This function may filter
+   * duplicate calls with the same metrics. This function must be called on the
+   * main thread.
+   */
+  void RequestContentRepaint(const ParentLayerPoint& aVelocity,
+                             const ScreenMargin& aDisplayportMargins,
+                             RepaintUpdateType aUpdateType);
+
+  /**
+   * Gets the current frame metrics. This is *not* the Gecko copy stored in the
+   * layers code.
+   */
+  const FrameMetrics& GetFrameMetrics() const;
+
+  /**
+   * Gets the current scroll metadata. This is *not* the Gecko copy stored in
+   * the layers code/
+   */
+  const ScrollMetadata& GetScrollMetadata() const;
+
+  /**
+   * Gets the pointer to the apzc tree manager. All the access to tree manager
+   * should be made via this method and not via private variable since this
+   * method ensures that no lock is set.
+   */
+  APZCTreeManager* GetApzcTreeManager() const;
+
+  void AssertOnSamplerThread() const;
+  void AssertOnUpdaterThread() const;
+
+  /**
+   * Convert ScreenPoint relative to the screen to LayoutDevicePoint relative
+   * to the parent document. This excludes the transient compositor transform.
+   * NOTE: This must be converted to LayoutDevicePoint relative to the child
+   * document before sending over IPC to a child process.
+   */
+  Maybe<LayoutDevicePoint> ConvertToGecko(const ScreenIntPoint& aPoint);
+
+  enum AxisLockMode {
+    FREE,     /* No locking at all */
+    STANDARD, /* Default axis locking mode that remains locked until pan ends */
+    STICKY,   /* Allow lock to be broken, with hysteresis */
+    DOMINANT_AXIS, /* Only allow movement on one axis */
+  };
+
+  static AxisLockMode GetAxisLockMode();
+
+  bool UsingStatefulAxisLock() const;
+
+  enum PinchLockMode {
+    PINCH_FREE,     /* No locking at all */
+    PINCH_STANDARD, /* Default pinch locking mode that remains locked until
+                       pinch gesture ends*/
+    PINCH_STICKY,   /* Allow lock to be broken, with hysteresis */
+  };
+
+  static PinchLockMode GetPinchLockMode();
+
+  // Helper function for OnSingleTapUp(), OnSingleTapConfirmed(), and
+  // OnLongPressUp().
+  nsEventStatus GenerateSingleTap(GeckoContentController::TapType aType,
+                                  const ScreenIntPoint& aPoint,
+                                  mozilla::Modifiers aModifiers);
+
+  // Common processing at the end of a touch block.
+  void OnTouchEndOrCancel();
+
+  LayersId mLayersId;
+  RefPtr<CompositorController> mCompositorController;
+
+  /* Access to the following two fields is protected by the mRefPtrMonitor,
+     since they are accessed on the UI thread but can be cleared on the
+     updater thread. */
+  RefPtr<GeckoContentController> mGeckoContentController
+      MOZ_GUARDED_BY(mRefPtrMonitor);
+  RefPtr<GestureEventListener> mGestureEventListener
+      MOZ_GUARDED_BY(mRefPtrMonitor);
+  mutable Monitor mRefPtrMonitor;
+
+  // This is a raw pointer to avoid introducing a reference cycle between
+  // AsyncPanZoomController and APZCTreeManager. Since these objects don't
+  // live on the main thread, we can't use the cycle collector with them.
+  // The APZCTreeManager owns the lifetime of the APZCs, so nulling this
+  // pointer out in Destroy() will prevent accessing deleted memory.
+  Atomic<APZCTreeManager*> mTreeManager;
+
+  /* Utility functions that return a addrefed pointer to the corresponding
+   * fields. */
+  already_AddRefed<GeckoContentController> GetGeckoContentController() const;
+  already_AddRefed<GestureEventListener> GetGestureEventListener() const;
+
+  PlatformSpecificStateBase* GetPlatformSpecificState();
+
+  /**
+   * Convenience functions to get the corresponding fields of mZoomContraints
+   * while holding mRecursiveMutex.
+   */
+  bool ZoomConstraintsAllowZoom() const;
+  bool ZoomConstraintsAllowDoubleTapZoom() const;
+
+ protected:
+  // Both |mScrollMetadata| and |mLastContentPaintMetrics| are protected by the
+  // monitor. Do not read from or modify them without locking.
+  ScrollMetadata mScrollMetadata;
+
+  // Protects |mScrollMetadata|, |mLastContentPaintMetrics|, |mState| and
+  // |mLastSnapTargetIds|.  Before manipulating |mScrollMetadata|,
+  // |mLastContentPaintMetrics| or |mLastSnapTargetIds| the monitor should be
+  // held. When setting |mState|, either the SetState() function can be used, or
+  // the monitor can be held and then |mState| updated.
+  // IMPORTANT: See the note about lock ordering at the top of
+  // APZCTreeManager.h. This is mutable to allow entering it from 'const'
+  // methods; doing otherwise would significantly limit what methods could be
+  // 'const'.
+  // FIXME: Please keep in mind that due to some existing coupled relationships
+  // among the class members, we should be aware of indirect usage of the
+  // monitor-protected members. That is, although this monitor isn't required to
+  // be held before manipulating non-protected class members, some functions on
+  // those members might indirectly manipulate the protected members; in such
+  // cases, the monitor should still be held. Let's take mX.CanScroll for
+  // example:
+  // Axis::CanScroll(ParentLayerCoord) calls Axis::CanScroll() which calls
+  // Axis::GetPageLength() which calls Axis::GetFrameMetrics() which calls
+  // AsyncPanZoomController::GetFrameMetrics(), therefore, this monitor should
+  // be held before calling the CanScroll function of |mX| and |mY|. These
+  // coupled relationships bring us the burden of taking care of when the
+  // monitor should be held, so they should be decoupled in the future.
+  mutable RecursiveMutex mRecursiveMutex;
+
+ private:
+  // Metadata of the container layer corresponding to this APZC. This is
+  // stored here so that it is accessible from the UI/controller thread.
+  // These are the metrics at last content paint, the most recent
+  // values we were notified of in NotifyLayersUpdate(). Since it represents
+  // the Gecko state, it should be used as a basis for untransformation when
+  // sending messages back to Gecko.
+  ScrollMetadata mLastContentPaintMetadata;
+  FrameMetrics& mLastContentPaintMetrics;  // for convenience, refers to
+                                           // mLastContentPaintMetadata.mMetrics
+  // The last content repaint request.
+  RepaintRequest mLastPaintRequestMetrics;
+  // The metrics that we expect content to have. This is updated when we
+  // request a content repaint, and when we receive a shadow layers update.
+  // This allows us to transform events into Gecko's coordinate space.
+  ExpectedGeckoMetrics mExpectedGeckoMetrics;
+
+  // This holds important state from the Metrics() at previous times
+  // SampleCompositedAsyncTransform() was called. This will always have at least
+  // one item. mRecursiveMutex must be held when using or modifying this member.
+  // Samples should be inserted to the "back" of the deque and extracted from
+  // the "front".
+  std::deque<SampledAPZCState> mSampledState;
+
+  // Groups state variables that are specific to a platform.
+  // Initialized on first use.
+  UniquePtr<PlatformSpecificStateBase> mPlatformSpecificState;
+
+  // This flag is set to true when we are in a axis-locked pan as a result of
+  // the touch-action CSS property.
+  bool mPanDirRestricted;
+
+  // This flag is set to true when we are in a pinch-locked state. ie: user
+  // is performing a two-finger pan rather than a pinch gesture
+  bool mPinchLocked;
+
+  // Stores the pinch events that occured within a given timeframe. Used to
+  // calculate the focusChange and spanDistance within a fixed timeframe.
+  // RecentEventsBuffer is not threadsafe. Should only be accessed on the
+  // controller thread.
+  RecentEventsBuffer<PinchGestureInput> mPinchEventBuffer;
+
+  // Most up-to-date constraints on zooming. These should always be reasonable
+  // values; for example, allowing a min zoom of 0.0 can cause very bad things
+  // to happen. Hold mRecursiveMutex when accessing this.
+  ZoomConstraints mZoomConstraints;
+
+  // The last time the compositor has sampled the content transform for this
+  // frame.
+  SampleTime mLastSampleTime;
+
+  // The last sample time at which we submitted a checkerboarding report.
+  SampleTime mLastCheckerboardReport;
+
+  // Stores the previous focus point if there is a pinch gesture happening. Used
+  // to allow panning by moving multiple fingers (thus moving the focus point).
+  ParentLayerPoint mLastZoomFocus;
+
+  // Stores the previous zoom level at which we last sent a ScaleGestureComplete
+  // notification.
+  CSSToParentLayerScale mLastNotifiedZoom;
+
+  // Accessing mAnimation needs to be protected by mRecursiveMutex
+  RefPtr<AsyncPanZoomAnimation> mAnimation;
+
+  UniquePtr<OverscrollEffectBase> mOverscrollEffect;
+
+  // Zoom animation id, used for zooming in WebRender. This should only be
+  // set on the APZC instance for the root content document (i.e. the one we
+  // support zooming on), and is only used if WebRender is enabled. The
+  // animation id itself refers to the transform animation id that was set on
+  // the stacking context in the WR display list. By changing the transform
+  // associated with this id, we can adjust the scaling that WebRender applies,
+  // thereby controlling the zoom.
+  Maybe<uint64_t> mZoomAnimationId;
+
+  // Position on screen where user first put their finger down.
+  ExternalPoint mStartTouch;
+
+  // Accessing mScrollPayload needs to be protected by mRecursiveMutex
+  Maybe<CompositionPayload> mScrollPayload;
+
+  // Representing sampled scroll offset generation, this value is bumped up
+  // every time this APZC sampled new scroll offset.
+  APZScrollGeneration mScrollGeneration;
+
+  friend class Axis;
+
+ public:
+  Maybe<CompositionPayload> NotifyScrollSampling();
+
+  /**
+   * Invoke |callable|, passing |mLastContentPaintMetrics| as argument,
+   * while holding the APZC lock required to access |mLastContentPaintMetrics|.
+   * This allows code outside of an AsyncPanZoomController method implementation
+   * to access |mLastContentPaintMetrics| without having to make a copy of it.
+   * Passes through the return value of |callable|.
+   */
+  template <typename Callable>
+  auto CallWithLastContentPaintMetrics(const Callable& callable) const
+      -> decltype(callable(mLastContentPaintMetrics)) {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return callable(mLastContentPaintMetrics);
+  }
+
+  void SetZoomAnimationId(const Maybe<uint64_t>& aZoomAnimationId);
+  Maybe<uint64_t> GetZoomAnimationId() const;
+
+  /* ===================================================================
+   * The functions and members in this section are used to expose
+   * the current async transform state to callers.
+   */
+ public:
+  static const AsyncTransformConsumer eForEventHandling =
+      AsyncTransformConsumer::eForEventHandling;
+  static const AsyncTransformConsumer eForCompositing =
+      AsyncTransformConsumer::eForCompositing;
+
+  /**
+   * Get the current scroll offset of the scrollable frame corresponding
+   * to this APZC, including the effects of any asynchronous panning and
+   * zooming, in ParentLayer pixels.
+   */
+  ParentLayerPoint GetCurrentAsyncScrollOffset(
+      AsyncTransformConsumer aMode) const;
+
+  /**
+   * Get the current visual viewport of the scrollable frame corresponding
+   * to this APZC, including the effects of any asynchronous panning and
+   * zooming, in CSS pixels.
+   */
+  CSSRect GetCurrentAsyncVisualViewport(AsyncTransformConsumer aMode) const;
+
+  /**
+   * Return a visual effect that reflects this apzc's
+   * overscrolled state, if any.
+   */
+  AsyncTransformComponentMatrix GetOverscrollTransform(
+      AsyncTransformConsumer aMode) const;
+
+  /**
+   * Returns the incremental transformation corresponding to the async pan/zoom
+   * in progress. That is, when this transform is multiplied with the layer's
+   * existing transform, it will make the layer appear with the desired pan/zoom
+   * amount.
+   * The transform can have both scroll and zoom components; the caller can
+   * request just one or the other, or both, via the |aComponents| parameter.
+   * When only the eLayout component is requested, the returned translation
+   * should really be a LayerPoint, rather than a ParentLayerPoint, as it will
+   * not be scaled by the asynchronous zoom.
+   * |aMode| specifies whether the async transform is queried for the purpose of
+   * hit testing (eHitTesting) in which case the latest values from |Metrics()|
+   * are used, or for compositing (eCompositing) in which case a sampled value
+   * from |mSampledState| is used.
+   * |aSampleIndex| specifies which sample in |mSampledState| to use.
+   */
+  AsyncTransform GetCurrentAsyncTransform(
+      AsyncTransformConsumer aMode,
+      AsyncTransformComponents aComponents = LayoutAndVisual,
+      std::size_t aSampleIndex = 0) const;
+
+  /**
+   * A variant of GetCurrentAsyncTransform() intended for use when computing
+   * the screen-to-apzc and apzc-to-gecko transforms. This includes the
+   * overscroll transform, and additionally requires the caller to pass in
+   * the LayersId of the APZC whose screen-to-apzc or apzc-to-gecko transform
+   * is being computed in |aForLayersId|; if that APZC is in an out-of-process
+   * subdocument, and we are the zoomable (root-content) APZC, the returned
+   * transform includes the resolution at painting time, not just the async
+   * change to the resolution since painting. This is done because
+   * out-of-process content expects the screen-to-apzc and apzc-to-gecko
+   * transforms to include the painted resolution (see bug 1827330 for details).
+   *
+   * Should only be called from
+   * APZCTreeManager::{GetScreenToApzcTransform(),GetApzcToGeckoTransform()}.
+   */
+  AsyncTransformComponentMatrix GetAsyncTransformForInputTransformation(
+      AsyncTransformComponents aComponents, LayersId aForLayersId) const;
+
+  /**
+   * Returns a transform that scales by the resolution that was in place
+   * at "painting" (display list building) time. This is used as a
+   * component of some transforms related to APZCs in out-of-process
+   * subdocuments.
+   *
+   * Can only be called for the root content APZC.
+   */
+  Matrix4x4 GetPaintedResolutionTransform() const;
+
+  AutoTArray<wr::SampledScrollOffset, 2> GetSampledScrollOffsets() const;
+
+  /**
+   * Returns the "zoom" bits of the transform. This includes both the rasterized
+   * (layout device to layer scale) and async (layer scale to parent layer
+   * scale) components of the zoom.
+   */
+  LayoutDeviceToParentLayerScale GetCurrentPinchZoomScale(
+      AsyncTransformConsumer aMode) const;
+
+  ParentLayerRect GetCompositionBounds() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mScrollMetadata.GetMetrics().GetCompositionBounds();
+  }
+
+  LayoutDeviceToLayerScale GetCumulativeResolution() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mScrollMetadata.GetMetrics().GetCumulativeResolution();
+  }
+
+  CSSRect GetScrollableRect() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mScrollMetadata.GetMetrics().GetScrollableRect();
+  }
+
+  CSSToParentLayerScale GetZoom() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return Metrics().GetZoom();
+  }
+
+  CSSRect GetVisualViewport() const {
+    MOZ_ASSERT(IsRootContent());
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return Metrics().GetVisualViewport();
+  }
+
+  CSSPoint GetLayoutScrollOffset() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return Metrics().GetLayoutScrollOffset();
+  }
+
+  // Returns the delta for the given InputData.
+  ParentLayerPoint GetDeltaForEvent(const InputData& aEvent) const;
+
+  /**
+   * Get the current scroll range of the scrollable frame coreesponding to this
+   * APZC.
+   */
+  CSSRect GetCurrentScrollRangeInCssPixels() const;
+
+ private:
+  /**
+   * Advances to the next sample, if there is one, the list of sampled states
+   * stored in mSampledState. This will make the result of
+   * |GetCurrentAsyncTransform(eForCompositing)| and similar functions reflect
+   * the async scroll offset and zoom of the next sample. See also
+   * SampleCompositedAsyncTransform which creates the samples.
+   */
+  void AdvanceToNextSample();
+
+  /**
+   * Returns whether we have changes to the scroll offsets which need to be
+   * sampled in the next couple of frames (it depends on how many offsets we
+   * have, currently it's two).
+   */
+  bool HavePendingFrameDelayedOffset() const;
+
+  /**
+   * Samples the composited async transform, storing the result into
+   * mSampledState. This will make the result of
+   * |GetCurrentAsyncTransform(eForCompositing)| and similar functions reflect
+   * the async scroll offset and zoom stored in |Metrics()| when the sample
+   * is activated via some future call to |AdvanceToNextSample|.
+   *
+   * Returns true if the newly sampled value is different from the last
+   * sampled value.
+   */
+  bool SampleCompositedAsyncTransform(
+      const RecursiveMutexAutoLock& aProofOfLock);
+
+  /**
+   * Updates the sample at the front of mSampledState with the latest
+   * metrics. This makes the result of
+   * |GetCurrentAsyncTransform(eForCompositing)| reflect the current Metrics().
+   */
+  void ResampleCompositedAsyncTransform(
+      const RecursiveMutexAutoLock& aProofOfLock);
+
+  /*
+   * Helper functions to query the async layout viewport, scroll offset, and
+   * zoom either directly from |Metrics()|, or from cached variables that
+   * store the required value from the last time it was sampled by calling
+   * SampleCompositedAsyncTransform(), depending on who is asking.
+   */
+  CSSRect GetEffectiveLayoutViewport(AsyncTransformConsumer aMode,
+                                     const RecursiveMutexAutoLock& aProofOfLock,
+                                     std::size_t aSampleIndex = 0) const;
+  CSSPoint GetEffectiveScrollOffset(AsyncTransformConsumer aMode,
+                                    const RecursiveMutexAutoLock& aProofOfLock,
+                                    std::size_t aSampleIndex = 0) const;
+  CSSToParentLayerScale GetEffectiveZoom(
+      AsyncTransformConsumer aMode, const RecursiveMutexAutoLock& aProofOfLock,
+      std::size_t aSampleIndex = 0) const;
+
+  /**
+   * Returns the visible portion of the content scrolled by this APZC, in
+   * CSS pixels. The caller must have acquired the mRecursiveMutex lock.
+   */
+  CSSRect GetVisibleRect(const RecursiveMutexAutoLock& aProofOfLock) const;
+
+  /**
+   * Returns a pair of displacements both in logical/physical units for
+   * |aEvent|.
+   */
+  std::tuple<ParentLayerPoint, ScreenPoint> GetDisplacementsForPanGesture(
+      const PanGestureInput& aEvent);
+
+ private:
+  friend class AutoApplyAsyncTestAttributes;
+
+  bool SuppressAsyncScrollOffset() const;
+
+  /**
+   * Applies |mTestAsyncScrollOffset| and |mTestAsyncZoom| to this
+   * AsyncPanZoomController. Calls |SampleCompositedAsyncTransform| to ensure
+   * that the GetCurrentAsync* functions consider the test offset and zoom in
+   * their computations.
+   */
+  void ApplyAsyncTestAttributes(const RecursiveMutexAutoLock& aProofOfLock);
+
+  /**
+   * Sets this AsyncPanZoomController's FrameMetrics to |aPrevFrameMetrics| and
+   * calls |SampleCompositedAsyncTransform| to unapply any test values applied
+   * by |ApplyAsyncTestAttributes|.
+   */
+  void UnapplyAsyncTestAttributes(const RecursiveMutexAutoLock& aProofOfLock,
+                                  const FrameMetrics& aPrevFrameMetrics,
+                                  const ParentLayerPoint& aPrevOverscroll);
+
+  /* ===================================================================
+   * The functions and members in this section are used to manage
+   * the state that tracks what this APZC is doing with the input events.
+   */
+ protected:
+  enum PanZoomState {
+    NOTHING,  /* no touch-start events received */
+    FLING,    /* all touches removed, but we're still scrolling page */
+    TOUCHING, /* one touch-start event received */
+
+    PANNING,          /* panning the frame */
+    PANNING_LOCKED_X, /* touch-start followed by move (i.e. panning with axis
+                         lock) X axis */
+    PANNING_LOCKED_Y, /* as above for Y axis */
+
+    PAN_MOMENTUM, /* like PANNING, but controlled by momentum PanGestureInput
+                     events */
+
+    PINCHING, /* nth touch-start, where n > 1. this mode allows pan and zoom */
+    ANIMATING_ZOOM,       /* animated zoom to a new rect */
+    OVERSCROLL_ANIMATION, /* Spring-based animation used to relieve overscroll
+                             once the finger is lifted. */
+    SMOOTH_SCROLL,        /* Smooth scrolling to destination, with physics
+                             controlled by prefs specific to the scroll origin. */
+    SMOOTHMSD_SCROLL,     /* SmoothMSD scrolling to destination. Used by
+                             CSSOM-View smooth scroll-behavior */
+    WHEEL_SCROLL,    /* Smooth scrolling to a destination for a wheel event. */
+    KEYBOARD_SCROLL, /* Smooth scrolling to a destination for a keyboard event.
+                      */
+    AUTOSCROLL,      /* Autoscroll animation. */
+    SCROLLBAR_DRAG   /* Async scrollbar drag. */
+  };
+  // This is in theory protected by |mRecursiveMutex|; that is, it should be
+  // held whenever this is updated. In practice though... see bug 897017.
+  PanZoomState mState;
+
+  AxisX mX;
+  AxisY mY;
+
+  /**
+   * Returns wheter the given input state is a user pan-gesture.
+   *
+   * Note: momentum pan gesture states are not considered a panning state.
+   */
+  static bool IsPanningState(PanZoomState aState);
+
+  /**
+   * Returns wheter a delayed transform end is queued.
+   */
+  bool IsDelayedTransformEndSet();
+
+  /**
+   * Returns wheter a delayed transform end is queued.
+   */
+  void SetDelayedTransformEnd(bool aDelayedTransformEnd);
+
+  /**
+   * Returns whether the specified PanZoomState does not need to be reset when
+   * a scroll offset update is processed.
+   */
+  static bool CanHandleScrollOffsetUpdate(PanZoomState aState);
+
+  /**
+   * Determine whether a main-thread scroll offset update should result in
+   * a call to CancelAnimation() (which interrupts in-progress animations and
+   * gestures).
+   *
+   * If the update is a relative update, |aRelativeDelta| contains its amount.
+   * If the update is not a relative update, GetMetrics() should already reflect
+   * the new offset at the time of the call.
+   */
+  bool ShouldCancelAnimationForScrollUpdate(
+      const Maybe<CSSPoint>& aRelativeDelta);
+
+ private:
+  friend class StateChangeNotificationBlocker;
+  /**
+   * A counter of how many StateChangeNotificationBlockers are active.
+   * A non-zero count will prevent state change notifications from
+   * being dispatched. Only code that holds mRecursiveMutex should touch this.
+   */
+  int mNotificationBlockers;
+
+  /**
+   * Helper to set the current state, without content controller events
+   * for the state change. This is useful in cases where the content
+   * controller events may need to be delayed.
+   */
+  PanZoomState SetStateNoContentControllerDispatch(PanZoomState aNewState);
+
+  /**
+   * Helper to set the current state. Holds mRecursiveMutex before actually
+   * setting it and fires content controller events based on state changes.
+   * Always set the state using this call, do not set it directly.
+   */
+  void SetState(PanZoomState aNewState);
+  /**
+   * Helper for getting the current state which acquires mRecursiveMutex
+   * before accessing the field.
+   */
+  PanZoomState GetState() const;
+  /**
+   * Fire content controller notifications about state changes, assuming no
+   * StateChangeNotificationBlocker has been activated.
+   */
+  void DispatchStateChangeNotification(PanZoomState aOldState,
+                                       PanZoomState aNewState);
+
+  /**
+   * Send a TransformBegin notification followed by a TransformEnd
+   * notification.
+   */
+  void SendTransformBeginAndEnd();
+
+  /**
+   * Internal helpers for checking general state of this apzc.
+   */
+  bool IsInTransformingState() const;
+  static bool IsTransformingState(PanZoomState aState);
+
+  /* ===================================================================
+   * The functions and members in this section are used to manage
+   * blocks of touch events and the state needed to deal with content
+   * listeners.
+   */
+ public:
+  /**
+   * Flush a repaint request if one is needed, without throttling it with the
+   * paint throttler.
+   */
+  void FlushRepaintForNewInputBlock();
+
+  /**
+   * Given an input event and the touch block it belongs to, check if the
+   * event can lead to a panning/zooming behavior.
+   * This is used for logic related to the pointer events spec (figuring out
+   * when to dispatch the pointercancel event), as well as an input to the
+   * computation of the APZHandledResult for the event (used on Android to
+   * govern dynamic toolbar and pull-to-refresh behaviour).
+   */
+  PointerEventsConsumableFlags ArePointerEventsConsumable(
+      TouchBlockState* aBlock, const MultiTouchInput& aInput);
+
+  /**
+   * Clear internal state relating to touch input handling.
+   */
+  void ResetTouchInputState();
+
+  /**
+     Clear internal state relating to pan gesture input handling.
+   */
+  void ResetPanGestureInputState();
+
+  /**
+   * Gets a ref to the input queue that is shared across the entire tree
+   * manager.
+   */
+  const RefPtr<InputQueue>& GetInputQueue() const;
+
+ private:
+  void CancelAnimationAndGestureState();
+
+  RefPtr<InputQueue> mInputQueue;
+  InputBlockState* GetCurrentInputBlock() const;
+  TouchBlockState* GetCurrentTouchBlock() const;
+  bool HasReadyTouchBlock() const;
+
+  PanGestureBlockState* GetCurrentPanGestureBlock() const;
+  PinchGestureBlockState* GetCurrentPinchGestureBlock() const;
+
+ private:
+  /* ===================================================================
+   * The functions and members in this section are used to manage
+   * fling animations, smooth scroll animations, and overscroll
+   * during a fling or smooth scroll.
+   */
+ public:
+  /**
+   * Attempt a fling with the velocity specified in |aHandoffState|.
+   * |aHandoffState.mIsHandoff| should be true iff. the fling was handed off
+   * from a previous APZC, and determines whether acceleration is applied
+   * to the fling.
+   * We only accept the fling in the direction(s) in which we are pannable.
+   * Returns the "residual velocity", i.e. the portion of
+   * |aHandoffState.mVelocity| that this APZC did not consume.
+   */
+  ParentLayerPoint AttemptFling(const FlingHandoffState& aHandoffState);
+
+  ParentLayerPoint AdjustHandoffVelocityForOverscrollBehavior(
+      ParentLayerPoint& aHandoffVelocity) const;
+
+ private:
+  friend class StackScrollerFlingAnimation;
+  friend class AutoscrollAnimation;
+  template <typename FlingPhysics>
+  friend class GenericFlingAnimation;
+  friend class AndroidFlingPhysics;
+  friend class DesktopFlingPhysics;
+  friend class OverscrollAnimation;
+  friend class SmoothMsdScrollAnimation;
+  friend class GenericScrollAnimation;
+  friend class WheelScrollAnimation;
+  friend class ZoomAnimation;
+
+  friend class GenericOverscrollEffect;
+  friend class WidgetOverscrollEffect;
+  friend struct apz::AsyncScrollThumbTransformer;
+
+  FlingAccelerator mFlingAccelerator;
+
+  // Indicates if the repaint-during-pinch timer is currently set
+  bool mPinchPaintTimerSet;
+
+  // Indicates a delayed transform end notification is queued, and the
+  // transform-end timer is currently set. mRecursiveMutex must be held
+  // when using or modifying this member.
+  bool mDelayedTransformEnd;
+
+  // Deal with overscroll resulting from a fling animation. This is only ever
+  // called on APZC instances that were actually performing a fling.
+  // The overscroll is handled by trying to hand the fling off to an APZC
+  // later in the handoff chain, or if there are no takers, continuing the
+  // fling and entering an overscrolled state.
+  void HandleFlingOverscroll(
+      const ParentLayerPoint& aVelocity, SideBits aOverscrollSideBits,
+      const RefPtr<const OverscrollHandoffChain>& aOverscrollHandoffChain,
+      const RefPtr<const AsyncPanZoomController>& aScrolledApzc);
+
+  void HandleSmoothScrollOverscroll(const ParentLayerPoint& aVelocity,
+                                    SideBits aOverscrollSideBits);
+
+  // Start an overscroll animation with the given initial velocity.
+  void StartOverscrollAnimation(const ParentLayerPoint& aVelocity,
+                                SideBits aOverscrollSideBits);
+
+  // Start a smooth-scrolling animation to the given destination, with physics
+  // based on the prefs for the indicated origin.
+  void SmoothScrollTo(CSSSnapDestination&& aDestination,
+                      ScrollTriggeredByScript aTriggeredByScript,
+                      const ScrollOrigin& aOrigin);
+
+  ParentLayerPoint ConvertDestinationToDelta(CSSPoint& aDestination) const;
+
+  // Start a smooth-scrolling animation to the given destination, with MSD
+  // physics that is suited for scroll-snapping.
+  void SmoothMsdScrollTo(CSSSnapDestination&& aDestination,
+                         ScrollTriggeredByScript aTriggeredByScript);
+
+  // Returns whether overscroll is allowed during an event.
+  bool AllowScrollHandoffInCurrentBlock() const;
+
+  // Invoked by the pinch repaint timer.
+  void DoDelayedRequestContentRepaint();
+
+  // Invoked by the on pan-end handler to ensure that scrollend is only
+  // fired once when a momentum pan or scroll snap is triggered as a
+  // result of the pan gesture.
+  void DoDelayedTransformEndNotification(PanZoomState aOldState);
+
+  // Compute the number of ParentLayer pixels per (Screen) inch at the given
+  // point and in the given direction.
+  float ComputePLPPI(ParentLayerPoint aPoint,
+                     ParentLayerPoint aDirection) const;
+
+  Maybe<CSSPoint> GetCurrentAnimationDestination(
+      const RecursiveMutexAutoLock& aProofOfLock) const;
+
+  /* ===================================================================
+   * The functions and members in this section are used to make ancestor chains
+   * out of APZC instances. These chains can only be walked or manipulated
+   * while holding the lock in the associated APZCTreeManager instance.
+   */
+ public:
+  void SetParent(AsyncPanZoomController* aParent) { mParent = aParent; }
+
+  AsyncPanZoomController* GetParent() const { return mParent; }
+
+  /* Returns true if there is no APZC higher in the tree with the same
+   * layers id.
+   */
+  bool HasNoParentWithSameLayersId() const {
+    return !mParent || (mParent->mLayersId != mLayersId);
+  }
+
+  bool IsRootForLayersId() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mScrollMetadata.IsLayersIdRoot();
+  }
+
+  bool IsRootContent() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return Metrics().IsRootContent();
+  }
+
+ private:
+  // |mTreeManager| belongs in this section but it's declaration is a bit
+  // further above due to initialization-order constraints.
+
+  RefPtr<AsyncPanZoomController> mParent;
+
+  /* ===================================================================
+   * The functions and members in this section are used for scrolling,
+   * including handing off scroll to another APZC, and overscrolling.
+   */
+
+  ScrollableLayerGuid::ViewID GetScrollId() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return Metrics().GetScrollId();
+  }
+
+ public:
+  ScrollableLayerGuid::ViewID GetScrollHandoffParentId() const {
+    return mScrollMetadata.GetScrollParentId();
+  }
+
+  /**
+   * Attempt to scroll in response to a touch-move from |aStartPoint| to
+   * |aEndPoint|, which are in our (transformed) screen coordinates.
+   * Due to overscroll handling, there may not actually have been a touch-move
+   * at these points, but this function will scroll as if there had been.
+   * If this attempt causes overscroll (i.e. the layer cannot be scrolled
+   * by the entire amount requested), the overscroll is passed back to the
+   * tree manager via APZCTreeManager::DispatchScroll(). If the tree manager
+   * does not find an APZC further in the handoff chain to accept the
+   * overscroll, and this APZC is pannable, this APZC enters an overscrolled
+   * state.
+   * |aOverscrollHandoffChain| and |aOverscrollHandoffChainIndex| are used by
+   * the tree manager to keep track of which APZC to hand off the overscroll
+   * to; this function increments the chain and the index and passes it on to
+   * APZCTreeManager::DispatchScroll() in the event of overscroll.
+   * Returns true iff. this APZC, or an APZC further down the
+   * handoff chain, accepted the scroll (possibly entering an overscrolled
+   * state). If this returns false, the caller APZC knows that it should enter
+   * an overscrolled state itself if it can.
+   * aStartPoint and aEndPoint are modified depending on how much of the
+   * scroll gesture was consumed by APZCs in the handoff chain.
+   */
+  bool AttemptScroll(ParentLayerPoint& aStartPoint, ParentLayerPoint& aEndPoint,
+                     OverscrollHandoffState& aOverscrollHandoffState);
+
+  void FlushRepaintForOverscrollHandoff();
+
+  /**
+   * If overscrolled, start a snap-back animation and return true. Even if not
+   * overscrolled, this function tries to snap back to if there's an applicable
+   * scroll snap point.
+   * Otherwise return false.
+   */
+  bool SnapBackIfOverscrolled();
+
+  /**
+   * NOTE: Similar to above but this function doesn't snap back to the scroll
+   * snap point.
+   */
+  bool SnapBackIfOverscrolledForMomentum(const ParentLayerPoint& aVelocity);
+
+  /**
+   * Build the chain of APZCs along which scroll will be handed off when
+   * this APZC receives input events.
+   *
+   * Notes on lifetime and const-correctness:
+   *   - The returned handoff chain is |const|, to indicate that it cannot be
+   *     changed after being built.
+   *   - When passing the chain to a function that uses it without storing it,
+   *     pass it by reference-to-const (as in |const OverscrollHandoffChain&|).
+   *   - When storing the chain, store it by RefPtr-to-const (as in
+   *     |RefPtr<const OverscrollHandoffChain>|). This ensures the chain is
+   *     kept alive. Note that queueing a task that uses the chain as an
+   *     argument constitutes storing, as the task may outlive its queuer.
+   *   - When passing the chain to a function that will store it, pass it as
+   *     |const RefPtr<const OverscrollHandoffChain>&|. This allows the
+   *     function to copy it into the |RefPtr<const OverscrollHandoffChain>|
+   *     that will store it, while avoiding an unnecessary copy (and thus
+   *     AddRef() and Release()) when passing it.
+   */
+  RefPtr<const OverscrollHandoffChain> BuildOverscrollHandoffChain();
+
+ private:
+  /**
+   * A helper function for calling APZCTreeManager::DispatchScroll().
+   * Guards against the case where the APZC is being concurrently destroyed
+   * (and thus mTreeManager is being nulled out).
+   */
+  bool CallDispatchScroll(ParentLayerPoint& aStartPoint,
+                          ParentLayerPoint& aEndPoint,
+                          OverscrollHandoffState& aOverscrollHandoffState);
+
+  void RecordScrollPayload(const TimeStamp& aTimeStamp);
+
+  /**
+   * A helper function for overscrolling during panning. This is a wrapper
+   * around OverscrollBy() that also implements restrictions on entering
+   * overscroll based on the pan angle.
+   */
+  void OverscrollForPanning(ParentLayerPoint& aOverscroll,
+                            const ScreenPoint& aPanDistance);
+
+  /**
+   * Try to overscroll by 'aOverscroll'.
+   * If we are pannable on a particular axis, that component of 'aOverscroll'
+   * is transferred to any existing overscroll.
+   */
+  void OverscrollBy(ParentLayerPoint& aOverscroll);
+
+  /* ===================================================================
+   * The functions and members in this section are used to maintain the
+   * area that this APZC instance is responsible for. This is used when
+   * hit-testing to see which APZC instance should handle touch events.
+   */
+ public:
+  void SetAncestorTransform(const AncestorTransform& aAncestorTransform) {
+    mAncestorTransform = aAncestorTransform;
+  }
+
+  Matrix4x4 GetAncestorTransform() const {
+    return mAncestorTransform.CombinedTransform();
+  }
+
+  bool AncestorTransformContainsPerspective() const {
+    return mAncestorTransform.ContainsPerspectiveTransform();
+  }
+
+  // Return the perspective transform component of the ancestor transform.
+  Matrix4x4 GetAncestorTransformPerspective() const {
+    return mAncestorTransform.GetPerspectiveTransform();
+  }
+
+  // Returns whether or not this apzc contains the given screen point within
+  // its composition bounds.
+  bool Contains(const ScreenIntPoint& aPoint) const;
+
+  bool IsInOverscrollGutter(const ScreenPoint& aHitTestPoint) const;
+  bool IsInOverscrollGutter(const ParentLayerPoint& aHitTestPoint) const;
+
+  bool IsOverscrolled() const;
+
+  bool IsOverscrollAnimationRunning() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mState == OVERSCROLL_ANIMATION;
+  }
+
+  // IsPhysicallyOverscrolled() checks whether the APZC is overscrolled
+  // by an overscroll effect which applies a transform to the APZC's contents.
+  bool IsPhysicallyOverscrolled() const;
+
+ private:
+  bool IsInInvalidOverscroll() const;
+
+ public:
+  bool IsInPanningState() const;
+
+  // Returns whether being in the middle of a gesture. E.g., this APZC has
+  // started handling a pan gesture but hasn't yet received pan-end, etc.
+  bool IsInScrollingGesture() const;
+
+ private:
+  /* This is the cumulative CSS transform for all the layers from (and
+   * including) the parent APZC down to (but excluding) this one, and excluding
+   * any perspective transforms. */
+  AncestorTransform mAncestorTransform;
+
+  /* ===================================================================
+   * The functions and members in this section are used for testing
+   * and assertion purposes only.
+   */
+ public:
+  /**
+   * Gets whether this APZC has performed async key scrolling.
+   */
+  bool TestHasAsyncKeyScrolled() const { return mTestHasAsyncKeyScrolled; }
+
+  /**
+   * Set an extra offset for testing async scrolling.
+   */
+  void SetTestAsyncScrollOffset(const CSSPoint& aPoint);
+  /**
+   * Set an extra offset for testing async scrolling.
+   */
+  void SetTestAsyncZoom(const LayerToParentLayerScale& aZoom);
+
+  LayersId GetLayersId() const { return mLayersId; }
+
+  bool IsAsyncZooming() const {
+    RecursiveMutexAutoLock lock(mRecursiveMutex);
+    return mState == PINCHING || mState == ANIMATING_ZOOM;
+  }
+
+ private:
+  // The timestamp of the latest touch start event.
+  TimeStamp mTouchStartTime;
+  // Used for interpolating touch events that cross the touch-start
+  // tolerance threshold.
+  struct TouchSample {
+    ExternalPoint mPosition;
+    TimeStamp mTimeStamp;
+  };
+  // Information about the latest touch event.
+  // This is only populated when we're in the TOUCHING state
+  // (and thus the last touch event has only one touch point).
+  TouchSample mLastTouch;
+  // The time duration between mTouchStartTime and the touchmove event that
+  // started the pan (the touchmove event that transitioned this APZC from the
+  // TOUCHING state to one of the PANNING* states). Only valid while this APZC
+  // is in a panning state.
+  TimeDuration mTouchStartRestingTimeBeforePan;
+  Maybe<ParentLayerCoord> mMinimumVelocityDuringPan;
+  // This variable needs to be protected by |mRecursiveMutex|.
+  ScrollSnapTargetIds mLastSnapTargetIds;
+  // Extra offset to add to the async scroll position for testing
+  CSSPoint mTestAsyncScrollOffset;
+  // Extra zoom to include in the aync zoom for testing
+  LayerToParentLayerScale mTestAsyncZoom;
+  uint8_t mTestAttributeAppliers;
+  // Flag to track whether or not this APZC has ever async key scrolled.
+  bool mTestHasAsyncKeyScrolled;
+
+  /* ===================================================================
+   * The functions and members in this section are used for checkerboard
+   * recording.
+   */
+ private:
+  // Helper function to update the in-progress checkerboard event, if any.
+  void UpdateCheckerboardEvent(const MutexAutoLock& aProofOfLock,
+                               uint32_t aMagnitude);
+
+  // Mutex protecting mCheckerboardEvent
+  Mutex mCheckerboardEventLock;
+  // This is created when this APZC instance is first included as part of a
+  // composite. If a checkerboard event takes place, this is destroyed at the
+  // end of the event, and a new one is created on the next composite.
+  UniquePtr<CheckerboardEvent> mCheckerboardEvent;
+  // This is used to track the total amount of time that we could reasonably
+  // be checkerboarding. Combined with other info, this allows us to
+  // meaningfully say how frequently users actually encounter checkerboarding.
+  PotentialCheckerboardDurationTracker mPotentialCheckerboardTracker;
+
+  /* ===================================================================
+   * The functions in this section are used for CSS scroll snapping.
+   */
+
+  // If moving |aStartPosition| by |aDelta| should trigger scroll snapping,
+  // adjust |aDelta| to reflect the snapping (that is, make it a delta that will
+  // take us to the desired snap point). The delta is interpreted as being
+  // relative to |aStartPosition|, and if a target snap point is found,
+  // |aStartPosition| is also updated, to the value of the snap point.
+  // |aUnit| affects the snapping behaviour (see ScrollSnapUtils::
+  // GetSnapPointForDestination).
+  // Returns true iff. a target snap point was found.
+  Maybe<CSSSnapDestination> MaybeAdjustDeltaForScrollSnapping(
+      ScrollUnit aUnit, ScrollSnapFlags aSnapFlags, ParentLayerPoint& aDelta,
+      CSSPoint& aStartPosition);
+
+  // A wrapper function of MaybeAdjustDeltaForScrollSnapping for
+  // ScrollWheelInput.
+  Maybe<CSSSnapDestination> MaybeAdjustDeltaForScrollSnappingOnWheelInput(
+      const ScrollWheelInput& aEvent, ParentLayerPoint& aDelta,
+      CSSPoint& aStartPosition);
+
+  Maybe<CSSSnapDestination> MaybeAdjustDestinationForScrollSnapping(
+      const KeyboardInput& aEvent, CSSPoint& aDestination,
+      ScrollSnapFlags aSnapFlags);
+
+  // Snap to a snap position nearby the current scroll position, if appropriate.
+  void ScrollSnap(ScrollSnapFlags aSnapFlags);
+
+  // Snap to a snap position nearby the destination predicted based on the
+  // current velocity, if appropriate.
+  void ScrollSnapToDestination();
+
+  // Snap to a snap position nearby the provided destination, if appropriate.
+  void ScrollSnapNear(const CSSPoint& aDestination, ScrollSnapFlags aSnapFlags);
+
+  // Find a snap point near |aDestination| that we should snap to.
+  // Returns the snap point if one was found, or an empty Maybe otherwise.
+  // |aUnit| affects the snapping behaviour (see ScrollSnapUtils::
+  // GetSnapPointForDestination). It should generally be determined by the
+  // type of event that's triggering the scroll.
+  Maybe<CSSSnapDestination> FindSnapPointNear(const CSSPoint& aDestination,
+                                              ScrollUnit aUnit,
+                                              ScrollSnapFlags aSnapFlags);
+
+  // If |aOriginalEvent| crosses the touch-start tolerance threshold, split it
+  // into two events: one that just reaches the threshold, and the remainder.
+  //
+  // |aPanThreshold| is the touch-start tolerance, and |aVectorLength| is
+  // the length of the vector from the touch-start position to |aOriginalEvent|.
+  // These values could be computed from |aOriginalEvent| but they are
+  // passed in for convenience since the caller also needs to compute them.
+  //
+  // |aExtPoint| is the position of |aOriginalEvent| in External coordinates,
+  // and in case of a split is modified by the function to reflect the position
+  // of of the first event. This is a workaround for the fact that recomputing
+  // the External position from the returned event would require a round-trip
+  // through |mScreenPoint| which is an integer.
+  Maybe<std::pair<MultiTouchInput, MultiTouchInput>> MaybeSplitTouchMoveEvent(
+      const MultiTouchInput& aOriginalEvent, ScreenCoord aPanThreshold,
+      float aVectorLength, ExternalPoint& aExtPoint);
+
+  friend std::ostream& operator<<(
+      std::ostream& aOut, const AsyncPanZoomController::PanZoomState& aState);
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_PanZoomController_h
diff --git a/gfx/layers/apz/src/AutoDirWheelDeltaAdjuster.h b/gfx/layers/apz/src/AutoDirWheelDeltaAdjuster.h
new file mode 100644
index 0000000000..187641514a
--- /dev/null
+++ b/gfx/layers/apz/src/AutoDirWheelDeltaAdjuster.h
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __mozilla_layers_AutoDirWheelDeltaAdjuster_h__
+#define __mozilla_layers_AutoDirWheelDeltaAdjuster_h__
+
+#include "Axis.h"                         // for AxisX, AxisY, Side
+#include "mozilla/WheelHandlingHelper.h"  // for AutoDirWheelDeltaAdjuster
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * About AutoDirWheelDeltaAdjuster:
+ * For an AutoDir wheel scroll, there's some situations where we should adjust a
+ * wheel event's delta values. AutoDirWheelDeltaAdjuster converts delta values
+ * for AutoDir scrolling. An AutoDir wheel scroll lets the user scroll a frame
+ * with only one scrollbar, using either a vertical or a horzizontal wheel.
+ * For more detail about the concept of AutoDir scrolling, see the comments in
+ * AutoDirWheelDeltaAdjuster.
+ *
+ * This is the APZ implementation of AutoDirWheelDeltaAdjuster.
+ */
+class MOZ_STACK_CLASS APZAutoDirWheelDeltaAdjuster final
+    : public AutoDirWheelDeltaAdjuster {
+ public:
+  /**
+   * @param aDeltaX            DeltaX for a wheel event whose delta values will
+   *                           be adjusted upon calling adjust() when
+   *                           ShouldBeAdjusted() returns true.
+   * @param aDeltaY            DeltaY for a wheel event, like DeltaX.
+   * @param aAxisX             The X axis information provider for the current
+   *                           frame, such as whether the frame can be scrolled
+   *                           horizontally, leftwards or rightwards.
+   * @param aAxisY             The Y axis information provider for the current
+   *                           frame, such as whether the frame can be scrolled
+   *                           vertically, upwards or downwards.
+   * @param aIsHorizontalContentRightToLeft
+   *                           Indicates whether the horizontal content starts
+   *                           at rightside. This value will decide which edge
+   *                           the adjusted scroll goes towards, in other words,
+   *                           it will decide the sign of the adjusted delta
+   *                           values). For detailed information, see
+   *                           IsHorizontalContentRightToLeft() in
+   *                           the base class AutoDirWheelDeltaAdjuster.
+   */
+  APZAutoDirWheelDeltaAdjuster(double& aDeltaX, double& aDeltaY,
+                               const AxisX& aAxisX, const AxisY& aAxisY,
+                               bool aIsHorizontalContentRightToLeft)
+      : AutoDirWheelDeltaAdjuster(aDeltaX, aDeltaY),
+        mAxisX(aAxisX),
+        mAxisY(aAxisY),
+        mIsHorizontalContentRightToLeft(aIsHorizontalContentRightToLeft) {}
+
+ private:
+  virtual bool CanScrollAlongXAxis() const override {
+    return mAxisX.CanScroll();
+  }
+  virtual bool CanScrollAlongYAxis() const override {
+    return mAxisY.CanScroll();
+  }
+  virtual bool CanScrollUpwards() const override {
+    return mAxisY.CanScrollTo(eSideTop);
+  }
+  virtual bool CanScrollDownwards() const override {
+    return mAxisY.CanScrollTo(eSideBottom);
+  }
+  virtual bool CanScrollLeftwards() const override {
+    return mAxisX.CanScrollTo(eSideLeft);
+  }
+  virtual bool CanScrollRightwards() const override {
+    return mAxisX.CanScrollTo(eSideRight);
+  }
+  virtual bool IsHorizontalContentRightToLeft() const override {
+    return mIsHorizontalContentRightToLeft;
+  }
+
+  const AxisX& mAxisX;
+  const AxisY& mAxisY;
+  bool mIsHorizontalContentRightToLeft;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // __mozilla_layers_AutoDirWheelDeltaAdjuster_h__
diff --git a/gfx/layers/apz/src/AutoscrollAnimation.cpp b/gfx/layers/apz/src/AutoscrollAnimation.cpp
new file mode 100644
index 0000000000..8d4b8fca10
--- /dev/null
+++ b/gfx/layers/apz/src/AutoscrollAnimation.cpp
@@ -0,0 +1,93 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AutoscrollAnimation.h"
+
+#include <cmath>  // for sqrtf()
+
+#include "AsyncPanZoomController.h"
+#include "APZCTreeManager.h"
+#include "FrameMetrics.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+
+namespace mozilla {
+namespace layers {
+
+// Helper function for AutoscrollAnimation::DoSample().
+// Basically copied as-is from toolkit/actors/AutoScrollChild.jsm.
+static float Accelerate(ScreenCoord curr, ScreenCoord start) {
+  static const int speed = 12;
+  float val = (curr - start) / speed;
+  if (val > 1) {
+    return val * sqrtf(val) - 1;
+  }
+  if (val < -1) {
+    return val * sqrtf(-val) + 1;
+  }
+  return 0;
+}
+
+AutoscrollAnimation::AutoscrollAnimation(AsyncPanZoomController& aApzc,
+                                         const ScreenPoint& aAnchorLocation)
+    : mApzc(aApzc), mAnchorLocation(aAnchorLocation) {}
+
+bool AutoscrollAnimation::DoSample(FrameMetrics& aFrameMetrics,
+                                   const TimeDuration& aDelta) {
+  APZCTreeManager* treeManager = mApzc.GetApzcTreeManager();
+  if (!treeManager) {
+    return false;
+  }
+
+  ScreenPoint mouseLocation = treeManager->GetCurrentMousePosition();
+
+  // The implementation of this function closely mirrors that of its main-
+  // thread equivalent, the autoscrollLoop() function in
+  // toolkit/actors/AutoScrollChild.jsm.
+
+  // Avoid long jumps when the browser hangs for more than |maxTimeDelta| ms.
+  static const TimeDuration maxTimeDelta = TimeDuration::FromMilliseconds(100);
+  TimeDuration timeDelta = TimeDuration::Min(aDelta, maxTimeDelta);
+
+  float timeCompensation = timeDelta.ToMilliseconds() / 20;
+
+  // Notes:
+  //   - The main-thread implementation rounds the scroll delta to an integer,
+  //     and keeps track of the fractional part as an "error". It does this
+  //     because it uses Window.scrollBy() or Element.scrollBy() to perform
+  //     the scrolling, and those functions truncate the fractional part of
+  //     the offset. APZ does no such truncation, so there's no need to keep
+  //     track of the fractional part separately.
+  //   - The Accelerate() function takes Screen coordinates as inputs, but
+  //     its output is interpreted as CSS coordinates. This is intentional,
+  //     insofar as autoscrollLoop() does the same thing.
+  CSSPoint scrollDelta{
+      Accelerate(mouseLocation.x, mAnchorLocation.x) * timeCompensation,
+      Accelerate(mouseLocation.y, mAnchorLocation.y) * timeCompensation};
+
+  mApzc.ScrollByAndClamp(scrollDelta);
+
+  // An autoscroll animation never ends of its own accord.
+  // It can be stopped in response to various input events, in which case
+  // AsyncPanZoomController::StopAutoscroll() will stop it via
+  // CancelAnimation().
+  return true;
+}
+
+void AutoscrollAnimation::Cancel(CancelAnimationFlags aFlags) {
+  // The cancellation was initiated by browser.js, so there's no need to
+  // notify it.
+  if (aFlags & TriggeredExternally) {
+    return;
+  }
+
+  if (RefPtr<GeckoContentController> controller =
+          mApzc.GetGeckoContentController()) {
+    controller->CancelAutoscroll(mApzc.GetGuid());
+  }
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/AutoscrollAnimation.h b/gfx/layers/apz/src/AutoscrollAnimation.h
new file mode 100644
index 0000000000..a37f6d473a
--- /dev/null
+++ b/gfx/layers/apz/src/AutoscrollAnimation.h
@@ -0,0 +1,42 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_AutocrollAnimation_h_
+#define mozilla_layers_AutocrollAnimation_h_
+
+#include "AsyncPanZoomAnimation.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+class AutoscrollAnimation : public AsyncPanZoomAnimation {
+ public:
+  AutoscrollAnimation(AsyncPanZoomController& aApzc,
+                      const ScreenPoint& aAnchorLocation);
+
+  bool DoSample(FrameMetrics& aFrameMetrics,
+                const TimeDuration& aDelta) override;
+
+  bool HandleScrollOffsetUpdate(
+      const Maybe<CSSPoint>& aRelativeDelta) override {
+    // Autoscroll works using screen space coordinates, so there's no work we
+    // need to do to handle either a relative or an absolute scroll update.
+    return true;
+  }
+
+  void Cancel(CancelAnimationFlags aFlags) override;
+
+ private:
+  AsyncPanZoomController& mApzc;
+  ScreenPoint mAnchorLocation;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_AutoscrollAnimation_h_
diff --git a/gfx/layers/apz/src/Axis.cpp b/gfx/layers/apz/src/Axis.cpp
new file mode 100644
index 0000000000..ddc1e09691
--- /dev/null
+++ b/gfx/layers/apz/src/Axis.cpp
@@ -0,0 +1,733 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "Axis.h"
+
+#include <math.h>     // for fabsf, pow, powf
+#include <algorithm>  // for max
+
+#include "APZCTreeManager.h"                // for APZCTreeManager
+#include "AsyncPanZoomController.h"         // for AsyncPanZoomController
+#include "FrameMetrics.h"                   // for FrameMetrics
+#include "SimpleVelocityTracker.h"          // for FrameMetrics
+#include "mozilla/Attributes.h"             // for final
+#include "mozilla/Preferences.h"            // for Preferences
+#include "mozilla/gfx/Rect.h"               // for RoundedIn
+#include "mozilla/layers/APZThreadUtils.h"  // for AssertOnControllerThread
+#include "mozilla/mozalloc.h"               // for operator new
+#include "nsMathUtils.h"                    // for NS_lround
+#include "nsPrintfCString.h"                // for nsPrintfCString
+#include "nsThreadUtils.h"                  // for NS_DispatchToMainThread, etc
+#include "nscore.h"                         // for NS_IMETHOD
+
+static mozilla::LazyLogModule sApzAxsLog("apz.axis");
+#define AXIS_LOG(...) MOZ_LOG(sApzAxsLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+bool FuzzyEqualsCoordinate(CSSCoord aValue1, CSSCoord aValue2) {
+  return FuzzyEqualsAdditive(aValue1, aValue2, COORDINATE_EPSILON) ||
+         FuzzyEqualsMultiplicative(aValue1, aValue2);
+}
+
+Axis::Axis(AsyncPanZoomController* aAsyncPanZoomController)
+    : mPos(0),
+      mVelocity(0.0f, "Axis::mVelocity"),
+      mAxisLocked(false),
+      mAsyncPanZoomController(aAsyncPanZoomController),
+      mOverscroll(0),
+      mMSDModel(0.0, 0.0, 0.0, StaticPrefs::apz_overscroll_spring_stiffness(),
+                StaticPrefs::apz_overscroll_damping()),
+      mVelocityTracker(mAsyncPanZoomController->GetPlatformSpecificState()
+                           ->CreateVelocityTracker(this)) {}
+
+float Axis::ToLocalVelocity(float aVelocityInchesPerMs) const {
+  ScreenPoint velocity =
+      MakePoint(aVelocityInchesPerMs * mAsyncPanZoomController->GetDPI());
+  // Use ToScreenCoordinates() to convert a point rather than a vector by
+  // treating the point as a vector, and using (0, 0) as the anchor.
+  ScreenPoint panStart = mAsyncPanZoomController->ToScreenCoordinates(
+      mAsyncPanZoomController->PanStart(), ParentLayerPoint());
+  ParentLayerPoint localVelocity =
+      mAsyncPanZoomController->ToParentLayerCoordinates(velocity, panStart);
+  return localVelocity.Length();
+}
+
+void Axis::UpdateWithTouchAtDevicePoint(ParentLayerCoord aPos,
+                                        TimeStamp aTimestamp) {
+  // mVelocityTracker is controller-thread only
+  APZThreadUtils::AssertOnControllerThread();
+
+  mPos = aPos;
+
+  AXIS_LOG("%p|%s got position %f\n", mAsyncPanZoomController, Name(),
+           mPos.value);
+  if (Maybe<float> newVelocity =
+          mVelocityTracker->AddPosition(aPos, aTimestamp)) {
+    DoSetVelocity(mAxisLocked ? 0 : *newVelocity);
+    AXIS_LOG("%p|%s velocity from tracker is %f%s\n", mAsyncPanZoomController,
+             Name(), *newVelocity,
+             mAxisLocked ? ", but we are axis locked" : "");
+  }
+}
+
+void Axis::StartTouch(ParentLayerCoord aPos, TimeStamp aTimestamp) {
+  mStartPos = aPos;
+  mPos = aPos;
+  mVelocityTracker->StartTracking(aPos, aTimestamp);
+  mAxisLocked = false;
+}
+
+bool Axis::AdjustDisplacement(ParentLayerCoord aDisplacement,
+                              ParentLayerCoord& aDisplacementOut,
+                              ParentLayerCoord& aOverscrollAmountOut,
+                              bool aForceOverscroll /* = false */) {
+  if (mAxisLocked) {
+    aOverscrollAmountOut = 0;
+    aDisplacementOut = 0;
+    return false;
+  }
+  if (aForceOverscroll) {
+    aOverscrollAmountOut = aDisplacement;
+    aDisplacementOut = 0;
+    return false;
+  }
+
+  ParentLayerCoord displacement = aDisplacement;
+
+  // First consume any overscroll in the opposite direction along this axis.
+  ParentLayerCoord consumedOverscroll = 0;
+  if (mOverscroll > 0 && aDisplacement < 0) {
+    consumedOverscroll = std::min(mOverscroll, -aDisplacement);
+  } else if (mOverscroll < 0 && aDisplacement > 0) {
+    consumedOverscroll = 0.f - std::min(-mOverscroll, aDisplacement);
+  }
+  mOverscroll -= consumedOverscroll;
+  displacement += consumedOverscroll;
+
+  if (consumedOverscroll != 0.0f) {
+    AXIS_LOG("%p|%s changed overscroll amount to %f\n", mAsyncPanZoomController,
+             Name(), mOverscroll.value);
+  }
+
+  // Split the requested displacement into an allowed displacement that does
+  // not overscroll, and an overscroll amount.
+  aOverscrollAmountOut = DisplacementWillOverscrollAmount(displacement);
+  if (aOverscrollAmountOut != 0.0f) {
+    // No need to have a velocity along this axis anymore; it won't take us
+    // anywhere, so we're just spinning needlessly.
+    AXIS_LOG("%p|%s has overscrolled, clearing velocity\n",
+             mAsyncPanZoomController, Name());
+    DoSetVelocity(0.0f);
+    displacement -= aOverscrollAmountOut;
+  }
+  aDisplacementOut = displacement;
+  return fabsf(consumedOverscroll) > EPSILON;
+}
+
+ParentLayerCoord Axis::ApplyResistance(
+    ParentLayerCoord aRequestedOverscroll) const {
+  // 'resistanceFactor' is a value between 0 and 1/16, which:
+  //   - tends to 1/16 as the existing overscroll tends to 0
+  //   - tends to 0 as the existing overscroll tends to the composition length
+  // The actual overscroll is the requested overscroll multiplied by this
+  // factor.
+  float resistanceFactor =
+      (1 - fabsf(GetOverscroll()) / GetCompositionLength()) / 16;
+  float result = resistanceFactor < 0 ? ParentLayerCoord(0)
+                                      : aRequestedOverscroll * resistanceFactor;
+  result = clamped(result, -8.0f, 8.0f);
+  return result;
+}
+
+void Axis::OverscrollBy(ParentLayerCoord aOverscroll) {
+  MOZ_ASSERT(CanScroll());
+  // We can get some spurious calls to OverscrollBy() with near-zero values
+  // due to rounding error. Ignore those (they might trip the asserts below.)
+  if (mAsyncPanZoomController->IsZero(aOverscroll)) {
+    return;
+  }
+  EndOverscrollAnimation();
+  aOverscroll = ApplyResistance(aOverscroll);
+  if (aOverscroll > 0) {
+#ifdef DEBUG
+    if (!IsScrolledToEnd()) {
+      nsPrintfCString message(
+          "composition end (%f) is not equal (within error) to page end (%f)\n",
+          GetCompositionEnd().value, GetPageEnd().value);
+      NS_ASSERTION(false, message.get());
+      MOZ_CRASH("GFX: Overscroll issue > 0");
+    }
+#endif
+    MOZ_ASSERT(mOverscroll >= 0);
+  } else if (aOverscroll < 0) {
+#ifdef DEBUG
+    if (!IsScrolledToStart()) {
+      nsPrintfCString message(
+          "composition origin (%f) is not equal (within error) to page origin "
+          "(%f)\n",
+          GetOrigin().value, GetPageStart().value);
+      NS_ASSERTION(false, message.get());
+      MOZ_CRASH("GFX: Overscroll issue < 0");
+    }
+#endif
+    MOZ_ASSERT(mOverscroll <= 0);
+  }
+  mOverscroll += aOverscroll;
+
+  AXIS_LOG("%p|%s changed overscroll amount to %f\n", mAsyncPanZoomController,
+           Name(), mOverscroll.value);
+}
+
+ParentLayerCoord Axis::GetOverscroll() const { return mOverscroll; }
+
+void Axis::RestoreOverscroll(ParentLayerCoord aOverscroll) {
+  mOverscroll = aOverscroll;
+}
+
+void Axis::StartOverscrollAnimation(float aVelocity) {
+  const float maxVelocity = StaticPrefs::apz_overscroll_max_velocity();
+  aVelocity = clamped(aVelocity / 2.0f, -maxVelocity, maxVelocity);
+  SetVelocity(aVelocity);
+  mMSDModel.SetPosition(mOverscroll);
+  // Convert velocity from ParentLayerCoords/millisecond to
+  // ParentLayerCoords/second.
+  mMSDModel.SetVelocity(DoGetVelocity() * 1000.0);
+
+  AXIS_LOG(
+      "%p|%s beginning overscroll animation with amount %f and velocity %f\n",
+      mAsyncPanZoomController, Name(), mOverscroll.value, DoGetVelocity());
+}
+
+void Axis::EndOverscrollAnimation() {
+  mMSDModel.SetPosition(0.0);
+  mMSDModel.SetVelocity(0.0);
+}
+
+bool Axis::SampleOverscrollAnimation(const TimeDuration& aDelta,
+                                     SideBits aOverscrollSideBits) {
+  mMSDModel.Simulate(aDelta);
+  mOverscroll = mMSDModel.GetPosition();
+
+  if (((aOverscrollSideBits & (SideBits::eTop | SideBits::eLeft)) &&
+       mOverscroll > 0) ||
+      ((aOverscrollSideBits & (SideBits::eBottom | SideBits::eRight)) &&
+       mOverscroll < 0)) {
+    // Stop the overscroll model immediately if it's going to get across the
+    // boundary.
+    mMSDModel.SetPosition(0.0);
+    mMSDModel.SetVelocity(0.0);
+  }
+
+  AXIS_LOG("%p|%s changed overscroll amount to %f\n", mAsyncPanZoomController,
+           Name(), mOverscroll.value);
+
+  if (mMSDModel.IsFinished(1.0)) {
+    // "Jump" to the at-rest state. The jump shouldn't be noticeable as the
+    // velocity and overscroll are already low.
+    AXIS_LOG("%p|%s oscillation dropped below threshold, going to rest\n",
+             mAsyncPanZoomController, Name());
+    ClearOverscroll();
+    DoSetVelocity(0);
+    return false;
+  }
+
+  // Otherwise, continue the animation.
+  return true;
+}
+
+bool Axis::IsOverscrollAnimationRunning() const {
+  return !mMSDModel.IsFinished(1.0);
+}
+
+bool Axis::IsOverscrollAnimationAlive() const {
+  // Unlike IsOverscrollAnimationRunning, check the position and the velocity to
+  // be sure that the animation has started but hasn't yet finished.
+  return mMSDModel.GetPosition() != 0.0 || mMSDModel.GetVelocity() != 0.0;
+}
+
+bool Axis::IsOverscrolled() const { return mOverscroll != 0.f; }
+
+bool Axis::IsScrolledToStart() const {
+  const auto zoom = GetFrameMetrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return FuzzyEqualsCoordinate(GetOrigin() / zoom, GetPageStart() / zoom);
+}
+
+bool Axis::IsScrolledToEnd() const {
+  const auto zoom = GetFrameMetrics().GetZoom();
+
+  if (zoom == CSSToParentLayerScale(0)) {
+    return true;
+  }
+
+  return FuzzyEqualsCoordinate(GetCompositionEnd() / zoom, GetPageEnd() / zoom);
+}
+
+bool Axis::IsInInvalidOverscroll() const {
+  if (mOverscroll > 0) {
+    return !IsScrolledToEnd();
+  } else if (mOverscroll < 0) {
+    return !IsScrolledToStart();
+  }
+  return false;
+}
+
+void Axis::ClearOverscroll() {
+  EndOverscrollAnimation();
+  mOverscroll = 0;
+}
+
+ParentLayerCoord Axis::PanStart() const { return mStartPos; }
+
+ParentLayerCoord Axis::PanDistance() const { return fabs(mPos - mStartPos); }
+
+ParentLayerCoord Axis::PanDistance(ParentLayerCoord aPos) const {
+  return fabs(aPos - mStartPos);
+}
+
+void Axis::EndTouch(TimeStamp aTimestamp, ClearAxisLock aClearAxisLock) {
+  // mVelocityQueue is controller-thread only
+  APZThreadUtils::AssertOnControllerThread();
+
+  // If the velocity tracker wasn't able to compute a velocity, zero out
+  // the velocity to make sure we don't get a fling based on some old and
+  // no-longer-relevant value of mVelocity. Also if the axis is locked then
+  // just reset the velocity to 0 since we don't need any velocity to carry
+  // into the fling.
+  if (mAxisLocked) {
+    DoSetVelocity(0);
+  } else if (Maybe<float> velocity =
+                 mVelocityTracker->ComputeVelocity(aTimestamp)) {
+    DoSetVelocity(*velocity);
+  } else {
+    DoSetVelocity(0);
+  }
+  if (aClearAxisLock == ClearAxisLock::Yes) {
+    mAxisLocked = false;
+  }
+  AXIS_LOG("%p|%s ending touch, computed velocity %f\n",
+           mAsyncPanZoomController, Name(), DoGetVelocity());
+}
+
+void Axis::CancelGesture() {
+  // mVelocityQueue is controller-thread only
+  APZThreadUtils::AssertOnControllerThread();
+
+  AXIS_LOG("%p|%s cancelling touch, clearing velocity queue\n",
+           mAsyncPanZoomController, Name());
+  DoSetVelocity(0.0f);
+  mVelocityTracker->Clear();
+  SetAxisLocked(false);
+}
+
+bool Axis::CanScroll() const {
+  return mAsyncPanZoomController->FuzzyGreater(GetPageLength(),
+                                               GetCompositionLength());
+}
+
+bool Axis::CanScroll(CSSCoord aDelta) const {
+  return CanScroll(aDelta * GetFrameMetrics().GetZoom());
+}
+
+bool Axis::CanScroll(ParentLayerCoord aDelta) const {
+  if (!CanScroll()) {
+    return false;
+  }
+
+  const auto zoom = GetFrameMetrics().GetZoom();
+  CSSCoord availableToScroll = 0;
+
+  if (zoom != CSSToParentLayerScale(0)) {
+    availableToScroll =
+        ParentLayerCoord(
+            fabs(DisplacementWillOverscrollAmount(aDelta) - aDelta)) /
+        zoom;
+  }
+
+  return availableToScroll > COORDINATE_EPSILON;
+}
+
+CSSCoord Axis::ClampOriginToScrollableRect(CSSCoord aOrigin) const {
+  CSSToParentLayerScale zoom = GetFrameMetrics().GetZoom();
+  ParentLayerCoord origin = aOrigin * zoom;
+  ParentLayerCoord result;
+  if (origin < GetPageStart()) {
+    result = GetPageStart();
+  } else if (origin + GetCompositionLength() > GetPageEnd()) {
+    result = GetPageEnd() - GetCompositionLength();
+  } else {
+    return aOrigin;
+  }
+  if (zoom == CSSToParentLayerScale(0)) {
+    return aOrigin;
+  }
+  return result / zoom;
+}
+
+bool Axis::CanScrollNow() const { return !mAxisLocked && CanScroll(); }
+
+ParentLayerCoord Axis::DisplacementWillOverscrollAmount(
+    ParentLayerCoord aDisplacement) const {
+  ParentLayerCoord newOrigin = GetOrigin() + aDisplacement;
+  ParentLayerCoord newCompositionEnd = GetCompositionEnd() + aDisplacement;
+  // If the current pan plus a displacement takes the window to the left of or
+  // above the current page rect.
+  bool minus = newOrigin < GetPageStart();
+  // If the current pan plus a displacement takes the window to the right of or
+  // below the current page rect.
+  bool plus = newCompositionEnd > GetPageEnd();
+  if (minus && plus) {
+    // Don't handle overscrolled in both directions; a displacement can't cause
+    // this, it must have already been zoomed out too far.
+    return 0;
+  }
+  if (minus) {
+    return newOrigin - GetPageStart();
+  }
+  if (plus) {
+    return newCompositionEnd - GetPageEnd();
+  }
+  return 0;
+}
+
+CSSCoord Axis::ScaleWillOverscrollAmount(float aScale, CSSCoord aFocus) const {
+  // Internally, do computations in ParentLayer coordinates *before* the scale
+  // is applied.
+  CSSToParentLayerScale zoom = GetFrameMetrics().GetZoom();
+  ParentLayerCoord focus = aFocus * zoom;
+  ParentLayerCoord originAfterScale = (GetOrigin() + focus) - (focus / aScale);
+
+  bool both = ScaleWillOverscrollBothSides(aScale);
+  bool minus = GetPageStart() - originAfterScale > COORDINATE_EPSILON;
+  bool plus =
+      (originAfterScale + (GetCompositionLength() / aScale)) - GetPageEnd() >
+      COORDINATE_EPSILON;
+
+  if ((minus && plus) || both) {
+    // If we ever reach here it's a bug in the client code.
+    MOZ_ASSERT(false,
+               "In an OVERSCROLL_BOTH condition in ScaleWillOverscrollAmount");
+    return 0;
+  }
+  if (minus && zoom != CSSToParentLayerScale(0)) {
+    return (originAfterScale - GetPageStart()) / zoom;
+  }
+  if (plus && zoom != CSSToParentLayerScale(0)) {
+    return (originAfterScale + (GetCompositionLength() / aScale) -
+            GetPageEnd()) /
+           zoom;
+  }
+  return 0;
+}
+
+bool Axis::IsAxisLocked() const { return mAxisLocked; }
+
+float Axis::GetVelocity() const { return mAxisLocked ? 0 : DoGetVelocity(); }
+
+void Axis::SetVelocity(float aVelocity) {
+  AXIS_LOG("%p|%s direct-setting velocity to %f\n", mAsyncPanZoomController,
+           Name(), aVelocity);
+  DoSetVelocity(aVelocity);
+}
+
+ParentLayerCoord Axis::GetCompositionEnd() const {
+  return GetOrigin() + GetCompositionLength();
+}
+
+ParentLayerCoord Axis::GetPageEnd() const {
+  return GetPageStart() + GetPageLength();
+}
+
+ParentLayerCoord Axis::GetScrollRangeEnd() const {
+  return GetPageEnd() - GetCompositionLength();
+}
+
+ParentLayerCoord Axis::GetOrigin() const {
+  ParentLayerPoint origin =
+      GetFrameMetrics().GetVisualScrollOffset() * GetFrameMetrics().GetZoom();
+  return GetPointOffset(origin);
+}
+
+ParentLayerCoord Axis::GetCompositionLength() const {
+  return GetRectLength(GetFrameMetrics().GetCompositionBounds());
+}
+
+ParentLayerCoord Axis::GetPageStart() const {
+  ParentLayerRect pageRect = GetFrameMetrics().GetExpandedScrollableRect() *
+                             GetFrameMetrics().GetZoom();
+  return GetRectOffset(pageRect);
+}
+
+ParentLayerCoord Axis::GetPageLength() const {
+  ParentLayerRect pageRect = GetFrameMetrics().GetExpandedScrollableRect() *
+                             GetFrameMetrics().GetZoom();
+  return GetRectLength(pageRect);
+}
+
+bool Axis::ScaleWillOverscrollBothSides(float aScale) const {
+  const FrameMetrics& metrics = GetFrameMetrics();
+  ParentLayerRect screenCompositionBounds =
+      metrics.GetCompositionBounds() / ParentLayerToParentLayerScale(aScale);
+  return GetRectLength(screenCompositionBounds) - GetPageLength() >
+         COORDINATE_EPSILON;
+}
+
+float Axis::DoGetVelocity() const {
+  auto velocity = mVelocity.Lock();
+  return velocity.ref();
+}
+void Axis::DoSetVelocity(float aVelocity) {
+  auto velocity = mVelocity.Lock();
+  velocity.ref() = aVelocity;
+}
+
+const FrameMetrics& Axis::GetFrameMetrics() const {
+  return mAsyncPanZoomController->GetFrameMetrics();
+}
+
+const ScrollMetadata& Axis::GetScrollMetadata() const {
+  return mAsyncPanZoomController->GetScrollMetadata();
+}
+
+bool Axis::OverscrollBehaviorAllowsHandoff() const {
+  // Scroll handoff is a "non-local" overscroll behavior, so it's allowed
+  // with "auto" and disallowed with "contain" and "none".
+  return GetOverscrollBehavior() == OverscrollBehavior::Auto;
+}
+
+bool Axis::OverscrollBehaviorAllowsOverscrollEffect() const {
+  // An overscroll effect is a "local" overscroll behavior, so it's allowed
+  // with "auto" and "contain" and disallowed with "none".
+  return GetOverscrollBehavior() != OverscrollBehavior::None;
+}
+
+AxisX::AxisX(AsyncPanZoomController* aAsyncPanZoomController)
+    : Axis(aAsyncPanZoomController) {}
+
+CSSCoord AxisX::GetPointOffset(const CSSPoint& aPoint) const {
+  return aPoint.x;
+}
+
+OuterCSSCoord AxisX::GetPointOffset(const OuterCSSPoint& aPoint) const {
+  return aPoint.x;
+}
+
+ParentLayerCoord AxisX::GetPointOffset(const ParentLayerPoint& aPoint) const {
+  return aPoint.x;
+}
+
+CSSToParentLayerScale AxisX::GetAxisScale(
+    const CSSToParentLayerScale2D& aScale) const {
+  return CSSToParentLayerScale(aScale.xScale);
+}
+
+ParentLayerCoord AxisX::GetRectLength(const ParentLayerRect& aRect) const {
+  return aRect.Width();
+}
+
+CSSCoord AxisX::GetRectLength(const CSSRect& aRect) const {
+  return aRect.Width();
+}
+
+ParentLayerCoord AxisX::GetRectOffset(const ParentLayerRect& aRect) const {
+  return aRect.X();
+}
+
+CSSCoord AxisX::GetRectOffset(const CSSRect& aRect) const { return aRect.X(); }
+
+float AxisX::GetTransformScale(
+    const AsyncTransformComponentMatrix& aMatrix) const {
+  return aMatrix._11;
+}
+
+ParentLayerCoord AxisX::GetTransformTranslation(
+    const AsyncTransformComponentMatrix& aMatrix) const {
+  return aMatrix._41;
+}
+
+void AxisX::PostScale(AsyncTransformComponentMatrix& aMatrix,
+                      float aScale) const {
+  aMatrix.PostScale(aScale, 1.f, 1.f);
+}
+
+void AxisX::PostTranslate(AsyncTransformComponentMatrix& aMatrix,
+                          ParentLayerCoord aTranslation) const {
+  aMatrix.PostTranslate(aTranslation, 0, 0);
+}
+
+ScreenPoint AxisX::MakePoint(ScreenCoord aCoord) const {
+  return ScreenPoint(aCoord, 0);
+}
+
+const char* AxisX::Name() const { return "X"; }
+
+bool AxisX::CanScrollTo(Side aSide) const {
+  switch (aSide) {
+    case eSideLeft:
+      return CanScroll(CSSCoord(-COORDINATE_EPSILON * 2));
+    case eSideRight:
+      return CanScroll(CSSCoord(COORDINATE_EPSILON * 2));
+    default:
+      MOZ_ASSERT_UNREACHABLE("aSide is out of valid values");
+      return false;
+  }
+}
+
+SideBits AxisX::ScrollableDirections() const {
+  SideBits directions = SideBits::eNone;
+
+  if (CanScrollTo(eSideLeft)) {
+    directions |= SideBits::eLeft;
+  }
+  if (CanScrollTo(eSideRight)) {
+    directions |= SideBits::eRight;
+  }
+
+  return directions;
+}
+
+OverscrollBehavior AxisX::GetOverscrollBehavior() const {
+  return GetScrollMetadata().GetOverscrollBehavior().mBehaviorX;
+}
+
+AxisY::AxisY(AsyncPanZoomController* aAsyncPanZoomController)
+    : Axis(aAsyncPanZoomController) {}
+
+CSSCoord AxisY::GetPointOffset(const CSSPoint& aPoint) const {
+  return aPoint.y;
+}
+
+OuterCSSCoord AxisY::GetPointOffset(const OuterCSSPoint& aPoint) const {
+  return aPoint.y;
+}
+
+ParentLayerCoord AxisY::GetPointOffset(const ParentLayerPoint& aPoint) const {
+  return aPoint.y;
+}
+
+CSSToParentLayerScale AxisY::GetAxisScale(
+    const CSSToParentLayerScale2D& aScale) const {
+  return CSSToParentLayerScale(aScale.yScale);
+}
+
+ParentLayerCoord AxisY::GetRectLength(const ParentLayerRect& aRect) const {
+  return aRect.Height();
+}
+
+CSSCoord AxisY::GetRectLength(const CSSRect& aRect) const {
+  return aRect.Height();
+}
+
+ParentLayerCoord AxisY::GetRectOffset(const ParentLayerRect& aRect) const {
+  return aRect.Y();
+}
+
+CSSCoord AxisY::GetRectOffset(const CSSRect& aRect) const { return aRect.Y(); }
+
+float AxisY::GetTransformScale(
+    const AsyncTransformComponentMatrix& aMatrix) const {
+  return aMatrix._22;
+}
+
+ParentLayerCoord AxisY::GetTransformTranslation(
+    const AsyncTransformComponentMatrix& aMatrix) const {
+  return aMatrix._42;
+}
+
+void AxisY::PostScale(AsyncTransformComponentMatrix& aMatrix,
+                      float aScale) const {
+  aMatrix.PostScale(1.f, aScale, 1.f);
+}
+
+void AxisY::PostTranslate(AsyncTransformComponentMatrix& aMatrix,
+                          ParentLayerCoord aTranslation) const {
+  aMatrix.PostTranslate(0, aTranslation, 0);
+}
+
+ScreenPoint AxisY::MakePoint(ScreenCoord aCoord) const {
+  return ScreenPoint(0, aCoord);
+}
+
+const char* AxisY::Name() const { return "Y"; }
+
+bool AxisY::CanScrollTo(Side aSide) const {
+  switch (aSide) {
+    case eSideTop:
+      return CanScroll(CSSCoord(-COORDINATE_EPSILON * 2));
+    case eSideBottom:
+      return CanScroll(CSSCoord(COORDINATE_EPSILON * 2));
+    default:
+      MOZ_ASSERT_UNREACHABLE("aSide is out of valid values");
+      return false;
+  }
+}
+
+SideBits AxisY::ScrollableDirections() const {
+  SideBits directions = SideBits::eNone;
+
+  if (CanScrollTo(eSideTop)) {
+    directions |= SideBits::eTop;
+  }
+  if (CanScrollTo(eSideBottom)) {
+    directions |= SideBits::eBottom;
+  }
+
+  return directions;
+}
+
+bool AxisY::HasDynamicToolbar() const {
+  return GetCompositionLengthWithoutDynamicToolbar() != ParentLayerCoord(0);
+}
+
+SideBits AxisY::ScrollableDirectionsWithDynamicToolbar(
+    const ScreenMargin& aFixedLayerMargins) const {
+  MOZ_ASSERT(mAsyncPanZoomController->IsRootContent());
+
+  SideBits directions = ScrollableDirections();
+
+  if (HasDynamicToolbar()) {
+    ParentLayerCoord toolbarHeight =
+        GetCompositionLength() - GetCompositionLengthWithoutDynamicToolbar();
+
+    ParentLayerMargin fixedLayerMargins = ViewAs<ParentLayerPixel>(
+        aFixedLayerMargins, PixelCastJustification::ScreenIsParentLayerForRoot);
+
+    if (!mAsyncPanZoomController->IsZero(fixedLayerMargins.bottom)) {
+      directions |= SideBits::eTop;
+    }
+    if (mAsyncPanZoomController->FuzzyGreater(
+            fixedLayerMargins.bottom + toolbarHeight, 0)) {
+      directions |= SideBits::eBottom;
+    }
+  }
+
+  return directions;
+}
+
+bool AxisY::CanVerticalScrollWithDynamicToolbar() const {
+  return !HasDynamicToolbar()
+             ? CanScroll()
+             : mAsyncPanZoomController->FuzzyGreater(
+                   GetPageLength(),
+                   GetCompositionLengthWithoutDynamicToolbar());
+}
+
+OverscrollBehavior AxisY::GetOverscrollBehavior() const {
+  return GetScrollMetadata().GetOverscrollBehavior().mBehaviorY;
+}
+
+ParentLayerCoord AxisY::GetCompositionLengthWithoutDynamicToolbar() const {
+  return GetFrameMetrics().GetCompositionSizeWithoutDynamicToolbar().Height();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/Axis.h b/gfx/layers/apz/src/Axis.h
new file mode 100644
index 0000000000..072c0a297b
--- /dev/null
+++ b/gfx/layers/apz/src/Axis.h
@@ -0,0 +1,462 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_Axis_h
+#define mozilla_layers_Axis_h
+
+#include <sys/types.h>  // for int32_t
+
+#include "APZUtils.h"
+#include "AxisPhysicsMSDModel.h"
+#include "mozilla/DataMutex.h"  // for DataMutex
+#include "mozilla/gfx/Types.h"  // for Side
+#include "mozilla/TimeStamp.h"  // for TimeDuration
+#include "nsTArray.h"           // for nsTArray
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+const float EPSILON = 0.0001f;
+
+/**
+ * Compare two coordinates for equality, accounting for rounding error.
+ * Use both FuzzyEqualsAdditive() with COORDINATE_EPISLON, which accounts for
+ * things like the error introduced by rounding during a round-trip to app
+ * units, and FuzzyEqualsMultiplicative(), which accounts for accumulated error
+ * due to floating-point operations (which can be larger than COORDINATE_EPISLON
+ * for sufficiently large coordinate values).
+ */
+bool FuzzyEqualsCoordinate(CSSCoord aValue1, CSSCoord aValue2);
+
+struct FrameMetrics;
+class AsyncPanZoomController;
+
+/**
+ * Interface for computing velocities along the axis based on
+ * position samples.
+ */
+class VelocityTracker {
+ public:
+  virtual ~VelocityTracker() = default;
+
+  /**
+   * Start tracking velocity along this axis, starting with the given
+   * initial position and corresponding timestamp.
+   */
+  virtual void StartTracking(ParentLayerCoord aPos, TimeStamp aTimestamp) = 0;
+  /**
+   * Record a new position along this axis, at the given timestamp.
+   * Returns the average velocity between the last sample and this one, or
+   * or Nothing() if a reasonable average cannot be computed.
+   */
+  virtual Maybe<float> AddPosition(ParentLayerCoord aPos,
+                                   TimeStamp aTimestamp) = 0;
+  /**
+   * Compute an estimate of the axis's current velocity, based on recent
+   * position samples. It's up to implementation how many samples to consider
+   * and how to perform the computation.
+   * If the tracker doesn't have enough samples to compute a result, it
+   * may return Nothing{}.
+   */
+  virtual Maybe<float> ComputeVelocity(TimeStamp aTimestamp) = 0;
+  /**
+   * Clear all state in the velocity tracker.
+   */
+  virtual void Clear() = 0;
+};
+
+/**
+ * Helper class to maintain each axis of movement (X,Y) for panning and zooming.
+ * Note that everything here is specific to one axis; that is, the X axis knows
+ * nothing about the Y axis and vice versa.
+ */
+class Axis {
+ public:
+  explicit Axis(AsyncPanZoomController* aAsyncPanZoomController);
+
+  /**
+   * Notify this Axis that a new touch has been received, including a timestamp
+   * for when the touch was received. This triggers a recalculation of velocity.
+   * This can also used for pan gesture events. For those events, |aPos| is
+   * an invented position corresponding to the mouse position plus any
+   * accumulated displacements over the course of the pan gesture.
+   */
+  void UpdateWithTouchAtDevicePoint(ParentLayerCoord aPos,
+                                    TimeStamp aTimestamp);
+
+ public:
+  /**
+   * Notify this Axis that a touch has begun, i.e. the user has put their finger
+   * on the screen but has not yet tried to pan.
+   */
+  void StartTouch(ParentLayerCoord aPos, TimeStamp aTimestamp);
+
+  /**
+   * Helper enum class for specifying if EndTouch() should clear the axis lock.
+   */
+  enum class ClearAxisLock { Yes, No };
+
+  /**
+   * Notify this Axis that a touch has ended gracefully. This may perform
+   * recalculations of the axis velocity.
+   */
+  void EndTouch(TimeStamp aTimestamp, ClearAxisLock aClearAxisLock);
+
+  /**
+   * Notify this Axis that the gesture has ended forcefully. Useful for stopping
+   * flings when a user puts their finger down in the middle of one (i.e. to
+   * stop a previous touch including its fling so that a new one can take its
+   * place).
+   */
+  void CancelGesture();
+
+  /**
+   * Takes a requested displacement to the position of this axis, and adjusts it
+   * to account for overscroll (which might decrease the displacement; this is
+   * to prevent the viewport from overscrolling the page rect), and axis locking
+   * (which might prevent any displacement from happening). If overscroll
+   * ocurred, its amount is written to |aOverscrollAmountOut|.
+   * The |aDisplacementOut| parameter is set to the adjusted displacement, and
+   * the function returns true if and only if internal overscroll amounts were
+   * changed.
+   */
+  bool AdjustDisplacement(ParentLayerCoord aDisplacement,
+                          ParentLayerCoord& aDisplacementOut,
+                          ParentLayerCoord& aOverscrollAmountOut,
+                          bool aForceOverscroll = false);
+
+  /**
+   * Overscrolls this axis by the requested amount in the requested direction.
+   * The axis must be at the end of its scroll range in this direction.
+   */
+  void OverscrollBy(ParentLayerCoord aOverscroll);
+
+  /**
+   * Return the amount of overscroll on this axis, in ParentLayer pixels.
+   *
+   * If this amount is nonzero, the relevant component of
+   * mAsyncPanZoomController->Metrics().mScrollOffset must be at its
+   * extreme allowed value in the relevant direction (that is, it must be at
+   * its maximum value if we are overscrolled at our composition length, and
+   * at its minimum value if we are overscrolled at the origin).
+   */
+  ParentLayerCoord GetOverscroll() const;
+
+  /**
+   * Restore the amount by which this axis is overscrolled to the specified
+   * amount. This is for test-related use; overscrolling as a result of user
+   * input should happen via OverscrollBy().
+   */
+  void RestoreOverscroll(ParentLayerCoord aOverscroll);
+
+  /**
+   * Start an overscroll animation with the given initial velocity.
+   */
+  void StartOverscrollAnimation(float aVelocity);
+
+  /**
+   * Sample the snap-back animation to relieve overscroll.
+   * |aDelta| is the time since the last sample, |aOverscrollSideBits| is
+   * the direction where the overscroll happens on this axis.
+   */
+  bool SampleOverscrollAnimation(const TimeDuration& aDelta,
+                                 SideBits aOverscrollSideBits);
+
+  /**
+   * Stop an overscroll animation.
+   */
+  void EndOverscrollAnimation();
+
+  /**
+   * Return whether this axis is overscrolled in either direction.
+   */
+  bool IsOverscrolled() const;
+
+  /**
+   * Return true if this axis is overscrolled but its scroll offset
+   * has changed in a way that makes the oversrolled state no longer
+   * valid (for example, it is overscrolled at the top but the
+   * scroll offset is no longer zero).
+   */
+  bool IsInInvalidOverscroll() const;
+
+  /**
+   * Clear any overscroll amount on this axis.
+   */
+  void ClearOverscroll();
+
+  /**
+   * Returns whether the overscroll animation is alive.
+   */
+  bool IsOverscrollAnimationAlive() const;
+
+  /**
+   * Returns whether the overscroll animation is running.
+   * Note that unlike the above IsOverscrollAnimationAlive, this function
+   * returns false even if the animation is still there but is very close to
+   * the destination position and its velocity is quite low, i.e. it's time to
+   * finish.
+   */
+  bool IsOverscrollAnimationRunning() const;
+
+  /**
+   * Gets the starting position of the touch supplied in StartTouch().
+   */
+  ParentLayerCoord PanStart() const;
+
+  /**
+   * Gets the distance between the starting position of the touch supplied in
+   * StartTouch() and the current touch from the last
+   * UpdateWithTouchAtDevicePoint().
+   */
+  ParentLayerCoord PanDistance() const;
+
+  /**
+   * Gets the distance between the starting position of the touch supplied in
+   * StartTouch() and the supplied position.
+   */
+  ParentLayerCoord PanDistance(ParentLayerCoord aPos) const;
+
+  /**
+   * Returns true if the page has room to be scrolled along this axis.
+   */
+  bool CanScroll() const;
+
+  /**
+   * Returns whether this axis can scroll any more in a particular direction.
+   */
+  bool CanScroll(CSSCoord aDelta) const;
+  bool CanScroll(ParentLayerCoord aDelta) const;
+
+  /**
+   * Returns true if the page has room to be scrolled along this axis
+   * and this axis is not scroll-locked.
+   */
+  bool CanScrollNow() const;
+
+  /**
+   * Clamp a point to the page's scrollable bounds. That is, a scroll
+   * destination to the returned point will not contain any overscroll.
+   */
+  CSSCoord ClampOriginToScrollableRect(CSSCoord aOrigin) const;
+
+  void SetAxisLocked(bool aAxisLocked) { mAxisLocked = aAxisLocked; }
+
+  /**
+   * Gets the raw velocity of this axis at this moment.
+   */
+  float GetVelocity() const;
+
+  /**
+   * Sets the raw velocity of this axis at this moment.
+   * Intended to be called only when the axis "takes over" a velocity from
+   * another APZC, in which case there are no touch points available to call
+   * UpdateWithTouchAtDevicePoint. In other circumstances,
+   * UpdateWithTouchAtDevicePoint should be used and the velocity calculated
+   * there.
+   */
+  void SetVelocity(float aVelocity);
+
+  /**
+   * If a displacement will overscroll the axis, this returns the amount and in
+   * what direction.
+   */
+  ParentLayerCoord DisplacementWillOverscrollAmount(
+      ParentLayerCoord aDisplacement) const;
+
+  /**
+   * If a scale will overscroll the axis, this returns the amount and in what
+   * direction.
+   *
+   * |aFocus| is the point at which the scale is focused at. We will offset the
+   * scroll offset in such a way that it remains in the same place on the page
+   * relative.
+   *
+   * Note: Unlike most other functions in Axis, this functions operates in
+   *       CSS coordinates so there is no confusion as to whether the
+   *       ParentLayer coordinates it operates in are before or after the scale
+   *       is applied.
+   */
+  CSSCoord ScaleWillOverscrollAmount(float aScale, CSSCoord aFocus) const;
+
+  /**
+   * Checks if an axis will overscroll in both directions by computing the
+   * content rect and checking that its height/width (depending on the axis)
+   * does not overextend past the viewport.
+   *
+   * This gets called by ScaleWillOverscroll().
+   */
+  bool ScaleWillOverscrollBothSides(float aScale) const;
+
+  /**
+   * Returns true if movement on this axis is locked.
+   */
+  bool IsAxisLocked() const;
+
+  ParentLayerCoord GetOrigin() const;
+  ParentLayerCoord GetCompositionLength() const;
+  ParentLayerCoord GetPageStart() const;
+  ParentLayerCoord GetPageLength() const;
+  ParentLayerCoord GetCompositionEnd() const;
+  ParentLayerCoord GetPageEnd() const;
+  ParentLayerCoord GetScrollRangeEnd() const;
+
+  bool IsScrolledToStart() const;
+  bool IsScrolledToEnd() const;
+
+  ParentLayerCoord GetPos() const { return mPos; }
+
+  bool OverscrollBehaviorAllowsHandoff() const;
+  bool OverscrollBehaviorAllowsOverscrollEffect() const;
+
+  virtual CSSToParentLayerScale GetAxisScale(
+      const CSSToParentLayerScale2D& aScale) const = 0;
+  virtual CSSCoord GetPointOffset(const CSSPoint& aPoint) const = 0;
+  virtual OuterCSSCoord GetPointOffset(const OuterCSSPoint& aPoint) const = 0;
+  virtual ParentLayerCoord GetPointOffset(
+      const ParentLayerPoint& aPoint) const = 0;
+  virtual ParentLayerCoord GetRectLength(
+      const ParentLayerRect& aRect) const = 0;
+  virtual CSSCoord GetRectLength(const CSSRect& aRect) const = 0;
+  virtual ParentLayerCoord GetRectOffset(
+      const ParentLayerRect& aRect) const = 0;
+  virtual CSSCoord GetRectOffset(const CSSRect& aRect) const = 0;
+  virtual float GetTransformScale(
+      const AsyncTransformComponentMatrix& aMatrix) const = 0;
+  virtual ParentLayerCoord GetTransformTranslation(
+      const AsyncTransformComponentMatrix& aMatrix) const = 0;
+  virtual void PostScale(AsyncTransformComponentMatrix& aMatrix,
+                         float aScale) const = 0;
+  virtual void PostTranslate(AsyncTransformComponentMatrix& aMatrix,
+                             ParentLayerCoord aTranslation) const = 0;
+
+  virtual ScreenPoint MakePoint(ScreenCoord aCoord) const = 0;
+
+  const void* OpaqueApzcPointer() const { return mAsyncPanZoomController; }
+
+  virtual const char* Name() const = 0;
+
+  // Convert a velocity from global inches/ms into ParentLayerCoords/ms.
+  float ToLocalVelocity(float aVelocityInchesPerMs) const;
+
+ protected:
+  // A position along the axis, used during input event processing to
+  // track velocities (and for touch gestures, to track the length of
+  // the gesture). For touch events, this represents the position of
+  // the finger (or in the case of two-finger scrolling, the midpoint
+  // of the two fingers). For pan gesture events, this represents an
+  // invented position corresponding to the mouse position at the start
+  // of the pan, plus deltas representing the displacement of the pan.
+  ParentLayerCoord mPos;
+
+  ParentLayerCoord mStartPos;
+  // The velocity can be accessed from multiple threads (e.g. APZ
+  // controller thread and APZ sampler thread), so needs to be
+  // protected by a mutex.
+  // Units: ParentLayerCoords per millisecond
+  mutable DataMutex<float> mVelocity;
+  bool mAxisLocked;  // Whether movement on this axis is locked.
+  AsyncPanZoomController* mAsyncPanZoomController;
+
+  // The amount by which we are overscrolled; see GetOverscroll().
+  ParentLayerCoord mOverscroll;
+
+  // The mass-spring-damper model for overscroll physics.
+  AxisPhysicsMSDModel mMSDModel;
+
+  // Used to track velocity over a series of input events and compute
+  // a resulting velocity to use for e.g. starting a fling animation.
+  // This member can only be accessed on the controller/UI thread.
+  UniquePtr<VelocityTracker> mVelocityTracker;
+
+  float DoGetVelocity() const;
+  void DoSetVelocity(float aVelocity);
+
+  const FrameMetrics& GetFrameMetrics() const;
+  const ScrollMetadata& GetScrollMetadata() const;
+
+  // Do not use this function directly, use
+  // AsyncPanZoomController::GetAllowedHandoffDirections instead.
+  virtual OverscrollBehavior GetOverscrollBehavior() const = 0;
+
+  // Adjust a requested overscroll amount for resistance, yielding a smaller
+  // actual overscroll amount.
+  ParentLayerCoord ApplyResistance(ParentLayerCoord aOverscroll) const;
+
+  // Helper function for SampleOverscrollAnimation().
+  void StepOverscrollAnimation(double aStepDurationMilliseconds);
+};
+
+class AxisX : public Axis {
+ public:
+  explicit AxisX(AsyncPanZoomController* mAsyncPanZoomController);
+  CSSToParentLayerScale GetAxisScale(
+      const CSSToParentLayerScale2D& aScale) const override;
+  CSSCoord GetPointOffset(const CSSPoint& aPoint) const override;
+  OuterCSSCoord GetPointOffset(const OuterCSSPoint& aPoint) const override;
+  ParentLayerCoord GetPointOffset(
+      const ParentLayerPoint& aPoint) const override;
+  ParentLayerCoord GetRectLength(const ParentLayerRect& aRect) const override;
+  CSSCoord GetRectLength(const CSSRect& aRect) const override;
+  ParentLayerCoord GetRectOffset(const ParentLayerRect& aRect) const override;
+  CSSCoord GetRectOffset(const CSSRect& aRect) const override;
+  float GetTransformScale(
+      const AsyncTransformComponentMatrix& aMatrix) const override;
+  ParentLayerCoord GetTransformTranslation(
+      const AsyncTransformComponentMatrix& aMatrix) const override;
+  void PostScale(AsyncTransformComponentMatrix& aMatrix,
+                 float aScale) const override;
+  void PostTranslate(AsyncTransformComponentMatrix& aMatrix,
+                     ParentLayerCoord aTranslation) const override;
+  ScreenPoint MakePoint(ScreenCoord aCoord) const override;
+  const char* Name() const override;
+  bool CanScrollTo(Side aSide) const;
+  SideBits ScrollableDirections() const;
+
+ private:
+  OverscrollBehavior GetOverscrollBehavior() const override;
+};
+
+class AxisY : public Axis {
+ public:
+  explicit AxisY(AsyncPanZoomController* mAsyncPanZoomController);
+  CSSCoord GetPointOffset(const CSSPoint& aPoint) const override;
+  OuterCSSCoord GetPointOffset(const OuterCSSPoint& aPoint) const override;
+  ParentLayerCoord GetPointOffset(
+      const ParentLayerPoint& aPoint) const override;
+  CSSToParentLayerScale GetAxisScale(
+      const CSSToParentLayerScale2D& aScale) const override;
+  ParentLayerCoord GetRectLength(const ParentLayerRect& aRect) const override;
+  CSSCoord GetRectLength(const CSSRect& aRect) const override;
+  ParentLayerCoord GetRectOffset(const ParentLayerRect& aRect) const override;
+  CSSCoord GetRectOffset(const CSSRect& aRect) const override;
+  float GetTransformScale(
+      const AsyncTransformComponentMatrix& aMatrix) const override;
+  ParentLayerCoord GetTransformTranslation(
+      const AsyncTransformComponentMatrix& aMatrix) const override;
+  void PostScale(AsyncTransformComponentMatrix& aMatrix,
+                 float aScale) const override;
+  void PostTranslate(AsyncTransformComponentMatrix& aMatrix,
+                     ParentLayerCoord aTranslation) const override;
+  ScreenPoint MakePoint(ScreenCoord aCoord) const override;
+  const char* Name() const override;
+  bool CanScrollTo(Side aSide) const;
+  bool CanVerticalScrollWithDynamicToolbar() const;
+  SideBits ScrollableDirections() const;
+  SideBits ScrollableDirectionsWithDynamicToolbar(
+      const ScreenMargin& aFixedLayerMargins) const;
+
+ private:
+  OverscrollBehavior GetOverscrollBehavior() const override;
+  ParentLayerCoord GetCompositionLengthWithoutDynamicToolbar() const;
+  bool HasDynamicToolbar() const;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/CheckerboardEvent.cpp b/gfx/layers/apz/src/CheckerboardEvent.cpp
new file mode 100644
index 0000000000..5716f2d11a
--- /dev/null
+++ b/gfx/layers/apz/src/CheckerboardEvent.cpp
@@ -0,0 +1,194 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "CheckerboardEvent.h"
+#include "mozilla/Logging.h"
+
+#include <algorithm>  // for std::sort
+
+static mozilla::LazyLogModule sApzCheckLog("apz.checkerboard");
+
+namespace mozilla {
+namespace layers {
+
+// Relatively arbitrary limit to prevent a perma-checkerboard event from
+// eating up gobs of memory. Ideally we shouldn't have perma-checkerboarding
+// but better to guard against it.
+#define LOG_LENGTH_LIMIT (50 * 1024)
+
+const char* CheckerboardEvent::sDescriptions[] = {
+    "page",
+    "painted displayport",
+    "requested displayport",
+    "viewport",
+};
+
+const char* CheckerboardEvent::sColors[] = {
+    "brown",
+    "lightgreen",
+    "yellow",
+    "red",
+};
+
+CheckerboardEvent::CheckerboardEvent(bool aRecordTrace)
+    : mRecordTrace(aRecordTrace),
+      mOriginTime(TimeStamp::Now()),
+      mCheckerboardingActive(false),
+      mLastSampleTime(mOriginTime),
+      mFrameCount(0),
+      mTotalPixelMs(0),
+      mPeakPixels(0),
+      mRendertraceLock("Rendertrace") {}
+
+uint32_t CheckerboardEvent::GetSeverity() {
+  // Scale the total into a 32-bit value
+  return (uint32_t)sqrt((double)mTotalPixelMs);
+}
+
+uint32_t CheckerboardEvent::GetPeak() { return mPeakPixels; }
+
+TimeDuration CheckerboardEvent::GetDuration() { return mEndTime - mStartTime; }
+
+std::string CheckerboardEvent::GetLog() {
+  MonitorAutoLock lock(mRendertraceLock);
+  return mRendertraceInfo.str();
+}
+
+bool CheckerboardEvent::IsRecordingTrace() { return mRecordTrace; }
+
+void CheckerboardEvent::UpdateRendertraceProperty(
+    RendertraceProperty aProperty, const CSSRect& aRect,
+    const std::string& aExtraInfo) {
+  if (!mRecordTrace) {
+    return;
+  }
+  MonitorAutoLock lock(mRendertraceLock);
+  if (!mCheckerboardingActive) {
+    mBufferedProperties[aProperty].Update(aProperty, aRect, aExtraInfo, lock);
+  } else {
+    LogInfo(aProperty, TimeStamp::Now(), aRect, aExtraInfo, lock);
+  }
+}
+
+void CheckerboardEvent::LogInfo(RendertraceProperty aProperty,
+                                const TimeStamp& aTimestamp,
+                                const CSSRect& aRect,
+                                const std::string& aExtraInfo,
+                                const MonitorAutoLock& aProofOfLock) {
+  MOZ_ASSERT(mRecordTrace);
+  if (mRendertraceInfo.tellp() >= LOG_LENGTH_LIMIT) {
+    // The log is already long enough, don't put more things into it. We'll
+    // append a truncation message when this event ends.
+    return;
+  }
+  // The log is consumed by the page at about:checkerboard. The format is not
+  // formally specced, but an informal description can be found at
+  // https://searchfox.org/mozilla-central/rev/d866b96d74ec2a63f09ee418f048d23f4fd379a2/toolkit/components/aboutcheckerboard/content/aboutCheckerboard.js#86
+  mRendertraceInfo << "RENDERTRACE "
+                   << (aTimestamp - mOriginTime).ToMilliseconds() << " rect "
+                   << sColors[aProperty] << " " << aRect.X() << " " << aRect.Y()
+                   << " " << aRect.Width() << " " << aRect.Height() << " "
+                   << "// " << sDescriptions[aProperty] << aExtraInfo << '\n';
+}
+
+bool CheckerboardEvent::RecordFrameInfo(uint32_t aCssPixelsCheckerboarded) {
+  TimeStamp sampleTime = TimeStamp::Now();
+  bool eventEnding = false;
+  if (aCssPixelsCheckerboarded > 0) {
+    if (!mCheckerboardingActive) {
+      StartEvent();
+    }
+    MOZ_ASSERT(mCheckerboardingActive);
+    MOZ_ASSERT(sampleTime >= mLastSampleTime);
+    mTotalPixelMs +=
+        (uint64_t)((sampleTime - mLastSampleTime).ToMilliseconds() *
+                   aCssPixelsCheckerboarded);
+    if (aCssPixelsCheckerboarded > mPeakPixels) {
+      mPeakPixels = aCssPixelsCheckerboarded;
+    }
+    mFrameCount++;
+  } else {
+    if (mCheckerboardingActive) {
+      StopEvent();
+      eventEnding = true;
+    }
+    MOZ_ASSERT(!mCheckerboardingActive);
+  }
+  mLastSampleTime = sampleTime;
+  return eventEnding;
+}
+
+void CheckerboardEvent::StartEvent() {
+  MOZ_LOG(sApzCheckLog, LogLevel::Debug, ("Starting checkerboard event"));
+  MOZ_ASSERT(!mCheckerboardingActive);
+  mCheckerboardingActive = true;
+  mStartTime = TimeStamp::Now();
+
+  if (!mRecordTrace) {
+    return;
+  }
+  MonitorAutoLock lock(mRendertraceLock);
+  std::vector<PropertyValue> history;
+  for (PropertyBuffer& bufferedProperty : mBufferedProperties) {
+    bufferedProperty.Flush(history, lock);
+  }
+  std::sort(history.begin(), history.end());
+  for (const PropertyValue& p : history) {
+    LogInfo(p.mProperty, p.mTimeStamp, p.mRect, p.mExtraInfo, lock);
+  }
+  mRendertraceInfo << " -- checkerboarding starts below --\n";
+}
+
+void CheckerboardEvent::StopEvent() {
+  MOZ_LOG(sApzCheckLog, LogLevel::Debug, ("Stopping checkerboard event"));
+  mCheckerboardingActive = false;
+  mEndTime = TimeStamp::Now();
+
+  if (!mRecordTrace) {
+    return;
+  }
+  MonitorAutoLock lock(mRendertraceLock);
+  if (mRendertraceInfo.tellp() >= LOG_LENGTH_LIMIT) {
+    mRendertraceInfo << "[logging aborted due to length limitations]\n";
+  }
+  mRendertraceInfo << "Checkerboarded for " << mFrameCount << " frames ("
+                   << (mEndTime - mStartTime).ToMilliseconds() << " ms), "
+                   << mPeakPixels << " peak, " << GetSeverity() << " severity."
+                   << '\n';
+}
+
+bool CheckerboardEvent::PropertyValue::operator<(
+    const PropertyValue& aOther) const {
+  if (mTimeStamp < aOther.mTimeStamp) {
+    return true;
+  } else if (mTimeStamp > aOther.mTimeStamp) {
+    return false;
+  }
+  return mProperty < aOther.mProperty;
+}
+
+CheckerboardEvent::PropertyBuffer::PropertyBuffer() : mIndex(0) {}
+
+void CheckerboardEvent::PropertyBuffer::Update(
+    RendertraceProperty aProperty, const CSSRect& aRect,
+    const std::string& aExtraInfo, const MonitorAutoLock& aProofOfLock) {
+  mValues[mIndex] = {aProperty, TimeStamp::Now(), aRect, aExtraInfo};
+  mIndex = (mIndex + 1) % BUFFER_SIZE;
+}
+
+void CheckerboardEvent::PropertyBuffer::Flush(
+    std::vector<PropertyValue>& aOut, const MonitorAutoLock& aProofOfLock) {
+  for (uint32_t i = 0; i < BUFFER_SIZE; i++) {
+    uint32_t ix = (mIndex + i) % BUFFER_SIZE;
+    if (!mValues[ix].mTimeStamp.IsNull()) {
+      aOut.push_back(mValues[ix]);
+      mValues[ix].mTimeStamp = TimeStamp();
+    }
+  }
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/CheckerboardEvent.h b/gfx/layers/apz/src/CheckerboardEvent.h
new file mode 100644
index 0000000000..8306813528
--- /dev/null
+++ b/gfx/layers/apz/src/CheckerboardEvent.h
@@ -0,0 +1,220 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_CheckerboardEvent_h
+#define mozilla_layers_CheckerboardEvent_h
+
+#include "mozilla/DefineEnum.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/TimeStamp.h"
+#include <sstream>
+#include "Units.h"
+#include <vector>
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * This class records information relevant to one "checkerboard event", which is
+ * a contiguous set of frames where a given APZC was checkerboarding. The intent
+ * of this class is to record enough information that it can provide actionable
+ * steps to reduce the occurrence of checkerboarding. Furthermore, it records
+ * information about the severity of the checkerboarding so as to allow
+ * prioritizing the debugging of some checkerboarding events over others.
+ */
+class CheckerboardEvent final {
+ public:
+  // clang-format off
+  MOZ_DEFINE_ENUM_AT_CLASS_SCOPE(
+    RendertraceProperty, (
+      Page,
+      PaintedDisplayPort,
+      RequestedDisplayPort,
+      UserVisible
+  ));
+  // clang-format on
+
+  static const char* sDescriptions[sRendertracePropertyCount];
+  static const char* sColors[sRendertracePropertyCount];
+
+ public:
+  explicit CheckerboardEvent(bool aRecordTrace);
+
+  /**
+   * Gets the "severity" of the checkerboard event. This doesn't have units,
+   * it's just useful for comparing two checkerboard events to see which one
+   * is worse, for some implementation-specific definition of "worse".
+   */
+  uint32_t GetSeverity();
+
+  /**
+   * Gets the number of CSS pixels that were checkerboarded at the peak of the
+   * checkerboard event.
+   */
+  uint32_t GetPeak();
+
+  /**
+   * Gets the length of the checkerboard event.
+   */
+  TimeDuration GetDuration();
+
+  /**
+   * Gets the raw log of the checkerboard event. This can be called any time,
+   * although it really only makes sense to pull once the event is done, after
+   * RecordFrameInfo returns true.
+   */
+  std::string GetLog();
+
+  /**
+   * Returns true iff this event is recording a detailed trace of the event.
+   * This is the argument passed in to the constructor.
+   */
+  bool IsRecordingTrace();
+
+  /**
+   * Provide a new value for one of the rects that is tracked for
+   * checkerboard events.
+   */
+  void UpdateRendertraceProperty(RendertraceProperty aProperty,
+                                 const CSSRect& aRect,
+                                 const std::string& aExtraInfo = std::string());
+
+  /**
+   * Provide the number of CSS pixels that are checkerboarded in a composite
+   * at the current time.
+   * @return true if the checkerboard event has completed. The caller should
+   * stop updating this object once this happens.
+   */
+  bool RecordFrameInfo(uint32_t aCssPixelsCheckerboarded);
+
+ private:
+  /**
+   * Helper method to do stuff when checkeboarding starts.
+   */
+  void StartEvent();
+  /**
+   * Helper method to do stuff when checkerboarding stops.
+   */
+  void StopEvent();
+
+  /**
+   * Helper method to log a rendertrace property and its value to the
+   * rendertrace info buffer (mRendertraceInfo).
+   */
+  void LogInfo(RendertraceProperty aProperty, const TimeStamp& aTimestamp,
+               const CSSRect& aRect, const std::string& aExtraInfo,
+               const MonitorAutoLock& aProofOfLock)
+      MOZ_REQUIRES(mRendertraceLock);
+
+  /**
+   * Helper struct that holds a single rendertrace property value.
+   */
+  struct PropertyValue {
+    RendertraceProperty mProperty;
+    TimeStamp mTimeStamp;
+    CSSRect mRect;
+    std::string mExtraInfo;
+
+    bool operator<(const PropertyValue& aOther) const;
+  };
+
+  /**
+   * A circular buffer that stores the most recent BUFFER_SIZE values of a
+   * given property.
+   */
+  class PropertyBuffer {
+   public:
+    PropertyBuffer();
+    /**
+     * Add a new value to the buffer, overwriting the oldest one if needed.
+     */
+    void Update(RendertraceProperty aProperty, const CSSRect& aRect,
+                const std::string& aExtraInfo,
+                const MonitorAutoLock& aProofOfLock);
+    /**
+     * Dump the recorded values, oldest to newest, to the given vector, and
+     * remove them from this buffer.
+     */
+    void Flush(std::vector<PropertyValue>& aOut,
+               const MonitorAutoLock& aProofOfLock);
+
+   private:
+    static const uint32_t BUFFER_SIZE = 5;
+
+    /**
+     * The index of the oldest value in the buffer. This is the next index
+     * that will be written to.
+     */
+    uint32_t mIndex;
+    PropertyValue mValues[BUFFER_SIZE];
+  };
+
+ private:
+  /**
+   * If true, we should log the various properties during the checkerboard
+   * event. If false, we only need to record things we need for telemetry
+   * measures.
+   */
+  const bool mRecordTrace;
+  /**
+   * A base time so that the other timestamps can be turned into durations.
+   */
+  const TimeStamp mOriginTime;
+  /**
+   * Whether or not a checkerboard event is currently occurring.
+   */
+  bool mCheckerboardingActive;
+
+  /**
+   * The start time of the checkerboard event.
+   */
+  TimeStamp mStartTime;
+  /**
+   * The end time of the checkerboard event.
+   */
+  TimeStamp mEndTime;
+  /**
+   * The sample time of the last frame recorded.
+   */
+  TimeStamp mLastSampleTime;
+  /**
+   * The number of contiguous frames with checkerboard.
+   */
+  uint32_t mFrameCount;
+  /**
+   * The total number of pixel-milliseconds of checkerboarding visible to
+   * the user during the checkerboarding event.
+   */
+  uint64_t mTotalPixelMs;
+  /**
+   * The largest number of pixels of checkerboarding visible to the user
+   * during any one frame, during this checkerboarding event.
+   */
+  uint32_t mPeakPixels;
+
+  /**
+   * Monitor that needs to be acquired before touching mBufferedProperties
+   * or mRendertraceInfo.
+   */
+  mutable Monitor mRendertraceLock;
+  /**
+   * A circular buffer to store some properties. This is used before the
+   * checkerboarding actually starts, so that we have some data on what
+   * was happening before the checkerboarding started.
+   */
+  PropertyBuffer mBufferedProperties[sRendertracePropertyCount] MOZ_GUARDED_BY(
+      mRendertraceLock);
+  /**
+   * The rendertrace info buffer that gives us info on what was happening
+   * during the checkerboard event.
+   */
+  std::ostringstream mRendertraceInfo MOZ_GUARDED_BY(mRendertraceLock);
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_CheckerboardEvent_h
diff --git a/gfx/layers/apz/src/DesktopFlingPhysics.h b/gfx/layers/apz/src/DesktopFlingPhysics.h
new file mode 100644
index 0000000000..e93cc07a23
--- /dev/null
+++ b/gfx/layers/apz/src/DesktopFlingPhysics.h
@@ -0,0 +1,67 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_DesktopFlingPhysics_h_
+#define mozilla_layers_DesktopFlingPhysics_h_
+
+#include "AsyncPanZoomController.h"
+#include "Units.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/StaticPrefs_apz.h"
+
+namespace mozilla {
+namespace layers {
+
+class DesktopFlingPhysics {
+ public:
+  void Init(const ParentLayerPoint& aStartingVelocity,
+            float aPLPPI /* unused */) {
+    mVelocity = aStartingVelocity;
+  }
+  void Sample(const TimeDuration& aDelta, ParentLayerPoint* aOutVelocity,
+              ParentLayerPoint* aOutOffset) {
+    float friction = StaticPrefs::apz_fling_friction();
+    float threshold = StaticPrefs::apz_fling_stopped_threshold();
+
+    mVelocity = ParentLayerPoint(
+        ApplyFrictionOrCancel(mVelocity.x, aDelta, friction, threshold),
+        ApplyFrictionOrCancel(mVelocity.y, aDelta, friction, threshold));
+
+    *aOutVelocity = mVelocity;
+    *aOutOffset = mVelocity * aDelta.ToMilliseconds();
+  }
+
+ private:
+  /**
+   * Applies friction to the given velocity and returns the result, or
+   * returns zero if the velocity is too low.
+   * |aVelocity| is the incoming velocity.
+   * |aDelta| is the amount of time that has passed since the last time
+   * friction was applied.
+   * |aFriction| is the amount of friction to apply.
+   * |aThreshold| is the velocity below which the fling is cancelled.
+   */
+  static float ApplyFrictionOrCancel(float aVelocity,
+                                     const TimeDuration& aDelta,
+                                     float aFriction, float aThreshold) {
+    if (fabsf(aVelocity) <= aThreshold) {
+      // If the velocity is very low, just set it to 0 and stop the fling,
+      // otherwise we'll just asymptotically approach 0 and the user won't
+      // actually see any changes.
+      return 0.0f;
+    }
+
+    aVelocity *= pow(1.0f - aFriction, float(aDelta.ToMilliseconds()));
+    return aVelocity;
+  }
+
+  ParentLayerPoint mVelocity;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_DesktopFlingPhysics_h_
diff --git a/gfx/layers/apz/src/DragTracker.cpp b/gfx/layers/apz/src/DragTracker.cpp
new file mode 100644
index 0000000000..aa3b2a34f7
--- /dev/null
+++ b/gfx/layers/apz/src/DragTracker.cpp
@@ -0,0 +1,59 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "DragTracker.h"
+
+#include "InputData.h"
+#include "mozilla/Logging.h"
+
+static mozilla::LazyLogModule sApzDrgLog("apz.drag");
+#define DRAG_LOG(...) MOZ_LOG(sApzDrgLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+DragTracker::DragTracker() : mInDrag(false) {}
+
+/*static*/
+bool DragTracker::StartsDrag(const MouseInput& aInput) {
+  return aInput.IsLeftButton() && aInput.mType == MouseInput::MOUSE_DOWN;
+}
+
+/*static*/
+bool DragTracker::EndsDrag(const MouseInput& aInput) {
+  // On Windows, we don't receive a MOUSE_UP at the end of a drag if an
+  // actual drag session took place. As a backup, we detect the end of the
+  // drag using the MOUSE_DRAG_END event, which normally is routed directly
+  // to content, but we're specially routing to APZ for this purpose. Bug
+  // 1265105 tracks a solution to this at the Windows widget layer; once
+  // that is implemented, this workaround can be removed.
+  return (aInput.IsLeftButton() && aInput.mType == MouseInput::MOUSE_UP) ||
+         aInput.mType == MouseInput::MOUSE_DRAG_END;
+}
+
+void DragTracker::Update(const MouseInput& aInput) {
+  if (StartsDrag(aInput)) {
+    DRAG_LOG("Starting drag\n");
+    mInDrag = true;
+  } else if (EndsDrag(aInput)) {
+    DRAG_LOG("Ending drag\n");
+    mInDrag = false;
+    mOnScrollbar = Nothing();
+  }
+}
+
+bool DragTracker::InDrag() const { return mInDrag; }
+
+bool DragTracker::IsOnScrollbar(bool aOnScrollbar) {
+  if (!mOnScrollbar) {
+    DRAG_LOG("Setting hitscrollbar %d\n", aOnScrollbar);
+    mOnScrollbar = Some(aOnScrollbar);
+  }
+  return mOnScrollbar.value();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/DragTracker.h b/gfx/layers/apz/src/DragTracker.h
new file mode 100644
index 0000000000..92678d53c1
--- /dev/null
+++ b/gfx/layers/apz/src/DragTracker.h
@@ -0,0 +1,39 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_DragTracker_h
+#define mozilla_layers_DragTracker_h
+
+#include "mozilla/EventForwards.h"
+#include "mozilla/Maybe.h"
+
+namespace mozilla {
+
+class MouseInput;
+
+namespace layers {
+
+// DragTracker simply tracks a sequence of mouse inputs and allows us to tell
+// if we are in a drag or not (i.e. the left mouse button went down and hasn't
+// gone up yet).
+class DragTracker {
+ public:
+  DragTracker();
+  static bool StartsDrag(const MouseInput& aInput);
+  static bool EndsDrag(const MouseInput& aInput);
+  void Update(const MouseInput& aInput);
+  bool InDrag() const;
+  bool IsOnScrollbar(bool aOnScrollbar);
+
+ private:
+  Maybe<bool> mOnScrollbar;
+  bool mInDrag;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* mozilla_layers_DragTracker_h */
diff --git a/gfx/layers/apz/src/ExpectedGeckoMetrics.cpp b/gfx/layers/apz/src/ExpectedGeckoMetrics.cpp
new file mode 100644
index 0000000000..26b94e94d4
--- /dev/null
+++ b/gfx/layers/apz/src/ExpectedGeckoMetrics.cpp
@@ -0,0 +1,26 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ExpectedGeckoMetrics.h"
+#include "FrameMetrics.h"
+
+namespace mozilla {
+namespace layers {
+
+void ExpectedGeckoMetrics::UpdateFrom(const FrameMetrics& aMetrics) {
+  mVisualScrollOffset = aMetrics.GetVisualScrollOffset();
+  mLayoutScrollOffset = aMetrics.GetLayoutScrollOffset();
+  mZoom = aMetrics.GetZoom();
+  mDevPixelsPerCSSPixel = aMetrics.GetDevPixelsPerCSSPixel();
+}
+
+void ExpectedGeckoMetrics::UpdateZoomFrom(const FrameMetrics& aMetrics) {
+  mZoom = aMetrics.GetZoom();
+  mDevPixelsPerCSSPixel = aMetrics.GetDevPixelsPerCSSPixel();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/ExpectedGeckoMetrics.h b/gfx/layers/apz/src/ExpectedGeckoMetrics.h
new file mode 100644
index 0000000000..ca5aa76eba
--- /dev/null
+++ b/gfx/layers/apz/src/ExpectedGeckoMetrics.h
@@ -0,0 +1,44 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_ExpectedGeckoMetrics_h
+#define mozilla_layers_ExpectedGeckoMetrics_h
+
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+struct FrameMetrics;
+
+// A class that stores a subset of the FrameMetrics information
+// than an APZC instance expects Gecko to have (either the
+// metrics that were most recently sent to Gecko, or the ones
+// most recently received from Gecko).
+class ExpectedGeckoMetrics {
+ public:
+  ExpectedGeckoMetrics() = default;
+  void UpdateFrom(const FrameMetrics& aMetrics);
+  void UpdateZoomFrom(const FrameMetrics& aMetrics);
+
+  const CSSPoint& GetVisualScrollOffset() const { return mVisualScrollOffset; }
+  const CSSPoint& GetLayoutScrollOffset() const { return mLayoutScrollOffset; }
+  const CSSToParentLayerScale& GetZoom() const { return mZoom; }
+  const CSSToLayoutDeviceScale& GetDevPixelsPerCSSPixel() const {
+    return mDevPixelsPerCSSPixel;
+  }
+
+ private:
+  CSSPoint mVisualScrollOffset;
+  CSSPoint mLayoutScrollOffset;
+  CSSToParentLayerScale mZoom;
+  CSSToLayoutDeviceScale mDevPixelsPerCSSPixel;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/FlingAccelerator.cpp b/gfx/layers/apz/src/FlingAccelerator.cpp
new file mode 100644
index 0000000000..f37d04c77b
--- /dev/null
+++ b/gfx/layers/apz/src/FlingAccelerator.cpp
@@ -0,0 +1,128 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FlingAccelerator.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+
+#include "GenericFlingAnimation.h"  // for FLING_LOG and FlingHandoffState
+
+namespace mozilla {
+namespace layers {
+
+void FlingAccelerator::Reset() {
+  mPreviousFlingStartingVelocity = ParentLayerPoint{};
+  mPreviousFlingCancelVelocity = ParentLayerPoint{};
+  mIsTracking = false;
+}
+
+static bool SameDirection(float aVelocity1, float aVelocity2) {
+  return (aVelocity1 == 0.0f) || (aVelocity2 == 0.0f) ||
+         (IsNegative(aVelocity1) == IsNegative(aVelocity2));
+}
+
+static float Accelerate(float aBase, float aSupplemental) {
+  return (aBase * StaticPrefs::apz_fling_accel_base_mult()) +
+         (aSupplemental * StaticPrefs::apz_fling_accel_supplemental_mult());
+}
+
+ParentLayerPoint FlingAccelerator::GetFlingStartingVelocity(
+    const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+    const FlingHandoffState& aHandoffState) {
+  // If the fling should be accelerated and is in the same direction as the
+  // previous fling, boost the velocity to be the sum of the two. Check separate
+  // axes separately because we could have two vertical flings with small
+  // horizontal components on the opposite side of zero, and we still want the
+  // y-fling to get accelerated.
+  ParentLayerPoint velocity = aVelocity;
+  if (ShouldAccelerate(aNow, aVelocity, aHandoffState)) {
+    if (velocity.x != 0 &&
+        SameDirection(velocity.x, mPreviousFlingStartingVelocity.x)) {
+      velocity.x = Accelerate(velocity.x, mPreviousFlingStartingVelocity.x);
+      FLING_LOG("%p Applying fling x-acceleration from %f to %f (delta %f)\n",
+                this, aVelocity.x.value, velocity.x.value,
+                mPreviousFlingStartingVelocity.x.value);
+    }
+    if (velocity.y != 0 &&
+        SameDirection(velocity.y, mPreviousFlingStartingVelocity.y)) {
+      velocity.y = Accelerate(velocity.y, mPreviousFlingStartingVelocity.y);
+      FLING_LOG("%p Applying fling y-acceleration from %f to %f (delta %f)\n",
+                this, aVelocity.y.value, velocity.y.value,
+                mPreviousFlingStartingVelocity.y.value);
+    }
+  }
+
+  Reset();
+
+  mPreviousFlingStartingVelocity = velocity;
+  mIsTracking = true;
+
+  return velocity;
+}
+
+bool FlingAccelerator::ShouldAccelerate(
+    const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+    const FlingHandoffState& aHandoffState) const {
+  if (!IsTracking()) {
+    FLING_LOG("%p Fling accelerator was reset, not accelerating.\n", this);
+    return false;
+  }
+
+  if (!aHandoffState.mTouchStartRestingTime) {
+    FLING_LOG("%p Don't have a touch start resting time, not accelerating.\n",
+              this);
+    return false;
+  }
+
+  double msBetweenTouchStartAndPanStart =
+      aHandoffState.mTouchStartRestingTime->ToMilliseconds();
+  FLING_LOG(
+      "%p ShouldAccelerate with pan velocity %f pixels/ms, min pan velocity %f "
+      "pixels/ms, previous fling cancel velocity %f pixels/ms, time elapsed "
+      "since starting previous time between touch start and pan "
+      "start %fms.\n",
+      this, float(aVelocity.Length()), float(aHandoffState.mMinPanVelocity),
+      float(mPreviousFlingCancelVelocity.Length()),
+      float(msBetweenTouchStartAndPanStart));
+
+  if (aVelocity.Length() < StaticPrefs::apz_fling_accel_min_fling_velocity()) {
+    FLING_LOG("%p Fling velocity too low (%f), not accelerating.\n", this,
+              float(aVelocity.Length()));
+    return false;
+  }
+
+  if (aHandoffState.mMinPanVelocity <
+      StaticPrefs::apz_fling_accel_min_pan_velocity()) {
+    FLING_LOG(
+        "%p Panning velocity was too slow at some point during the pan (%f), "
+        "not accelerating.\n",
+        this, float(aHandoffState.mMinPanVelocity));
+    return false;
+  }
+
+  if (mPreviousFlingCancelVelocity.Length() <
+      StaticPrefs::apz_fling_accel_min_fling_velocity()) {
+    FLING_LOG(
+        "%p The previous fling animation had slowed down too much when it was "
+        "interrupted (%f), not accelerating.\n",
+        this, float(mPreviousFlingCancelVelocity.Length()));
+    return false;
+  }
+
+  if (msBetweenTouchStartAndPanStart >=
+      StaticPrefs::apz_fling_accel_max_pause_interval_ms()) {
+    FLING_LOG(
+        "%p Too much time (%fms) elapsed between touch start and pan start, "
+        "not accelerating.\n",
+        this, msBetweenTouchStartAndPanStart);
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/FlingAccelerator.h b/gfx/layers/apz/src/FlingAccelerator.h
new file mode 100644
index 0000000000..49e9a7b257
--- /dev/null
+++ b/gfx/layers/apz/src/FlingAccelerator.h
@@ -0,0 +1,59 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_FlingAccelerator_h
+#define mozilla_layers_FlingAccelerator_h
+
+#include "mozilla/layers/SampleTime.h"
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+struct FlingHandoffState;
+
+/**
+ * This class is used to track state that is used when determining whether a
+ * fling should be accelerated.
+ */
+class FlingAccelerator final {
+ public:
+  FlingAccelerator() {}
+
+  // Resets state so that the next fling will not be accelerated.
+  void Reset();
+
+  // Returns false after a reset or before the first fling.
+  bool IsTracking() const { return mIsTracking; }
+
+  // Starts a new fling, and returns the (potentially accelerated) velocity that
+  // should be used for that fling.
+  ParentLayerPoint GetFlingStartingVelocity(
+      const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+      const FlingHandoffState& aHandoffState);
+
+  void ObserveFlingCanceled(const ParentLayerPoint& aVelocity) {
+    mPreviousFlingCancelVelocity = aVelocity;
+  }
+
+ protected:
+  bool ShouldAccelerate(const SampleTime& aNow,
+                        const ParentLayerPoint& aVelocity,
+                        const FlingHandoffState& aHandoffState) const;
+
+  // The initial velocity of the most recent fling.
+  ParentLayerPoint mPreviousFlingStartingVelocity;
+  // The velocity that the previous fling animation had at the point it was
+  // interrupted.
+  ParentLayerPoint mPreviousFlingCancelVelocity;
+  // Whether the upcoming fling is eligible for acceleration.
+  bool mIsTracking = false;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_FlingAccelerator_h
diff --git a/gfx/layers/apz/src/FocusState.cpp b/gfx/layers/apz/src/FocusState.cpp
new file mode 100644
index 0000000000..0230676e7e
--- /dev/null
+++ b/gfx/layers/apz/src/FocusState.cpp
@@ -0,0 +1,237 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FocusState.h"
+
+#include "mozilla/Logging.h"
+#include "mozilla/layers/APZThreadUtils.h"
+
+static mozilla::LazyLogModule sApzFstLog("apz.focusstate");
+#define FS_LOG(...) MOZ_LOG(sApzFstLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+FocusState::FocusState()
+    : mMutex("FocusStateMutex"),
+      mLastAPZProcessedEvent(1),
+      mLastContentProcessedEvent(0),
+      mFocusHasKeyEventListeners(false),
+      mReceivedUpdate(false),
+      mFocusLayersId{0},
+      mFocusHorizontalTarget(ScrollableLayerGuid::NULL_SCROLL_ID),
+      mFocusVerticalTarget(ScrollableLayerGuid::NULL_SCROLL_ID) {}
+
+uint64_t FocusState::LastAPZProcessedEvent() const {
+  APZThreadUtils::AssertOnControllerThread();
+  MutexAutoLock lock(mMutex);
+
+  return mLastAPZProcessedEvent;
+}
+
+bool FocusState::IsCurrent(const MutexAutoLock& aProofOfLock) const {
+  FS_LOG("Checking IsCurrent() with cseq=%" PRIu64 ", aseq=%" PRIu64 "\n",
+         mLastContentProcessedEvent, mLastAPZProcessedEvent);
+  MOZ_ASSERT(mLastContentProcessedEvent <= mLastAPZProcessedEvent);
+  return mLastContentProcessedEvent == mLastAPZProcessedEvent;
+}
+
+void FocusState::ReceiveFocusChangingEvent() {
+  APZThreadUtils::AssertOnControllerThread();
+  MutexAutoLock lock(mMutex);
+
+  if (!mReceivedUpdate) {
+    // In the initial state don't advance mLastAPZProcessedEvent because we
+    // might blow away the information that we're in a freshly-restarted GPU
+    // process. This information (i.e. that mLastAPZProcessedEvent == 1) needs
+    // to be preserved until the first call to Update() which will then advance
+    // mLastAPZProcessedEvent to match the content-side sequence number.
+    return;
+  }
+  mLastAPZProcessedEvent += 1;
+  FS_LOG("Focus changing event incremented aseq to %" PRIu64 ", (%p)\n",
+         mLastAPZProcessedEvent, this);
+}
+
+void FocusState::Update(LayersId aRootLayerTreeId,
+                        LayersId aOriginatingLayersId,
+                        const FocusTarget& aState) {
+  // This runs on the updater thread, it's not worth passing around extra raw
+  // pointers just to assert it.
+
+  MutexAutoLock lock(mMutex);
+
+  FS_LOG("Update with rlt=%" PRIu64 ", olt=%" PRIu64 ", ft=(%s, %" PRIu64 ")\n",
+         aRootLayerTreeId.mId, aOriginatingLayersId.mId, aState.Type(),
+         aState.mSequenceNumber);
+  mReceivedUpdate = true;
+
+  // Update the focus tree with the latest target
+  mFocusTree[aOriginatingLayersId] = aState;
+
+  // Reset our internal state so we can recalculate it
+  mFocusHasKeyEventListeners = false;
+  mFocusLayersId = aRootLayerTreeId;
+  mFocusHorizontalTarget = ScrollableLayerGuid::NULL_SCROLL_ID;
+  mFocusVerticalTarget = ScrollableLayerGuid::NULL_SCROLL_ID;
+
+  // To update the focus state for the entire APZCTreeManager, we need
+  // to traverse the focus tree to find the current leaf which is the global
+  // focus target we can use for async keyboard scrolling
+  while (true) {
+    auto currentNode = mFocusTree.find(mFocusLayersId);
+    if (currentNode == mFocusTree.end()) {
+      FS_LOG("Setting target to nil (cannot find lt=%" PRIu64 ")\n",
+             mFocusLayersId.mId);
+      return;
+    }
+
+    const FocusTarget& target = currentNode->second;
+
+    // Accumulate event listener flags on the path to the focus target
+    mFocusHasKeyEventListeners |= target.mFocusHasKeyEventListeners;
+
+    // Match on the data stored in mData
+    // The match functions return true or false depending on whether the
+    // enclosing method, FocusState::Update, should return or continue to the
+    // next iteration of the while loop, respectively.
+    struct FocusTargetDataMatcher {
+      FocusState& mFocusState;
+      const uint64_t mSequenceNumber;
+
+      bool operator()(const FocusTarget::NoFocusTarget& aNoFocusTarget) {
+        FS_LOG("Setting target to nil (reached a nil target) with seq=%" PRIu64
+               ", (%p)\n",
+               mSequenceNumber, &mFocusState);
+
+        // Mark what sequence number this target has for debugging purposes so
+        // we can always accurately report on whether we are stale or not
+        mFocusState.mLastContentProcessedEvent = mSequenceNumber;
+
+        // If this focus state was just created and content has experienced more
+        // events then us, then assume we were recreated and sync focus sequence
+        // numbers.
+        if (mFocusState.mLastAPZProcessedEvent == 1 &&
+            mFocusState.mLastContentProcessedEvent >
+                mFocusState.mLastAPZProcessedEvent) {
+          mFocusState.mLastAPZProcessedEvent =
+              mFocusState.mLastContentProcessedEvent;
+        }
+        return true;
+      }
+
+      bool operator()(const LayersId& aRefLayerId) {
+        // Guard against infinite loops
+        MOZ_ASSERT(mFocusState.mFocusLayersId != aRefLayerId);
+        if (mFocusState.mFocusLayersId == aRefLayerId) {
+          FS_LOG(
+              "Setting target to nil (bailing out of infinite loop, lt=%" PRIu64
+              ")\n",
+              mFocusState.mFocusLayersId.mId);
+          return true;
+        }
+
+        FS_LOG("Looking for target in lt=%" PRIu64 "\n", aRefLayerId.mId);
+
+        // The focus target is in a child layer tree
+        mFocusState.mFocusLayersId = aRefLayerId;
+        return false;
+      }
+
+      bool operator()(const FocusTarget::ScrollTargets& aScrollTargets) {
+        FS_LOG("Setting target to h=%" PRIu64 ", v=%" PRIu64
+               ", and seq=%" PRIu64 "(%p)\n",
+               aScrollTargets.mHorizontal, aScrollTargets.mVertical,
+               mSequenceNumber, &mFocusState);
+
+        // This is the global focus target
+        mFocusState.mFocusHorizontalTarget = aScrollTargets.mHorizontal;
+        mFocusState.mFocusVerticalTarget = aScrollTargets.mVertical;
+
+        // Mark what sequence number this target has so we can determine whether
+        // it is stale or not
+        mFocusState.mLastContentProcessedEvent = mSequenceNumber;
+
+        // If this focus state was just created and content has experienced more
+        // events then us, then assume we were recreated and sync focus sequence
+        // numbers.
+        if (mFocusState.mLastAPZProcessedEvent == 1 &&
+            mFocusState.mLastContentProcessedEvent >
+                mFocusState.mLastAPZProcessedEvent) {
+          mFocusState.mLastAPZProcessedEvent =
+              mFocusState.mLastContentProcessedEvent;
+        }
+        return true;
+      }
+    };  // struct FocusTargetDataMatcher
+
+    if (target.mData.match(
+            FocusTargetDataMatcher{*this, target.mSequenceNumber})) {
+      return;
+    }
+  }
+}
+
+void FocusState::RemoveFocusTarget(LayersId aLayersId) {
+  // This runs on the updater thread, it's not worth passing around extra raw
+  // pointers just to assert it.
+  MutexAutoLock lock(mMutex);
+
+  mFocusTree.erase(aLayersId);
+}
+
+Maybe<ScrollableLayerGuid> FocusState::GetHorizontalTarget() const {
+  APZThreadUtils::AssertOnControllerThread();
+  MutexAutoLock lock(mMutex);
+
+  // There is not a scrollable layer to async scroll if
+  //   1. We aren't current
+  //   2. There are event listeners that could change the focus
+  //   3. The target has not been layerized
+  if (!IsCurrent(lock) || mFocusHasKeyEventListeners ||
+      mFocusHorizontalTarget == ScrollableLayerGuid::NULL_SCROLL_ID) {
+    return Nothing();
+  }
+  return Some(ScrollableLayerGuid(mFocusLayersId, 0, mFocusHorizontalTarget));
+}
+
+Maybe<ScrollableLayerGuid> FocusState::GetVerticalTarget() const {
+  APZThreadUtils::AssertOnControllerThread();
+  MutexAutoLock lock(mMutex);
+
+  // There is not a scrollable layer to async scroll if:
+  //   1. We aren't current
+  //   2. There are event listeners that could change the focus
+  //   3. The target has not been layerized
+  if (!IsCurrent(lock) || mFocusHasKeyEventListeners ||
+      mFocusVerticalTarget == ScrollableLayerGuid::NULL_SCROLL_ID) {
+    return Nothing();
+  }
+  return Some(ScrollableLayerGuid(mFocusLayersId, 0, mFocusVerticalTarget));
+}
+
+bool FocusState::CanIgnoreKeyboardShortcutMisses() const {
+  APZThreadUtils::AssertOnControllerThread();
+  MutexAutoLock lock(mMutex);
+
+  return IsCurrent(lock) && !mFocusHasKeyEventListeners;
+}
+
+void FocusState::Reset() {
+  MutexAutoLock lock(mMutex);
+
+  mLastAPZProcessedEvent = 1;
+  mLastContentProcessedEvent = 0;
+  mFocusHasKeyEventListeners = false;
+  mReceivedUpdate = false;
+  mFocusLayersId = {0};
+  mFocusHorizontalTarget = ScrollableLayerGuid::NULL_SCROLL_ID;
+  mFocusVerticalTarget = ScrollableLayerGuid::NULL_SCROLL_ID;
+  mFocusTree = {};
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/FocusState.h b/gfx/layers/apz/src/FocusState.h
new file mode 100644
index 0000000000..4b5ec6b71e
--- /dev/null
+++ b/gfx/layers/apz/src/FocusState.h
@@ -0,0 +1,180 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_FocusState_h
+#define mozilla_layers_FocusState_h
+
+#include <unordered_map>  // for std::unordered_map
+#include <unordered_set>  // for std::unordered_set
+
+#include "mozilla/layers/FocusTarget.h"          // for FocusTarget
+#include "mozilla/layers/ScrollableLayerGuid.h"  // for ViewID
+#include "mozilla/Mutex.h"                       // for Mutex
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * This class is used for tracking chrome and content focus targets and
+ * calculating global focus information from them for use by APZCTreeManager
+ * for async keyboard scrolling.
+ *
+ * # Calculating the element to scroll
+ *
+ * Chrome and content processes have independently focused elements. This makes
+ * it difficult to calculate the global focused element and its scrollable
+ * frame from the chrome or content side. So instead we send the local focus
+ * information from each process to here and then calculate the global focus
+ * information. This local information resides in a `focus target`.
+ *
+ * A focus target indicates that either:
+ *    1. The focused element is a remote browser along with its layer tree ID
+ *    2. The focused element is not scrollable
+ *    3. The focused element is scrollable along with the ViewID's of its
+         scrollable layers
+ *
+ * Using this information we can determine the global focus information by
+ * starting at the focus target of the root layer tree ID and following remote
+ * browsers until we reach a scrollable or non-scrollable focus target.
+ *
+ * # Determinism and sequence numbers
+ *
+ * The focused element in content can be changed within any javascript code. And
+ * javascript can run in response to an event or at any moment from `setTimeout`
+ * and others. This makes it impossible to always have the current focus
+ * information in APZ as it can be changed asynchronously at any moment. If we
+ * don't have the latest focus information, we may incorrectly scroll a target
+ * when we shouldn't.
+ *
+ * A tradeoff is designed here whereby we will maintain deterministic focus
+ * changes for user input, but not for other javascript code. The reasoning
+ * here is that `setTimeout` and others are already non-deterministic and so it
+ * might not be as breaking to web content.
+ *
+ * To maintain deterministic focus changes for a given stream of user inputs,
+ * we invalidate our focus state whenever we receive a user input that may
+ * trigger event listeners. We then attach a new sequence number to these
+ * events and dispatch them to content. Content will then include the latest
+ * sequence number it has processed to every focus update. Using this we can
+ * determine whether any potentially focus changing events have yet to be
+ * handled by content.
+ *
+ * Once we have received the latest focus sequence number from content, we know
+ * that all event listeners triggered by user inputs, and their resulting focus
+ * changes, have been processed and so we have a current target that we can use
+ * again.
+ */
+class FocusState final {
+ public:
+  FocusState();
+
+  /**
+   * The sequence number of the last potentially focus changing event processed
+   * by APZ. This number starts at one and increases monotonically. This number
+   * will never be zero as that is used to catch uninitialized focus sequence
+   * numbers on input events.
+   */
+  uint64_t LastAPZProcessedEvent() const;
+
+  /**
+   * Notify focus state of a potentially focus changing event. This will
+   * increment the current focus sequence number. The new value can be gotten
+   * from LastAPZProcessedEvent().
+   */
+  void ReceiveFocusChangingEvent();
+
+  /**
+   * Update the internal focus tree and recalculate the global focus target for
+   * a focus target update received from chrome or content.
+   *
+   * @param aRootLayerTreeId the layer tree ID of the root layer for the
+                             parent APZCTreeManager
+   * @param aOriginatingLayersId the layer tree ID that this focus target
+                                 belongs to
+   */
+  void Update(LayersId aRootLayerTreeId, LayersId aOriginatingLayersId,
+              const FocusTarget& aTarget);
+
+  /**
+   * Removes a focus target by its layer tree ID.
+   */
+  void RemoveFocusTarget(LayersId aLayersId);
+
+  /**
+   * Gets the scrollable layer that should be horizontally scrolled for a key
+   * event, if any. The returned ScrollableLayerGuid doesn't contain a
+   * presShellId, and so it should not be used in comparisons.
+   *
+   * No scrollable layer is returned if any of the following are true:
+   *   1. We don't have a current focus target
+   *   2. There are event listeners that could change the focus
+   *   3. The target has not been layerized
+   */
+  Maybe<ScrollableLayerGuid> GetHorizontalTarget() const;
+  /**
+   * The same as GetHorizontalTarget() but for vertical scrolling.
+   */
+  Maybe<ScrollableLayerGuid> GetVerticalTarget() const;
+
+  /**
+   * Gets whether it is safe to not increment the focus sequence number for an
+   * unmatched keyboard event.
+   */
+  bool CanIgnoreKeyboardShortcutMisses() const;
+
+  /**
+   * Reset to initial state.
+   */
+  void Reset();
+
+ private:
+  /**
+   * Whether the current focus state is known to be current or else if an event
+   * has been processed that could change the focus but we have not received an
+   * update with a new confirmed target.
+   * This can only be called by methods that have already acquired mMutex; they
+   * have to pass their lock as compile-time proof.
+   */
+  bool IsCurrent(const MutexAutoLock& aLock) const;
+
+ private:
+  // All methods should hold this lock, since this class is accessed via both
+  // the updater and controller threads.
+  mutable Mutex mMutex MOZ_UNANNOTATED;
+
+  // The set of focus targets received indexed by their layer tree ID
+  std::unordered_map<LayersId, FocusTarget, LayersId::HashFn> mFocusTree;
+
+  // The focus sequence number of the last potentially focus changing event
+  // processed by APZ. This number starts at one and increases monotonically.
+  // We don't worry about wrap around here because at a pace of 100
+  // increments/sec, it would take 5.85*10^9 years before we would wrap around.
+  // This number will never be zero as that is used to catch uninitialized focus
+  // sequence numbers on input events.
+  uint64_t mLastAPZProcessedEvent;
+  // The focus sequence number last received in a focus update.
+  uint64_t mLastContentProcessedEvent;
+
+  // A flag whether there is a key listener on the event target chain for the
+  // focused element
+  bool mFocusHasKeyEventListeners;
+  // A flag that is false until the first call to Update().
+  bool mReceivedUpdate;
+
+  // The layer tree ID which contains the scrollable frame of the focused
+  // element
+  LayersId mFocusLayersId;
+  // The scrollable layer corresponding to the scrollable frame that is used to
+  // scroll the focused element. This depends on the direction the user is
+  // scrolling.
+  ScrollableLayerGuid::ViewID mFocusHorizontalTarget;
+  ScrollableLayerGuid::ViewID mFocusVerticalTarget;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_FocusState_h
diff --git a/gfx/layers/apz/src/FocusTarget.cpp b/gfx/layers/apz/src/FocusTarget.cpp
new file mode 100644
index 0000000000..f1f6463e5c
--- /dev/null
+++ b/gfx/layers/apz/src/FocusTarget.cpp
@@ -0,0 +1,233 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/FocusTarget.h"
+#include "mozilla/dom/BrowserBridgeChild.h"  // for BrowserBridgeChild
+#include "mozilla/dom/EventTarget.h"         // for EventTarget
+#include "mozilla/dom/RemoteBrowser.h"       // For RemoteBrowser
+#include "mozilla/EventDispatcher.h"         // for EventDispatcher
+#include "mozilla/PresShell.h"               // For PresShell
+#include "mozilla/StaticPrefs_apz.h"
+#include "nsIContentInlines.h"  // for nsINode::IsEditable()
+#include "nsLayoutUtils.h"      // for nsLayoutUtils
+
+static mozilla::LazyLogModule sApzFtgLog("apz.focustarget");
+#define FT_LOG(...) MOZ_LOG(sApzFtgLog, LogLevel::Debug, (__VA_ARGS__))
+
+using namespace mozilla::dom;
+using namespace mozilla::layout;
+
+namespace mozilla {
+namespace layers {
+
+static PresShell* GetRetargetEventPresShell(PresShell* aRootPresShell) {
+  MOZ_ASSERT(aRootPresShell);
+
+  // Use the last focused window in this PresShell and its
+  // associated PresShell
+  nsCOMPtr<nsPIDOMWindowOuter> window =
+      aRootPresShell->GetFocusedDOMWindowInOurWindow();
+  if (!window) {
+    return nullptr;
+  }
+
+  RefPtr<Document> retargetEventDoc = window->GetExtantDoc();
+  if (!retargetEventDoc) {
+    return nullptr;
+  }
+
+  return retargetEventDoc->GetPresShell();
+}
+
+// _BOUNDARY because Dispatch() with `targets` must not handle the event.
+MOZ_CAN_RUN_SCRIPT_BOUNDARY static bool HasListenersForKeyEvents(
+    nsIContent* aContent) {
+  if (!aContent) {
+    return false;
+  }
+
+  WidgetEvent event(true, eVoidEvent);
+  nsTArray<EventTarget*> targets;
+  nsresult rv = EventDispatcher::Dispatch(aContent, nullptr, &event, nullptr,
+                                          nullptr, nullptr, &targets);
+  NS_ENSURE_SUCCESS(rv, false);
+  for (size_t i = 0; i < targets.Length(); i++) {
+    if (targets[i]->HasNonSystemGroupListenersForUntrustedKeyEvents()) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// _BOUNDARY because Dispatch() with `targets` must not handle the event.
+MOZ_CAN_RUN_SCRIPT_BOUNDARY static bool HasListenersForNonPassiveKeyEvents(
+    nsIContent* aContent) {
+  if (!aContent) {
+    return false;
+  }
+
+  WidgetEvent event(true, eVoidEvent);
+  nsTArray<EventTarget*> targets;
+  nsresult rv = EventDispatcher::Dispatch(aContent, nullptr, &event, nullptr,
+                                          nullptr, nullptr, &targets);
+  NS_ENSURE_SUCCESS(rv, false);
+  for (size_t i = 0; i < targets.Length(); i++) {
+    if (targets[i]
+            ->HasNonPassiveNonSystemGroupListenersForUntrustedKeyEvents()) {
+      return true;
+    }
+  }
+  return false;
+}
+
+static bool IsEditableNode(nsINode* aNode) {
+  return aNode && aNode->IsEditable();
+}
+
+FocusTarget::FocusTarget()
+    : mSequenceNumber(0),
+      mFocusHasKeyEventListeners(false),
+      mData(AsVariant(NoFocusTarget())) {}
+
+FocusTarget::FocusTarget(PresShell* aRootPresShell,
+                         uint64_t aFocusSequenceNumber)
+    : mSequenceNumber(aFocusSequenceNumber),
+      mFocusHasKeyEventListeners(false),
+      mData(AsVariant(NoFocusTarget())) {
+  MOZ_ASSERT(aRootPresShell);
+  MOZ_ASSERT(NS_IsMainThread());
+
+  // Key events can be retargeted to a child PresShell when there is an iframe
+  RefPtr<PresShell> presShell = GetRetargetEventPresShell(aRootPresShell);
+
+  if (!presShell) {
+    FT_LOG("Creating nil target with seq=%" PRIu64
+           " (can't find retargeted presshell)\n",
+           aFocusSequenceNumber);
+
+    return;
+  }
+
+  RefPtr<Document> document = presShell->GetDocument();
+  if (!document) {
+    FT_LOG("Creating nil target with seq=%" PRIu64 " (no document)\n",
+           aFocusSequenceNumber);
+
+    return;
+  }
+
+  // Find the focused content and use it to determine whether there are key
+  // event listeners or whether key events will be targeted at a different
+  // process through a remote browser.
+  nsCOMPtr<nsIContent> focusedContent =
+      presShell->GetFocusedContentInOurWindow();
+  nsCOMPtr<nsIContent> keyEventTarget = focusedContent;
+
+  // If there is no focused element then event dispatch goes to the body of
+  // the page if it exists or the root element.
+  if (!keyEventTarget) {
+    keyEventTarget = document->GetUnfocusedKeyEventTarget();
+  }
+
+  // Check if there are key event listeners that could prevent default or change
+  // the focus or selection of the page.
+  if (StaticPrefs::apz_keyboard_passive_listeners()) {
+    mFocusHasKeyEventListeners =
+        HasListenersForNonPassiveKeyEvents(keyEventTarget.get());
+  } else {
+    mFocusHasKeyEventListeners = HasListenersForKeyEvents(keyEventTarget.get());
+  }
+
+  // Check if the key event target is content editable or if the document
+  // is in design mode.
+  if (IsEditableNode(keyEventTarget) || IsEditableNode(document)) {
+    FT_LOG("Creating nil target with seq=%" PRIu64
+           ", kl=%d (disabling for editable node)\n",
+           aFocusSequenceNumber, static_cast<int>(mFocusHasKeyEventListeners));
+
+    return;
+  }
+
+  // Check if the key event target is a remote browser
+  if (RemoteBrowser* remoteBrowser = RemoteBrowser::GetFrom(keyEventTarget)) {
+    LayersId layersId = remoteBrowser->GetLayersId();
+
+    // The globally focused element for scrolling is in a remote layer tree
+    if (layersId.IsValid()) {
+      FT_LOG("Creating reflayer target with seq=%" PRIu64 ", kl=%d, lt=%" PRIu64
+             "\n",
+             aFocusSequenceNumber, mFocusHasKeyEventListeners, layersId.mId);
+
+      mData = AsVariant<LayersId>(std::move(layersId));
+      return;
+    }
+
+    FT_LOG("Creating nil target with seq=%" PRIu64
+           ", kl=%d (remote browser missing layers id)\n",
+           aFocusSequenceNumber, mFocusHasKeyEventListeners);
+
+    return;
+  }
+
+  // The content to scroll is either the focused element or the focus node of
+  // the selection. It's difficult to determine if an element is an interactive
+  // element requiring async keyboard scrolling to be disabled. So we only
+  // allow async key scrolling based on the selection, which doesn't have
+  // this problem and is more common.
+  if (focusedContent) {
+    FT_LOG("Creating nil target with seq=%" PRIu64
+           ", kl=%d (disabling for focusing an element)\n",
+           aFocusSequenceNumber, mFocusHasKeyEventListeners);
+
+    return;
+  }
+
+  nsCOMPtr<nsIContent> selectedContent =
+      presShell->GetSelectedContentForScrolling();
+
+  // Gather the scrollable frames that would be scrolled in each direction
+  // for this scroll target
+  nsIScrollableFrame* horizontal =
+      presShell->GetScrollableFrameToScrollForContent(
+          selectedContent.get(), HorizontalScrollDirection);
+  nsIScrollableFrame* vertical =
+      presShell->GetScrollableFrameToScrollForContent(selectedContent.get(),
+                                                      VerticalScrollDirection);
+
+  // We might have the globally focused element for scrolling. Gather a ViewID
+  // for the horizontal and vertical scroll targets of this element.
+  ScrollTargets target;
+  target.mHorizontal = nsLayoutUtils::FindIDForScrollableFrame(horizontal);
+  target.mVertical = nsLayoutUtils::FindIDForScrollableFrame(vertical);
+  mData = AsVariant(target);
+
+  FT_LOG("Creating scroll target with seq=%" PRIu64 ", kl=%d, h=%" PRIu64
+         ", v=%" PRIu64 "\n",
+         aFocusSequenceNumber, mFocusHasKeyEventListeners, target.mHorizontal,
+         target.mVertical);
+}
+
+bool FocusTarget::operator==(const FocusTarget& aRhs) const {
+  return mSequenceNumber == aRhs.mSequenceNumber &&
+         mFocusHasKeyEventListeners == aRhs.mFocusHasKeyEventListeners &&
+         mData == aRhs.mData;
+}
+
+const char* FocusTarget::Type() const {
+  if (mData.is<LayersId>()) {
+    return "LayersId";
+  }
+  if (mData.is<ScrollTargets>()) {
+    return "ScrollTargets";
+  }
+  if (mData.is<NoFocusTarget>()) {
+    return "NoFocusTarget";
+  }
+  return "<unknown>";
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/FocusTarget.h b/gfx/layers/apz/src/FocusTarget.h
new file mode 100644
index 0000000000..f4caa5d070
--- /dev/null
+++ b/gfx/layers/apz/src/FocusTarget.h
@@ -0,0 +1,71 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_FocusTarget_h
+#define mozilla_layers_FocusTarget_h
+
+#include <stdint.h>  // for int32_t, uint32_t
+
+#include "mozilla/DefineEnum.h"                  // for MOZ_DEFINE_ENUM
+#include "mozilla/layers/ScrollableLayerGuid.h"  // for ViewID
+#include "mozilla/Variant.h"                     // for Variant
+#include "mozilla/Maybe.h"                       // for Maybe
+
+namespace mozilla {
+
+class PresShell;
+
+namespace layers {
+
+/**
+ * This class is used for communicating information about the currently focused
+ * element of a document and the scrollable frames to use when keyboard
+ * scrolling it. It is created on the main thread at paint-time, but is then
+ * passed over IPC to the compositor/APZ code.
+ */
+class FocusTarget final {
+ public:
+  struct ScrollTargets {
+    ScrollableLayerGuid::ViewID mHorizontal;
+    ScrollableLayerGuid::ViewID mVertical;
+
+    bool operator==(const ScrollTargets& aRhs) const {
+      return (mHorizontal == aRhs.mHorizontal && mVertical == aRhs.mVertical);
+    }
+  };
+
+  // We need this to represent the case where mData has no focus target data
+  // because we can't have an empty variant
+  struct NoFocusTarget {
+    bool operator==(const NoFocusTarget& aRhs) const { return true; }
+  };
+
+  FocusTarget();
+
+  /**
+   * Construct a focus target for the specified top level PresShell
+   */
+  FocusTarget(PresShell* aRootPresShell, uint64_t aFocusSequenceNumber);
+
+  bool operator==(const FocusTarget& aRhs) const;
+
+  const char* Type() const;
+
+ public:
+  // The content sequence number recorded at the time of this class's creation
+  uint64_t mSequenceNumber;
+
+  // Whether there are keydown, keypress, or keyup event listeners
+  // in the event target chain of the focused element
+  bool mFocusHasKeyEventListeners;
+
+  mozilla::Variant<LayersId, ScrollTargets, NoFocusTarget> mData;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_FocusTarget_h
diff --git a/gfx/layers/apz/src/GenericFlingAnimation.h b/gfx/layers/apz/src/GenericFlingAnimation.h
new file mode 100644
index 0000000000..82f981ae0a
--- /dev/null
+++ b/gfx/layers/apz/src/GenericFlingAnimation.h
@@ -0,0 +1,207 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_GenericFlingAnimation_h_
+#define mozilla_layers_GenericFlingAnimation_h_
+
+#include "APZUtils.h"
+#include "AsyncPanZoomAnimation.h"
+#include "AsyncPanZoomController.h"
+#include "FrameMetrics.h"
+#include "Units.h"
+#include "OverscrollHandoffState.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/ToString.h"
+#include "nsThreadUtils.h"
+
+static mozilla::LazyLogModule sApzFlgLog("apz.fling");
+#define FLING_LOG(...) MOZ_LOG(sApzFlgLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * The FlingPhysics template parameter determines the physics model
+ * that the fling animation follows. It must have the following methods:
+ *
+ *   - Default constructor.
+ *
+ *   - Init(const ParentLayerPoint& aStartingVelocity, float aPLPPI).
+ *     Called at the beginning of the fling, with the fling's starting velocity,
+ *     and the number of ParentLayer pixels per (Screen) inch at the point of
+ *     the fling's start in the fling's direction.
+ *
+ *   - Sample(const TimeDuration& aDelta,
+ *            ParentLayerPoint* aOutVelocity,
+ *            ParentLayerPoint* aOutOffset);
+ *     Called on each sample of the fling.
+ *     |aDelta| is the time elapsed since the last sample.
+ *     |aOutVelocity| should be the desired velocity after the current sample,
+ *                    in ParentLayer pixels per millisecond.
+ *     |aOutOffset| should be the desired _delta_ to the scroll offset after
+ *     the current sample. |aOutOffset| should _not_ be clamped to the APZC's
+ *     scrollable bounds; the caller will do the clamping, and it needs to
+ *     know the unclamped value to handle handoff/overscroll correctly.
+ */
+template <typename FlingPhysics>
+class GenericFlingAnimation : public AsyncPanZoomAnimation,
+                              public FlingPhysics {
+ public:
+  GenericFlingAnimation(AsyncPanZoomController& aApzc,
+                        const FlingHandoffState& aHandoffState, float aPLPPI)
+      : mApzc(aApzc),
+        mOverscrollHandoffChain(aHandoffState.mChain),
+        mScrolledApzc(aHandoffState.mScrolledApzc) {
+    MOZ_ASSERT(mOverscrollHandoffChain);
+
+    // Drop any velocity on axes where we don't have room to scroll anyways
+    // (in this APZC, or an APZC further in the handoff chain).
+    // This ensures that we don't take the 'overscroll' path in Sample()
+    // on account of one axis which can't scroll having a velocity.
+    if (!mOverscrollHandoffChain->CanScrollInDirection(
+            &mApzc, ScrollDirection::eHorizontal)) {
+      RecursiveMutexAutoLock lock(mApzc.mRecursiveMutex);
+      mApzc.mX.SetVelocity(0);
+    }
+    if (!mOverscrollHandoffChain->CanScrollInDirection(
+            &mApzc, ScrollDirection::eVertical)) {
+      RecursiveMutexAutoLock lock(mApzc.mRecursiveMutex);
+      mApzc.mY.SetVelocity(0);
+    }
+
+    if (aHandoffState.mIsHandoff) {
+      // Only apply acceleration in the APZC that originated the fling, not in
+      // APZCs further down the handoff chain during handoff.
+      mApzc.mFlingAccelerator.Reset();
+    }
+
+    ParentLayerPoint velocity =
+        mApzc.mFlingAccelerator.GetFlingStartingVelocity(
+            aApzc.GetFrameTime(), mApzc.GetVelocityVector(), aHandoffState);
+
+    mApzc.SetVelocityVector(velocity);
+
+    FlingPhysics::Init(mApzc.GetVelocityVector(), aPLPPI);
+  }
+
+  /**
+   * Advances a fling by an interpolated amount based on the passed in |aDelta|.
+   * This should be called whenever sampling the content transform for this
+   * frame. Returns true if the fling animation should be advanced by one frame,
+   * or false if there is no fling or the fling has ended.
+   */
+  virtual bool DoSample(FrameMetrics& aFrameMetrics,
+                        const TimeDuration& aDelta) override {
+    CSSToParentLayerScale zoom(aFrameMetrics.GetZoom());
+    if (zoom == CSSToParentLayerScale(0)) {
+      return false;
+    }
+
+    ParentLayerPoint velocity;
+    ParentLayerPoint offset;
+    FlingPhysics::Sample(aDelta, &velocity, &offset);
+
+    mApzc.SetVelocityVector(velocity);
+
+    // If we shouldn't continue the fling, let's just stop and repaint.
+    if (IsZero(velocity / zoom)) {
+      FLING_LOG("%p ending fling animation. overscrolled=%d\n", &mApzc,
+                mApzc.IsOverscrolled());
+      // This APZC or an APZC further down the handoff chain may be be
+      // overscrolled. Start a snap-back animation on the overscrolled APZC.
+      // Note:
+      //   This needs to be a deferred task even though it can safely run
+      //   while holding mRecursiveMutex, because otherwise, if the overscrolled
+      //   APZC is this one, then the SetState(NOTHING) in UpdateAnimation will
+      //   stomp on the SetState(SNAP_BACK) it does.
+      mDeferredTasks.AppendElement(NewRunnableMethod<AsyncPanZoomController*>(
+          "layers::OverscrollHandoffChain::SnapBackOverscrolledApzc",
+          mOverscrollHandoffChain.get(),
+          &OverscrollHandoffChain::SnapBackOverscrolledApzc, &mApzc));
+      return false;
+    }
+
+    // Ordinarily we might need to do a ScheduleComposite if either of
+    // the following AdjustDisplacement calls returns true, but this
+    // is already running as part of a FlingAnimation, so we'll be compositing
+    // per frame of animation anyway.
+    ParentLayerPoint overscroll;
+    ParentLayerPoint adjustedOffset;
+    mApzc.mX.AdjustDisplacement(offset.x, adjustedOffset.x, overscroll.x);
+    mApzc.mY.AdjustDisplacement(offset.y, adjustedOffset.y, overscroll.y);
+    if (aFrameMetrics.GetZoom() != CSSToParentLayerScale(0)) {
+      mApzc.ScrollBy(adjustedOffset / aFrameMetrics.GetZoom());
+    }
+
+    // The fling may have caused us to reach the end of our scroll range.
+    if (!IsZero(overscroll / zoom)) {
+      // Hand off the fling to the next APZC in the overscroll handoff chain.
+
+      // We may have reached the end of the scroll range along one axis but
+      // not the other. In such a case we only want to hand off the relevant
+      // component of the fling.
+      if (mApzc.IsZero(overscroll.x)) {
+        velocity.x = 0;
+      } else if (mApzc.IsZero(overscroll.y)) {
+        velocity.y = 0;
+      }
+
+      // To hand off the fling, we attempt to find a target APZC and start a new
+      // fling with the same velocity on that APZC. For simplicity, the actual
+      // overscroll of the current sample is discarded rather than being handed
+      // off. The compositor should sample animations sufficiently frequently
+      // that this is not noticeable. The target APZC is chosen by seeing if
+      // there is an APZC further in the handoff chain which is pannable; if
+      // there isn't, we take the new fling ourselves, entering an overscrolled
+      // state.
+      // Note: APZC is holding mRecursiveMutex, so directly calling
+      // HandleFlingOverscroll() (which acquires the tree lock) would violate
+      // the lock ordering. Instead we schedule HandleFlingOverscroll() to be
+      // called after mRecursiveMutex is released.
+      FLING_LOG("%p fling went into overscroll, handing off with velocity %s\n",
+                &mApzc, ToString(velocity).c_str());
+      mDeferredTasks.AppendElement(
+          NewRunnableMethod<ParentLayerPoint, SideBits,
+                            RefPtr<const OverscrollHandoffChain>,
+                            RefPtr<const AsyncPanZoomController>>(
+              "layers::AsyncPanZoomController::HandleFlingOverscroll", &mApzc,
+              &AsyncPanZoomController::HandleFlingOverscroll, velocity,
+              apz::GetOverscrollSideBits(overscroll), mOverscrollHandoffChain,
+              mScrolledApzc));
+
+      // If there is a remaining velocity on this APZC, continue this fling
+      // as well. (This fling and the handed-off fling will run concurrently.)
+      // Note that AdjustDisplacement() will have zeroed out the velocity
+      // along the axes where we're overscrolled.
+      return !IsZero(mApzc.GetVelocityVector() / zoom);
+    }
+
+    return true;
+  }
+
+  void Cancel(CancelAnimationFlags aFlags) override {
+    mApzc.mFlingAccelerator.ObserveFlingCanceled(mApzc.GetVelocityVector());
+  }
+
+  virtual bool HandleScrollOffsetUpdate(
+      const Maybe<CSSPoint>& aRelativeDelta) override {
+    return true;
+  }
+
+ private:
+  AsyncPanZoomController& mApzc;
+  RefPtr<const OverscrollHandoffChain> mOverscrollHandoffChain;
+  RefPtr<const AsyncPanZoomController> mScrolledApzc;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_GenericFlingAnimation_h_
diff --git a/gfx/layers/apz/src/GenericScrollAnimation.cpp b/gfx/layers/apz/src/GenericScrollAnimation.cpp
new file mode 100644
index 0000000000..04ca8c4697
--- /dev/null
+++ b/gfx/layers/apz/src/GenericScrollAnimation.cpp
@@ -0,0 +1,123 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "GenericScrollAnimation.h"
+
+#include "AsyncPanZoomController.h"
+#include "FrameMetrics.h"
+#include "mozilla/layers/APZPublicUtils.h"
+#include "nsPoint.h"
+#include "ScrollAnimationPhysics.h"
+#include "ScrollAnimationBezierPhysics.h"
+#include "ScrollAnimationMSDPhysics.h"
+#include "mozilla/StaticPrefs_general.h"
+
+namespace mozilla {
+namespace layers {
+
+GenericScrollAnimation::GenericScrollAnimation(AsyncPanZoomController& aApzc,
+                                               const nsPoint& aInitialPosition,
+                                               ScrollOrigin aOrigin)
+    : mApzc(aApzc), mFinalDestination(aInitialPosition) {
+  // ScrollAnimationBezierPhysics (despite its name) handles the case of
+  // general.smoothScroll being disabled whereas ScrollAnimationMSDPhysics does
+  // not (ie it scrolls smoothly).
+  if (StaticPrefs::general_smoothScroll() &&
+      StaticPrefs::general_smoothScroll_msdPhysics_enabled()) {
+    mAnimationPhysics = MakeUnique<ScrollAnimationMSDPhysics>(aInitialPosition);
+  } else {
+    mAnimationPhysics = MakeUnique<ScrollAnimationBezierPhysics>(
+        aInitialPosition,
+        apz::ComputeBezierAnimationSettingsForOrigin(aOrigin));
+  }
+}
+
+void GenericScrollAnimation::UpdateDelta(TimeStamp aTime, const nsPoint& aDelta,
+                                         const nsSize& aCurrentVelocity) {
+  mFinalDestination += aDelta;
+
+  Update(aTime, aCurrentVelocity);
+}
+
+void GenericScrollAnimation::UpdateDestination(TimeStamp aTime,
+                                               const nsPoint& aDestination,
+                                               const nsSize& aCurrentVelocity) {
+  mFinalDestination = aDestination;
+
+  Update(aTime, aCurrentVelocity);
+}
+
+void GenericScrollAnimation::Update(TimeStamp aTime,
+                                    const nsSize& aCurrentVelocity) {
+  // Clamp the final destination to the scrollable area.
+  CSSPoint clamped = CSSPoint::FromAppUnits(mFinalDestination);
+  clamped.x = mApzc.mX.ClampOriginToScrollableRect(clamped.x);
+  clamped.y = mApzc.mY.ClampOriginToScrollableRect(clamped.y);
+  mFinalDestination = CSSPoint::ToAppUnits(clamped);
+
+  mAnimationPhysics->Update(aTime, mFinalDestination, aCurrentVelocity);
+}
+
+bool GenericScrollAnimation::DoSample(FrameMetrics& aFrameMetrics,
+                                      const TimeDuration& aDelta) {
+  TimeStamp now = mApzc.GetFrameTime().Time();
+  CSSToParentLayerScale zoom(aFrameMetrics.GetZoom());
+  if (zoom == CSSToParentLayerScale(0)) {
+    return false;
+  }
+
+  // If the animation is finished, make sure the final position is correct by
+  // using one last displacement. Otherwise, compute the delta via the timing
+  // function as normal.
+  bool finished = mAnimationPhysics->IsFinished(now);
+  nsPoint sampledDest = mAnimationPhysics->PositionAt(now);
+  ParentLayerPoint displacement = (CSSPoint::FromAppUnits(sampledDest) -
+                                   aFrameMetrics.GetVisualScrollOffset()) *
+                                  zoom;
+
+  if (finished) {
+    mApzc.mX.SetVelocity(0);
+    mApzc.mY.SetVelocity(0);
+  } else if (!IsZero(displacement / zoom)) {
+    // Convert velocity from AppUnits/Seconds to ParentLayerCoords/Milliseconds
+    nsSize velocity = mAnimationPhysics->VelocityAt(now);
+    ParentLayerPoint velocityPL =
+        CSSPoint::FromAppUnits(nsPoint(velocity.width, velocity.height)) * zoom;
+    mApzc.mX.SetVelocity(velocityPL.x / 1000.0);
+    mApzc.mY.SetVelocity(velocityPL.y / 1000.0);
+  }
+  // Note: we ignore overscroll for generic animations.
+  ParentLayerPoint adjustedOffset, overscroll;
+  mApzc.mX.AdjustDisplacement(
+      displacement.x, adjustedOffset.x, overscroll.x,
+      mDirectionForcedToOverscroll == Some(ScrollDirection::eHorizontal));
+  mApzc.mY.AdjustDisplacement(
+      displacement.y, adjustedOffset.y, overscroll.y,
+      mDirectionForcedToOverscroll == Some(ScrollDirection::eVertical));
+  // If we expected to scroll, but there's no more scroll range on either axis,
+  // then end the animation early. Note that the initial displacement could be 0
+  // if the compositor ran very quickly (<1ms) after the animation was created.
+  // When that happens we want to make sure the animation continues.
+  if (!IsZero(displacement / zoom) && IsZero(adjustedOffset / zoom)) {
+    // Nothing more to do - end the animation.
+    return false;
+  }
+  mApzc.ScrollBy(adjustedOffset / zoom);
+  return !finished;
+}
+
+bool GenericScrollAnimation::HandleScrollOffsetUpdate(
+    const Maybe<CSSPoint>& aRelativeDelta) {
+  if (aRelativeDelta) {
+    mAnimationPhysics->ApplyContentShift(*aRelativeDelta);
+    mFinalDestination += CSSPoint::ToAppUnits(*aRelativeDelta);
+    return true;
+  }
+  return false;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/GenericScrollAnimation.h b/gfx/layers/apz/src/GenericScrollAnimation.h
new file mode 100644
index 0000000000..138cf664ad
--- /dev/null
+++ b/gfx/layers/apz/src/GenericScrollAnimation.h
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_GenericScrollAnimation_h_
+#define mozilla_layers_GenericScrollAnimation_h_
+
+#include "AsyncPanZoomAnimation.h"
+
+namespace mozilla {
+
+class ScrollAnimationPhysics;
+
+namespace layers {
+
+class AsyncPanZoomController;
+
+class GenericScrollAnimation : public AsyncPanZoomAnimation {
+ public:
+  GenericScrollAnimation(AsyncPanZoomController& aApzc,
+                         const nsPoint& aInitialPosition, ScrollOrigin aOrigin);
+
+  bool DoSample(FrameMetrics& aFrameMetrics,
+                const TimeDuration& aDelta) override;
+
+  bool HandleScrollOffsetUpdate(const Maybe<CSSPoint>& aRelativeDelta) override;
+
+  void UpdateDelta(TimeStamp aTime, const nsPoint& aDelta,
+                   const nsSize& aCurrentVelocity);
+  void UpdateDestination(TimeStamp aTime, const nsPoint& aDestination,
+                         const nsSize& aCurrentVelocity);
+
+  CSSPoint GetDestination() const {
+    return CSSPoint::FromAppUnits(mFinalDestination);
+  }
+
+ private:
+  void Update(TimeStamp aTime, const nsSize& aCurrentVelocity);
+
+ protected:
+  AsyncPanZoomController& mApzc;
+  UniquePtr<ScrollAnimationPhysics> mAnimationPhysics;
+  nsPoint mFinalDestination;
+  // If a direction is forced to overscroll, it means it's axis in that
+  // direction is locked, and scroll in that direction is treated as overscroll
+  // of an equal amount, which, for example, may then bubble up a scroll action
+  // to its parent, or may behave as whatever an overscroll occurence requires
+  // to behave
+  Maybe<ScrollDirection> mDirectionForcedToOverscroll;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_GenericScrollAnimation_h_
diff --git a/gfx/layers/apz/src/GestureEventListener.cpp b/gfx/layers/apz/src/GestureEventListener.cpp
new file mode 100644
index 0000000000..b54674b593
--- /dev/null
+++ b/gfx/layers/apz/src/GestureEventListener.cpp
@@ -0,0 +1,663 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "GestureEventListener.h"
+#include <algorithm>                 // for max
+#include <math.h>                    // for fabsf
+#include <stddef.h>                  // for size_t
+#include "AsyncPanZoomController.h"  // for AsyncPanZoomController
+#include "InputBlockState.h"         // for TouchBlockState
+#include "base/task.h"               // for CancelableTask, etc
+#include "InputBlockState.h"         // for TouchBlockState
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_ui.h"
+#include "nsDebug.h"      // for NS_WARNING
+#include "nsMathUtils.h"  // for NS_hypot
+
+static mozilla::LazyLogModule sApzGelLog("apz.gesture");
+#define GEL_LOG(...) MOZ_LOG(sApzGelLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * Amount of span or focus change needed to take us from the
+ * GESTURE_WAITING_PINCH state to the GESTURE_PINCH state. This is measured as
+ * either a change in distance between the fingers used to compute the span
+ * ratio, or the a change in position of the focus point between the two
+ * fingers.
+ */
+static const float PINCH_START_THRESHOLD = 35.0f;
+
+/**
+ * Determines how fast a one touch pinch zooms in and out. The greater the
+ * value, the faster it zooms.
+ */
+static const float ONE_TOUCH_PINCH_SPEED = 0.005f;
+
+static bool sLongTapEnabled = true;
+
+static ScreenPoint GetCurrentFocus(const MultiTouchInput& aEvent) {
+  const ScreenPoint& firstTouch = aEvent.mTouches[0].mScreenPoint;
+  const ScreenPoint& secondTouch = aEvent.mTouches[1].mScreenPoint;
+  return (firstTouch + secondTouch) / 2;
+}
+
+static ScreenCoord GetCurrentSpan(const MultiTouchInput& aEvent) {
+  const ScreenPoint& firstTouch = aEvent.mTouches[0].mScreenPoint;
+  const ScreenPoint& secondTouch = aEvent.mTouches[1].mScreenPoint;
+  ScreenPoint delta = secondTouch - firstTouch;
+  return delta.Length();
+}
+
+ScreenCoord GestureEventListener::GetYSpanFromGestureStartPoint() {
+  // use the position that began the one-touch-pinch gesture rather
+  // mTouchStartPosition
+  const ScreenPoint start = mOneTouchPinchStartPosition;
+  const ScreenPoint& current = mTouches[0].mScreenPoint;
+  return current.y - start.y;
+}
+
+static TapGestureInput CreateTapEvent(const MultiTouchInput& aTouch,
+                                      TapGestureInput::TapGestureType aType) {
+  return TapGestureInput(aType, aTouch.mTimeStamp,
+                         aTouch.mTouches[0].mScreenPoint, aTouch.modifiers);
+}
+
+GestureEventListener::GestureEventListener(
+    AsyncPanZoomController* aAsyncPanZoomController)
+    : mAsyncPanZoomController(aAsyncPanZoomController),
+      mState(GESTURE_NONE),
+      mSpanChange(0.0f),
+      mPreviousSpan(0.0f),
+      mFocusChange(0.0f),
+      mLastTouchInput(MultiTouchInput::MULTITOUCH_START, 0, TimeStamp(), 0),
+      mLastTapInput(MultiTouchInput::MULTITOUCH_START, 0, TimeStamp(), 0),
+      mLongTapTimeoutTask(nullptr),
+      mMaxTapTimeoutTask(nullptr) {}
+
+GestureEventListener::~GestureEventListener() = default;
+
+nsEventStatus GestureEventListener::HandleInputEvent(
+    const MultiTouchInput& aEvent) {
+  GEL_LOG("Receiving event type %d with %zu touches in state %d\n",
+          aEvent.mType, aEvent.mTouches.Length(), mState);
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  // Cache the current event since it may become the single or long tap that we
+  // send.
+  mLastTouchInput = aEvent;
+
+  switch (aEvent.mType) {
+    case MultiTouchInput::MULTITOUCH_START:
+      mTouches.Clear();
+      // Cache every touch.
+      for (size_t i = 0; i < aEvent.mTouches.Length(); i++) {
+        mTouches.AppendElement(aEvent.mTouches[i]);
+      }
+
+      if (aEvent.mTouches.Length() == 1) {
+        rv = HandleInputTouchSingleStart();
+      } else {
+        rv = HandleInputTouchMultiStart();
+      }
+      break;
+    case MultiTouchInput::MULTITOUCH_MOVE:
+      // Update the screen points of the cached touches.
+      for (size_t i = 0; i < aEvent.mTouches.Length(); i++) {
+        for (size_t j = 0; j < mTouches.Length(); j++) {
+          if (aEvent.mTouches[i].mIdentifier == mTouches[j].mIdentifier) {
+            mTouches[j].mScreenPoint = aEvent.mTouches[i].mScreenPoint;
+            mTouches[j].mLocalScreenPoint =
+                aEvent.mTouches[i].mLocalScreenPoint;
+          }
+        }
+      }
+      rv = HandleInputTouchMove();
+      break;
+    case MultiTouchInput::MULTITOUCH_END:
+      // Remove the cache of the touch that ended.
+      for (size_t i = 0; i < aEvent.mTouches.Length(); i++) {
+        for (size_t j = 0; j < mTouches.Length(); j++) {
+          if (aEvent.mTouches[i].mIdentifier == mTouches[j].mIdentifier) {
+            mTouches.RemoveElementAt(j);
+            break;
+          }
+        }
+      }
+
+      rv = HandleInputTouchEnd();
+      break;
+    case MultiTouchInput::MULTITOUCH_CANCEL:
+      mTouches.Clear();
+      rv = HandleInputTouchCancel();
+      break;
+  }
+
+  return rv;
+}
+
+int32_t GestureEventListener::GetLastTouchIdentifier() const {
+  if (mTouches.Length() != 1) {
+    NS_WARNING(
+        "GetLastTouchIdentifier() called when last touch event "
+        "did not have one touch");
+  }
+  return mTouches.IsEmpty() ? -1 : mTouches[0].mIdentifier;
+}
+
+/* static */
+void GestureEventListener::SetLongTapEnabled(bool aLongTapEnabled) {
+  sLongTapEnabled = aLongTapEnabled;
+}
+
+/* static */
+bool GestureEventListener::IsLongTapEnabled() { return sLongTapEnabled; }
+
+void GestureEventListener::EnterFirstSingleTouchDown() {
+  SetState(GESTURE_FIRST_SINGLE_TOUCH_DOWN);
+  mTouchStartPosition = mLastTouchInput.mTouches[0].mScreenPoint;
+  mTouchStartOffset = mLastTouchInput.mScreenOffset;
+
+  if (sLongTapEnabled) {
+    CreateLongTapTimeoutTask();
+  }
+  CreateMaxTapTimeoutTask();
+}
+
+nsEventStatus GestureEventListener::HandleInputTouchSingleStart() {
+  switch (mState) {
+    case GESTURE_NONE:
+      EnterFirstSingleTouchDown();
+      break;
+    case GESTURE_FIRST_SINGLE_TOUCH_UP:
+      if (SecondTapIsFar()) {
+        // If the second tap goes down far away from the first, then bail out
+        // of any gesture that includes the first tap.
+        CancelLongTapTimeoutTask();
+        CancelMaxTapTimeoutTask();
+        mSingleTapSent = Nothing();
+
+        // But still allow the second tap to participate in a gesture
+        // (e.g. lead to a single tap, or a double tap if an additional
+        // tap occurs near the same location).
+        EnterFirstSingleTouchDown();
+      } else {
+        // Otherwise, reset the touch start position so that, if this turns into
+        // a one-touch-pinch gesture, it uses the second tap's down position as
+        // the focus, rather than the first tap's.
+        mTouchStartPosition = mLastTouchInput.mTouches[0].mScreenPoint;
+        mTouchStartOffset = mLastTouchInput.mScreenOffset;
+        SetState(GESTURE_SECOND_SINGLE_TOUCH_DOWN);
+      }
+      break;
+    default:
+      NS_WARNING("Unhandled state upon single touch start");
+      SetState(GESTURE_NONE);
+      break;
+  }
+
+  return nsEventStatus_eIgnore;
+}
+
+nsEventStatus GestureEventListener::HandleInputTouchMultiStart() {
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (mState) {
+    case GESTURE_NONE:
+      SetState(GESTURE_MULTI_TOUCH_DOWN);
+      break;
+    case GESTURE_FIRST_SINGLE_TOUCH_DOWN:
+      CancelLongTapTimeoutTask();
+      CancelMaxTapTimeoutTask();
+      SetState(GESTURE_MULTI_TOUCH_DOWN);
+      // Prevent APZC::OnTouchStart() from handling MULTITOUCH_START event
+      rv = nsEventStatus_eConsumeNoDefault;
+      break;
+    case GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN:
+      CancelLongTapTimeoutTask();
+      SetState(GESTURE_MULTI_TOUCH_DOWN);
+      // Prevent APZC::OnTouchStart() from handling MULTITOUCH_START event
+      rv = nsEventStatus_eConsumeNoDefault;
+      break;
+    case GESTURE_FIRST_SINGLE_TOUCH_UP:
+    case GESTURE_SECOND_SINGLE_TOUCH_DOWN:
+      // Cancel wait for double tap
+      CancelMaxTapTimeoutTask();
+      MOZ_ASSERT(mSingleTapSent.isSome());
+      if (!mSingleTapSent.value()) {
+        TriggerSingleTapConfirmedEvent();
+      }
+      mSingleTapSent = Nothing();
+      SetState(GESTURE_MULTI_TOUCH_DOWN);
+      // Prevent APZC::OnTouchStart() from handling MULTITOUCH_START event
+      rv = nsEventStatus_eConsumeNoDefault;
+      break;
+    case GESTURE_LONG_TOUCH_DOWN:
+      SetState(GESTURE_MULTI_TOUCH_DOWN);
+      break;
+    case GESTURE_MULTI_TOUCH_DOWN:
+    case GESTURE_PINCH:
+      // Prevent APZC::OnTouchStart() from handling MULTITOUCH_START event
+      rv = nsEventStatus_eConsumeNoDefault;
+      break;
+    default:
+      NS_WARNING("Unhandled state upon multitouch start");
+      SetState(GESTURE_NONE);
+      break;
+  }
+
+  return rv;
+}
+
+bool GestureEventListener::MoveDistanceExceeds(ScreenCoord aThreshold) const {
+  ExternalPoint start = AsyncPanZoomController::ToExternalPoint(
+      mTouchStartOffset, mTouchStartPosition);
+  ExternalPoint end = AsyncPanZoomController::ToExternalPoint(
+      mLastTouchInput.mScreenOffset, mLastTouchInput.mTouches[0].mScreenPoint);
+  return (start - end).Length() > aThreshold;
+}
+
+bool GestureEventListener::MoveDistanceIsLarge() const {
+  return MoveDistanceExceeds(mAsyncPanZoomController->GetTouchStartTolerance());
+}
+
+bool GestureEventListener::SecondTapIsFar() const {
+  // Allow a little more room here, because the is actually lifting their finger
+  // off the screen before replacing it, and that tends to have more error than
+  // wiggling the finger while on the screen.
+  return MoveDistanceExceeds(mAsyncPanZoomController->GetSecondTapTolerance());
+}
+
+nsEventStatus GestureEventListener::HandleInputTouchMove() {
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (mState) {
+    case GESTURE_NONE:
+      // Ignore this input signal as the corresponding events get handled by
+      // APZC
+      break;
+
+    case GESTURE_LONG_TOUCH_DOWN:
+      if (MoveDistanceIsLarge()) {
+        // So that we don't fire a long-tap-up if the user moves around after a
+        // long-tap
+        SetState(GESTURE_NONE);
+      }
+      break;
+
+    case GESTURE_FIRST_SINGLE_TOUCH_DOWN:
+    case GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN: {
+      // If we move too much, bail out of the tap.
+      if (MoveDistanceIsLarge()) {
+        CancelLongTapTimeoutTask();
+        CancelMaxTapTimeoutTask();
+        mSingleTapSent = Nothing();
+        SetState(GESTURE_NONE);
+      }
+      break;
+    }
+
+    // The user has performed a double tap, but not lifted her finger.
+    case GESTURE_SECOND_SINGLE_TOUCH_DOWN: {
+      // If touch has moved noticeably (within StaticPrefs::apz_max_tap_time()),
+      // change state.
+      if (MoveDistanceIsLarge()) {
+        CancelLongTapTimeoutTask();
+        CancelMaxTapTimeoutTask();
+        mSingleTapSent = Nothing();
+        if (!StaticPrefs::apz_one_touch_pinch_enabled()) {
+          // If the one-touch-pinch feature is disabled, bail out of the double-
+          // tap gesture instead.
+          SetState(GESTURE_NONE);
+          break;
+        }
+
+        SetState(GESTURE_ONE_TOUCH_PINCH);
+
+        ScreenCoord currentSpan = 1.0f;
+        ScreenPoint currentFocus = mTouchStartPosition;
+
+        // save the position that the one-touch-pinch gesture actually begins
+        mOneTouchPinchStartPosition = mLastTouchInput.mTouches[0].mScreenPoint;
+
+        PinchGestureInput pinchEvent(
+            PinchGestureInput::PINCHGESTURE_START, PinchGestureInput::ONE_TOUCH,
+            mLastTouchInput.mTimeStamp, mLastTouchInput.mScreenOffset,
+            currentFocus, currentSpan, currentSpan, mLastTouchInput.modifiers);
+
+        rv = mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+
+        mPreviousSpan = currentSpan;
+        mPreviousFocus = currentFocus;
+      }
+      break;
+    }
+
+    case GESTURE_MULTI_TOUCH_DOWN: {
+      if (mLastTouchInput.mTouches.Length() < 2) {
+        NS_WARNING(
+            "Wrong input: less than 2 moving points in "
+            "GESTURE_MULTI_TOUCH_DOWN state");
+        break;
+      }
+
+      ScreenCoord currentSpan = GetCurrentSpan(mLastTouchInput);
+      ScreenPoint currentFocus = GetCurrentFocus(mLastTouchInput);
+
+      mSpanChange += fabsf(currentSpan - mPreviousSpan);
+      mFocusChange += (currentFocus - mPreviousFocus).Length();
+      if (mSpanChange > PINCH_START_THRESHOLD ||
+          mFocusChange > PINCH_START_THRESHOLD) {
+        SetState(GESTURE_PINCH);
+        PinchGestureInput pinchEvent(
+            PinchGestureInput::PINCHGESTURE_START, PinchGestureInput::TOUCH,
+            mLastTouchInput.mTimeStamp, mLastTouchInput.mScreenOffset,
+            currentFocus, currentSpan, currentSpan, mLastTouchInput.modifiers);
+
+        rv = mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+      } else {
+        // Prevent APZC::OnTouchMove from processing a move event when two
+        // touches are active
+        rv = nsEventStatus_eConsumeNoDefault;
+      }
+
+      mPreviousSpan = currentSpan;
+      mPreviousFocus = currentFocus;
+      break;
+    }
+
+    case GESTURE_PINCH: {
+      if (mLastTouchInput.mTouches.Length() < 2) {
+        NS_WARNING(
+            "Wrong input: less than 2 moving points in GESTURE_PINCH state");
+        // Prevent APZC::OnTouchMove() from handling this wrong input
+        rv = nsEventStatus_eConsumeNoDefault;
+        break;
+      }
+
+      ScreenCoord currentSpan = GetCurrentSpan(mLastTouchInput);
+
+      PinchGestureInput pinchEvent(
+          PinchGestureInput::PINCHGESTURE_SCALE, PinchGestureInput::TOUCH,
+          mLastTouchInput.mTimeStamp, mLastTouchInput.mScreenOffset,
+          GetCurrentFocus(mLastTouchInput), currentSpan, mPreviousSpan,
+          mLastTouchInput.modifiers);
+
+      rv = mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+      mPreviousSpan = currentSpan;
+
+      break;
+    }
+
+    case GESTURE_ONE_TOUCH_PINCH: {
+      ScreenCoord currentSpan = GetYSpanFromGestureStartPoint();
+      float effectiveSpan =
+          1.0f + (fabsf(currentSpan.value) * ONE_TOUCH_PINCH_SPEED);
+      ScreenPoint currentFocus = mTouchStartPosition;
+
+      // Invert zoom.
+      if (currentSpan.value < 0) {
+        effectiveSpan = 1.0f / effectiveSpan;
+      }
+
+      PinchGestureInput pinchEvent(
+          PinchGestureInput::PINCHGESTURE_SCALE, PinchGestureInput::ONE_TOUCH,
+          mLastTouchInput.mTimeStamp, mLastTouchInput.mScreenOffset,
+          currentFocus, effectiveSpan, mPreviousSpan,
+          mLastTouchInput.modifiers);
+
+      rv = mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+      mPreviousSpan = effectiveSpan;
+
+      break;
+    }
+
+    default:
+      NS_WARNING("Unhandled state upon touch move");
+      SetState(GESTURE_NONE);
+      break;
+  }
+
+  return rv;
+}
+
+nsEventStatus GestureEventListener::HandleInputTouchEnd() {
+  // We intentionally do not pass apzc return statuses up since
+  // it may cause apzc stay in the touching state even after
+  // gestures are completed (please see Bug 1013378 for reference).
+
+  nsEventStatus rv = nsEventStatus_eIgnore;
+
+  switch (mState) {
+    case GESTURE_NONE:
+      // GEL doesn't have a dedicated state for PANNING handled in APZC thus
+      // ignore.
+      break;
+
+    case GESTURE_FIRST_SINGLE_TOUCH_DOWN: {
+      CancelLongTapTimeoutTask();
+      CancelMaxTapTimeoutTask();
+      nsEventStatus tapupStatus = mAsyncPanZoomController->HandleGestureEvent(
+          CreateTapEvent(mLastTouchInput, TapGestureInput::TAPGESTURE_UP));
+      mSingleTapSent = Some(tapupStatus != nsEventStatus_eIgnore);
+      SetState(GESTURE_FIRST_SINGLE_TOUCH_UP);
+      CreateMaxTapTimeoutTask();
+      break;
+    }
+
+    case GESTURE_SECOND_SINGLE_TOUCH_DOWN: {
+      CancelMaxTapTimeoutTask();
+      MOZ_ASSERT(mSingleTapSent.isSome());
+      mAsyncPanZoomController->HandleGestureEvent(CreateTapEvent(
+          mLastTouchInput, mSingleTapSent.value()
+                               ? TapGestureInput::TAPGESTURE_SECOND
+                               : TapGestureInput::TAPGESTURE_DOUBLE));
+      mSingleTapSent = Nothing();
+      SetState(GESTURE_NONE);
+      break;
+    }
+
+    case GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN:
+      CancelLongTapTimeoutTask();
+      SetState(GESTURE_NONE);
+      TriggerSingleTapConfirmedEvent();
+      break;
+
+    case GESTURE_LONG_TOUCH_DOWN: {
+      SetState(GESTURE_NONE);
+      mAsyncPanZoomController->HandleGestureEvent(
+          CreateTapEvent(mLastTouchInput, TapGestureInput::TAPGESTURE_LONG_UP));
+      break;
+    }
+
+    case GESTURE_MULTI_TOUCH_DOWN:
+      if (mTouches.Length() < 2) {
+        SetState(GESTURE_NONE);
+      }
+      break;
+
+    case GESTURE_PINCH:
+      if (mTouches.Length() < 2) {
+        SetState(GESTURE_NONE);
+        PinchGestureInput::PinchGestureType type =
+            PinchGestureInput::PINCHGESTURE_END;
+        ScreenPoint point;
+        if (mTouches.Length() == 1) {
+          // As user still keeps one finger down the event's focus point should
+          // contain meaningful data.
+          type = PinchGestureInput::PINCHGESTURE_FINGERLIFTED;
+          point = mTouches[0].mScreenPoint;
+        }
+        PinchGestureInput pinchEvent(type, PinchGestureInput::TOUCH,
+                                     mLastTouchInput.mTimeStamp,
+                                     mLastTouchInput.mScreenOffset, point, 1.0f,
+                                     1.0f, mLastTouchInput.modifiers);
+        mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+      }
+
+      rv = nsEventStatus_eConsumeNoDefault;
+
+      break;
+
+    case GESTURE_ONE_TOUCH_PINCH: {
+      SetState(GESTURE_NONE);
+      PinchGestureInput pinchEvent(
+          PinchGestureInput::PINCHGESTURE_END, PinchGestureInput::ONE_TOUCH,
+          mLastTouchInput.mTimeStamp, mLastTouchInput.mScreenOffset,
+          ScreenPoint(), 1.0f, 1.0f, mLastTouchInput.modifiers);
+      mAsyncPanZoomController->HandleGestureEvent(pinchEvent);
+
+      rv = nsEventStatus_eConsumeNoDefault;
+
+      break;
+    }
+
+    default:
+      NS_WARNING("Unhandled state upon touch end");
+      SetState(GESTURE_NONE);
+      break;
+  }
+
+  return rv;
+}
+
+nsEventStatus GestureEventListener::HandleInputTouchCancel() {
+  mSingleTapSent = Nothing();
+  SetState(GESTURE_NONE);
+  CancelMaxTapTimeoutTask();
+  CancelLongTapTimeoutTask();
+  return nsEventStatus_eIgnore;
+}
+
+void GestureEventListener::HandleInputTimeoutLongTap() {
+  GEL_LOG("Running long-tap timeout task in state %d\n", mState);
+
+  mLongTapTimeoutTask = nullptr;
+
+  switch (mState) {
+    case GESTURE_FIRST_SINGLE_TOUCH_DOWN:
+      // just in case MaxTapTime > ContextMenuDelay cancel MaxTap timer
+      // and fall through
+      CancelMaxTapTimeoutTask();
+      [[fallthrough]];
+    case GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN: {
+      SetState(GESTURE_LONG_TOUCH_DOWN);
+      mAsyncPanZoomController->HandleGestureEvent(
+          CreateTapEvent(mLastTouchInput, TapGestureInput::TAPGESTURE_LONG));
+      break;
+    }
+    default:
+      NS_WARNING("Unhandled state upon long tap timeout");
+      SetState(GESTURE_NONE);
+      break;
+  }
+}
+
+void GestureEventListener::HandleInputTimeoutMaxTap(bool aDuringFastFling) {
+  GEL_LOG("Running max-tap timeout task in state %d\n", mState);
+
+  mMaxTapTimeoutTask = nullptr;
+
+  if (mState == GESTURE_FIRST_SINGLE_TOUCH_DOWN) {
+    SetState(GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN);
+  } else if (mState == GESTURE_FIRST_SINGLE_TOUCH_UP ||
+             mState == GESTURE_SECOND_SINGLE_TOUCH_DOWN) {
+    MOZ_ASSERT(mSingleTapSent.isSome());
+    if (!aDuringFastFling && !mSingleTapSent.value()) {
+      TriggerSingleTapConfirmedEvent();
+    }
+    mSingleTapSent = Nothing();
+    SetState(GESTURE_NONE);
+  } else {
+    NS_WARNING("Unhandled state upon MAX_TAP timeout");
+    SetState(GESTURE_NONE);
+  }
+}
+
+void GestureEventListener::TriggerSingleTapConfirmedEvent() {
+  mAsyncPanZoomController->HandleGestureEvent(
+      CreateTapEvent(mLastTapInput, TapGestureInput::TAPGESTURE_CONFIRMED));
+}
+
+void GestureEventListener::SetState(GestureState aState) {
+  mState = aState;
+
+  if (mState == GESTURE_NONE) {
+    mSpanChange = 0.0f;
+    mPreviousSpan = 0.0f;
+    mFocusChange = 0.0f;
+  } else if (mState == GESTURE_MULTI_TOUCH_DOWN) {
+    mPreviousSpan = GetCurrentSpan(mLastTouchInput);
+    mPreviousFocus = GetCurrentFocus(mLastTouchInput);
+  }
+}
+
+void GestureEventListener::CancelLongTapTimeoutTask() {
+  if (mState == GESTURE_SECOND_SINGLE_TOUCH_DOWN) {
+    // being in this state means the task has been canceled already
+    return;
+  }
+
+  if (mLongTapTimeoutTask) {
+    mLongTapTimeoutTask->Cancel();
+    mLongTapTimeoutTask = nullptr;
+  }
+}
+
+void GestureEventListener::CreateLongTapTimeoutTask() {
+  RefPtr<CancelableRunnable> task = NewCancelableRunnableMethod(
+      "layers::GestureEventListener::HandleInputTimeoutLongTap", this,
+      &GestureEventListener::HandleInputTimeoutLongTap);
+
+  mLongTapTimeoutTask = task;
+
+  TouchBlockState* block =
+      mAsyncPanZoomController->GetInputQueue()->GetCurrentTouchBlock();
+  MOZ_ASSERT(block);
+  long alreadyElapsed =
+      static_cast<long>(block->GetTimeSinceBlockStart().ToMilliseconds());
+  long remainingDelay =
+      StaticPrefs::ui_click_hold_context_menus_delay() - alreadyElapsed;
+  mAsyncPanZoomController->PostDelayedTask(task.forget(),
+                                           std::max(0L, remainingDelay));
+}
+
+void GestureEventListener::CancelMaxTapTimeoutTask() {
+  if (mState == GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN) {
+    // being in this state means the timer has just been triggered
+    return;
+  }
+
+  if (mMaxTapTimeoutTask) {
+    mMaxTapTimeoutTask->Cancel();
+    mMaxTapTimeoutTask = nullptr;
+  }
+}
+
+void GestureEventListener::CreateMaxTapTimeoutTask() {
+  mLastTapInput = mLastTouchInput;
+
+  TouchBlockState* block =
+      mAsyncPanZoomController->GetInputQueue()->GetCurrentTouchBlock();
+  MOZ_ASSERT(block);
+  RefPtr<CancelableRunnable> task = NewCancelableRunnableMethod<bool>(
+      "layers::GestureEventListener::HandleInputTimeoutMaxTap", this,
+      &GestureEventListener::HandleInputTimeoutMaxTap,
+      block->IsDuringFastFling());
+
+  mMaxTapTimeoutTask = task;
+
+  long alreadyElapsed =
+      static_cast<long>(block->GetTimeSinceBlockStart().ToMilliseconds());
+  long remainingDelay = StaticPrefs::apz_max_tap_time() - alreadyElapsed;
+  mAsyncPanZoomController->PostDelayedTask(task.forget(),
+                                           std::max(0L, remainingDelay));
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/GestureEventListener.h b/gfx/layers/apz/src/GestureEventListener.h
new file mode 100644
index 0000000000..aa51889fdd
--- /dev/null
+++ b/gfx/layers/apz/src/GestureEventListener.h
@@ -0,0 +1,285 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_GestureEventListener_h
+#define mozilla_layers_GestureEventListener_h
+
+#include "InputData.h"  // for MultiTouchInput, etc
+#include "Units.h"
+#include "mozilla/EventForwards.h"  // for nsEventStatus
+#include "mozilla/RefPtr.h"         // for RefPtr
+#include "nsISupportsImpl.h"
+#include "nsTArray.h"  // for nsTArray
+
+namespace mozilla {
+
+class CancelableRunnable;
+
+namespace layers {
+
+class AsyncPanZoomController;
+
+/**
+ * Platform-non-specific, generalized gesture event listener. This class
+ * intercepts all touches events on their way to AsyncPanZoomController and
+ * determines whether or not they are part of a gesture.
+ *
+ * For example, seeing that two fingers are on the screen means that the user
+ * wants to do a pinch gesture, so we don't forward the touches along to
+ * AsyncPanZoomController since it will think that they are just trying to pan
+ * the screen. Instead, we generate a PinchGestureInput and send that. If the
+ * touch event is not part of a gesture, we just return nsEventStatus_eIgnore
+ * and AsyncPanZoomController is expected to handle it.
+ */
+class GestureEventListener final {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(GestureEventListener)
+
+  explicit GestureEventListener(
+      AsyncPanZoomController* aAsyncPanZoomController);
+
+  // --------------------------------------------------------------------------
+  // These methods must only be called on the controller/UI thread.
+  //
+
+  /**
+   * General input handler for a touch event. If the touch event is not a part
+   * of a gesture, then we pass it along to AsyncPanZoomController. Otherwise,
+   * it gets consumed here and never forwarded along.
+   */
+  nsEventStatus HandleInputEvent(const MultiTouchInput& aEvent);
+
+  /**
+   * Returns the identifier of the touch in the last touch event processed by
+   * this GestureEventListener. This should only be called when the last touch
+   * event contained only one touch.
+   */
+  int32_t GetLastTouchIdentifier() const;
+
+  /**
+   * Function used to disable long tap gestures.
+   *
+   * On slow running tests, drags and touch events can be misinterpreted
+   * as a long tap. This allows tests to disable long tap gesture detection.
+   */
+  static void SetLongTapEnabled(bool aLongTapEnabled);
+  static bool IsLongTapEnabled();
+
+ private:
+  // Private destructor, to discourage deletion outside of Release():
+  ~GestureEventListener();
+
+  /**
+   * States of GEL finite-state machine.
+   */
+  enum GestureState {
+    // This is the initial and final state of any gesture.
+    // In this state there's no gesture going on, and we don't think we're
+    // about to enter one.
+    // Allowed next states: GESTURE_FIRST_SINGLE_TOUCH_DOWN,
+    // GESTURE_MULTI_TOUCH_DOWN.
+    GESTURE_NONE,
+
+    // A touch start with a single touch point has just happened.
+    // After having gotten into this state we start timers for MAX_TAP_TIME and
+    // StaticPrefs::ui_click_hold_context_menus_delay().
+    // Allowed next states: GESTURE_MULTI_TOUCH_DOWN, GESTURE_NONE,
+    //                      GESTURE_FIRST_SINGLE_TOUCH_UP,
+    //                      GESTURE_LONG_TOUCH_DOWN,
+    //                      GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN.
+    GESTURE_FIRST_SINGLE_TOUCH_DOWN,
+
+    // While in GESTURE_FIRST_SINGLE_TOUCH_DOWN state a MAX_TAP_TIME timer got
+    // triggered. Now we'll trigger either a single tap if a user lifts her
+    // finger or a long tap if StaticPrefs::ui_click_hold_context_menus_delay()
+    // happens first.
+    // Allowed next states: GESTURE_MULTI_TOUCH_DOWN, GESTURE_NONE,
+    //                      GESTURE_LONG_TOUCH_DOWN.
+    GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN,
+
+    // A user put her finger down and lifted it up quickly enough.
+    // After having gotten into this state we clear the timer for MAX_TAP_TIME.
+    // Allowed next states: GESTURE_SECOND_SINGLE_TOUCH_DOWN, GESTURE_NONE,
+    //                      GESTURE_MULTI_TOUCH_DOWN.
+    GESTURE_FIRST_SINGLE_TOUCH_UP,
+
+    // A user put down her finger again right after a single tap thus the
+    // gesture can't be a single tap, but rather a double tap. But we're
+    // still not sure about that until the user lifts her finger again.
+    // Allowed next states: GESTURE_MULTI_TOUCH_DOWN, GESTURE_ONE_TOUCH_PINCH,
+    //                      GESTURE_NONE.
+    GESTURE_SECOND_SINGLE_TOUCH_DOWN,
+
+    // A long touch has happened, but the user still keeps her finger down.
+    // We'll trigger a "long tap up" event when the finger is up.
+    // Allowed next states: GESTURE_NONE, GESTURE_MULTI_TOUCH_DOWN.
+    GESTURE_LONG_TOUCH_DOWN,
+
+    // We have detected that two or more fingers are on the screen, but there
+    // hasn't been enough movement yet to make us start actually zooming the
+    // screen.
+    // Allowed next states: GESTURE_PINCH, GESTURE_NONE
+    GESTURE_MULTI_TOUCH_DOWN,
+
+    // There are two or more fingers on the screen, and the user has already
+    // pinched enough for us to start zooming the screen.
+    // Allowed next states: GESTURE_NONE
+    GESTURE_PINCH,
+
+    // The user has double tapped, but not lifted her finger, and moved her
+    // finger more than PINCH_START_THRESHOLD.
+    // Allowed next states: GESTURE_NONE.
+    GESTURE_ONE_TOUCH_PINCH
+  };
+
+  /**
+   * These HandleInput* functions comprise input alphabet of the GEL
+   * finite-state machine triggering state transitions.
+   */
+  nsEventStatus HandleInputTouchSingleStart();
+  nsEventStatus HandleInputTouchMultiStart();
+  nsEventStatus HandleInputTouchEnd();
+  nsEventStatus HandleInputTouchMove();
+  nsEventStatus HandleInputTouchCancel();
+  void HandleInputTimeoutLongTap();
+  void HandleInputTimeoutMaxTap(bool aDuringFastFling);
+
+  void EnterFirstSingleTouchDown();
+
+  void TriggerSingleTapConfirmedEvent();
+
+  bool MoveDistanceExceeds(ScreenCoord aThreshold) const;
+  bool MoveDistanceIsLarge() const;
+  bool SecondTapIsFar() const;
+
+  /**
+   * Returns current vertical span, counting from the where the gesture first
+   * began (after a brief delay detecting the gesture from first touch).
+   */
+  ScreenCoord GetYSpanFromGestureStartPoint();
+
+  /**
+   * Do actual state transition and reset substates.
+   */
+  void SetState(GestureState aState);
+
+  RefPtr<AsyncPanZoomController> mAsyncPanZoomController;
+
+  /**
+   * Array containing all active touches. When a touch happens it, gets added to
+   * this array, even if we choose not to handle it. When it ends, we remove it.
+   * We need to maintain this array in order to detect the end of the
+   * "multitouch" states because touch start events contain all current touches,
+   * but touch end events contain only those touches that have gone.
+   */
+  nsTArray<SingleTouchData> mTouches;
+
+  /**
+   * Current state we're dealing with.
+   */
+  GestureState mState;
+
+  /**
+   * Total change in span since we detected a pinch gesture. Only used when we
+   * are in the |GESTURE_WAITING_PINCH| state and need to know how far zoomed
+   * out we are compared to our original pinch span. Note that this does _not_
+   * continue to be updated once we jump into the |GESTURE_PINCH| state.
+   */
+  ScreenCoord mSpanChange;
+
+  /**
+   * Previous span calculated for the purposes of setting inside a
+   * PinchGestureInput.
+   */
+  ScreenCoord mPreviousSpan;
+
+  /* Properties similar to mSpanChange and mPreviousSpan, but for the focus */
+  ScreenCoord mFocusChange;
+  ScreenPoint mPreviousFocus;
+
+  /**
+   * Cached copy of the last touch input.
+   */
+  MultiTouchInput mLastTouchInput;
+
+  /**
+   * Cached copy of the last tap gesture input.
+   * In the situation when we have a tap followed by a pinch we lose info
+   * about tap since we keep only last input and to dispatch it correctly
+   * we save last tap copy into this variable.
+   * For more info see bug 947892.
+   */
+  MultiTouchInput mLastTapInput;
+
+  /**
+   * Position of the last touch that exceeds the GetTouchStartTolerance when
+   * performing a one-touch-pinch gesture; using the mTouchStartPosition is
+   * slightly inaccurate because by the time the touch position has passed
+   * the threshold for the gesture, there is already a span that the zoom
+   * is calculated from, instead of starting at 1.0 when the threshold gets
+   * passed.
+   */
+  ScreenPoint mOneTouchPinchStartPosition;
+
+  /**
+   * Position of the last touch starting. This is only valid during an attempt
+   * to determine if a touch is a tap. If a touch point moves away from
+   * mTouchStartPosition to the distance greater than
+   * AsyncPanZoomController::GetTouchStartTolerance() while in
+   * GESTURE_FIRST_SINGLE_TOUCH_DOWN, GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN
+   * or GESTURE_SECOND_SINGLE_TOUCH_DOWN then we're certain the gesture is
+   * not tap.
+   */
+  ScreenPoint mTouchStartPosition;
+
+  /**
+   * We store the window/GeckoView's display offset as well, so we can
+   * track the user's physical touch movements in absolute display coordinates.
+   */
+  ExternalPoint mTouchStartOffset;
+
+  /**
+   * Task used to timeout a long tap. This gets posted to the UI thread such
+   * that it runs a time when a single tap happens. We cache it so that
+   * we can cancel it if any other touch event happens.
+   *
+   * The task is supposed to be non-null if in GESTURE_FIRST_SINGLE_TOUCH_DOWN
+   * and GESTURE_FIRST_SINGLE_TOUCH_MAX_TAP_DOWN states.
+   *
+   * CancelLongTapTimeoutTask: Cancel the mLongTapTimeoutTask and also set
+   * it to null.
+   */
+  RefPtr<CancelableRunnable> mLongTapTimeoutTask;
+  void CancelLongTapTimeoutTask();
+  void CreateLongTapTimeoutTask();
+
+  /**
+   * Task used to timeout a single tap or a double tap.
+   *
+   * The task is supposed to be non-null if in GESTURE_FIRST_SINGLE_TOUCH_DOWN,
+   * GESTURE_FIRST_SINGLE_TOUCH_UP and GESTURE_SECOND_SINGLE_TOUCH_DOWN states.
+   *
+   * CancelMaxTapTimeoutTask: Cancel the mMaxTapTimeoutTask and also set
+   * it to null.
+   */
+  RefPtr<CancelableRunnable> mMaxTapTimeoutTask;
+  void CancelMaxTapTimeoutTask();
+  void CreateMaxTapTimeoutTask();
+
+  /**
+   * Tracks whether the single-tap event was already sent to content. This is
+   * needed because it affects how the double-tap gesture, if detected, is
+   * handled. The value is only valid in states GESTURE_FIRST_SINGLE_TOUCH_UP
+   * and GESTURE_SECOND_SINGLE_TOUCH_DOWN; to more easily catch violations it is
+   * stored in a Maybe which is set to Nothing() at all other times.
+   */
+  Maybe<bool> mSingleTapSent;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/HitTestingTreeNode.cpp b/gfx/layers/apz/src/HitTestingTreeNode.cpp
new file mode 100644
index 0000000000..3396a71194
--- /dev/null
+++ b/gfx/layers/apz/src/HitTestingTreeNode.cpp
@@ -0,0 +1,422 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "HitTestingTreeNode.h"
+#include <stack>
+
+#include "AsyncPanZoomController.h"  // for AsyncPanZoomController
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/gfx/Point.h"        // for Point4D
+#include "mozilla/layers/APZUtils.h"  // for AsyncTransform, CompleteAsyncTransform
+#include "mozilla/layers/AsyncDragMetrics.h"  // for AsyncDragMetrics
+#include "mozilla/ToString.h"                 // for ToString
+#include "nsPrintfCString.h"                  // for nsPrintfCString
+#include "UnitTransforms.h"                   // for ViewAs
+
+static mozilla::LazyLogModule sApzMgrLog("apz.manager");
+
+namespace mozilla {
+namespace layers {
+
+using gfx::CompositorHitTestInfo;
+
+HitTestingTreeNode::HitTestingTreeNode(AsyncPanZoomController* aApzc,
+                                       bool aIsPrimaryHolder,
+                                       LayersId aLayersId)
+    : mApzc(aApzc),
+      mIsPrimaryApzcHolder(aIsPrimaryHolder),
+      mLockCount(0),
+      mLayersId(aLayersId),
+      mFixedPosTarget(ScrollableLayerGuid::NULL_SCROLL_ID),
+      mStickyPosTarget(ScrollableLayerGuid::NULL_SCROLL_ID),
+      mOverride(EventRegionsOverride::NoOverride) {
+  if (mIsPrimaryApzcHolder) {
+    MOZ_ASSERT(mApzc);
+  }
+  MOZ_ASSERT(!mApzc || mApzc->GetLayersId() == mLayersId);
+}
+
+void HitTestingTreeNode::RecycleWith(
+    const RecursiveMutexAutoLock& aProofOfTreeLock,
+    AsyncPanZoomController* aApzc, LayersId aLayersId) {
+  MOZ_ASSERT(IsRecyclable(aProofOfTreeLock));
+  Destroy();  // clear out tree pointers
+  mApzc = aApzc;
+  mLayersId = aLayersId;
+  MOZ_ASSERT(!mApzc || mApzc->GetLayersId() == mLayersId);
+  // The caller is expected to call appropriate setters (SetHitTestData,
+  // SetScrollbarData, SetFixedPosData, SetStickyPosData, etc.) to repopulate
+  // all the data fields before using this node for "real work". Otherwise
+  // those data fields may contain stale information from the previous use
+  // of this node object.
+}
+
+HitTestingTreeNode::~HitTestingTreeNode() = default;
+
+void HitTestingTreeNode::Destroy() {
+  // This runs on the updater thread, it's not worth passing around extra raw
+  // pointers just to assert it.
+
+  mPrevSibling = nullptr;
+  mLastChild = nullptr;
+  mParent = nullptr;
+
+  if (mApzc) {
+    if (mIsPrimaryApzcHolder) {
+      mApzc->Destroy();
+    }
+    mApzc = nullptr;
+  }
+}
+
+bool HitTestingTreeNode::IsRecyclable(
+    const RecursiveMutexAutoLock& aProofOfTreeLock) {
+  return !(IsPrimaryHolder() || (mLockCount > 0));
+}
+
+void HitTestingTreeNode::SetLastChild(HitTestingTreeNode* aChild) {
+  mLastChild = aChild;
+  if (aChild) {
+    aChild->mParent = this;
+
+    if (aChild->GetApzc()) {
+      AsyncPanZoomController* parent = GetNearestContainingApzc();
+      // We assume that HitTestingTreeNodes with an ancestor/descendant
+      // relationship cannot both point to the same APZC instance. This
+      // assertion only covers a subset of cases in which that might occur,
+      // but it's better than nothing.
+      MOZ_ASSERT(aChild->GetApzc() != parent);
+      aChild->SetApzcParent(parent);
+    }
+  }
+}
+
+void HitTestingTreeNode::SetScrollbarData(
+    const Maybe<uint64_t>& aScrollbarAnimationId,
+    const ScrollbarData& aScrollbarData) {
+  mScrollbarAnimationId = aScrollbarAnimationId;
+  mScrollbarData = aScrollbarData;
+}
+
+bool HitTestingTreeNode::MatchesScrollDragMetrics(
+    const AsyncDragMetrics& aDragMetrics, LayersId aLayersId) const {
+  return IsScrollThumbNode() && mLayersId == aLayersId &&
+         mScrollbarData.mDirection == aDragMetrics.mDirection &&
+         mScrollbarData.mTargetViewId == aDragMetrics.mViewId;
+}
+
+bool HitTestingTreeNode::IsScrollThumbNode() const {
+  return mScrollbarData.mScrollbarLayerType ==
+         layers::ScrollbarLayerType::Thumb;
+}
+
+bool HitTestingTreeNode::IsScrollbarNode() const {
+  return mScrollbarData.mScrollbarLayerType != layers::ScrollbarLayerType::None;
+}
+
+bool HitTestingTreeNode::IsScrollbarContainerNode() const {
+  return mScrollbarData.mScrollbarLayerType ==
+         layers::ScrollbarLayerType::Container;
+}
+
+ScrollDirection HitTestingTreeNode::GetScrollbarDirection() const {
+  MOZ_ASSERT(IsScrollbarNode());
+  MOZ_ASSERT(mScrollbarData.mDirection.isSome());
+  return *mScrollbarData.mDirection;
+}
+
+ScrollableLayerGuid::ViewID HitTestingTreeNode::GetScrollTargetId() const {
+  return mScrollbarData.mTargetViewId;
+}
+
+Maybe<uint64_t> HitTestingTreeNode::GetScrollbarAnimationId() const {
+  return mScrollbarAnimationId;
+}
+
+const ScrollbarData& HitTestingTreeNode::GetScrollbarData() const {
+  return mScrollbarData;
+}
+
+void HitTestingTreeNode::SetFixedPosData(
+    ScrollableLayerGuid::ViewID aFixedPosTarget, SideBits aFixedPosSides,
+    const Maybe<uint64_t>& aFixedPositionAnimationId) {
+  mFixedPosTarget = aFixedPosTarget;
+  mFixedPosSides = aFixedPosSides;
+  mFixedPositionAnimationId = aFixedPositionAnimationId;
+}
+
+ScrollableLayerGuid::ViewID HitTestingTreeNode::GetFixedPosTarget() const {
+  return mFixedPosTarget;
+}
+
+SideBits HitTestingTreeNode::GetFixedPosSides() const { return mFixedPosSides; }
+
+Maybe<uint64_t> HitTestingTreeNode::GetFixedPositionAnimationId() const {
+  return mFixedPositionAnimationId;
+}
+
+void HitTestingTreeNode::SetPrevSibling(HitTestingTreeNode* aSibling) {
+  mPrevSibling = aSibling;
+  if (aSibling) {
+    aSibling->mParent = mParent;
+
+    if (aSibling->GetApzc()) {
+      AsyncPanZoomController* parent =
+          mParent ? mParent->GetNearestContainingApzc() : nullptr;
+      aSibling->SetApzcParent(parent);
+    }
+  }
+}
+
+void HitTestingTreeNode::SetStickyPosData(
+    ScrollableLayerGuid::ViewID aStickyPosTarget,
+    const LayerRectAbsolute& aScrollRangeOuter,
+    const LayerRectAbsolute& aScrollRangeInner,
+    const Maybe<uint64_t>& aStickyPositionAnimationId) {
+  mStickyPosTarget = aStickyPosTarget;
+  mStickyScrollRangeOuter = aScrollRangeOuter;
+  mStickyScrollRangeInner = aScrollRangeInner;
+  mStickyPositionAnimationId = aStickyPositionAnimationId;
+}
+
+ScrollableLayerGuid::ViewID HitTestingTreeNode::GetStickyPosTarget() const {
+  return mStickyPosTarget;
+}
+
+const LayerRectAbsolute& HitTestingTreeNode::GetStickyScrollRangeOuter() const {
+  return mStickyScrollRangeOuter;
+}
+
+const LayerRectAbsolute& HitTestingTreeNode::GetStickyScrollRangeInner() const {
+  return mStickyScrollRangeInner;
+}
+
+Maybe<uint64_t> HitTestingTreeNode::GetStickyPositionAnimationId() const {
+  return mStickyPositionAnimationId;
+}
+
+void HitTestingTreeNode::MakeRoot() {
+  mParent = nullptr;
+
+  if (GetApzc()) {
+    SetApzcParent(nullptr);
+  }
+}
+
+HitTestingTreeNode* HitTestingTreeNode::GetFirstChild() const {
+  HitTestingTreeNode* child = GetLastChild();
+  while (child && child->GetPrevSibling()) {
+    child = child->GetPrevSibling();
+  }
+  return child;
+}
+
+HitTestingTreeNode* HitTestingTreeNode::GetLastChild() const {
+  return mLastChild;
+}
+
+HitTestingTreeNode* HitTestingTreeNode::GetPrevSibling() const {
+  return mPrevSibling;
+}
+
+HitTestingTreeNode* HitTestingTreeNode::GetParent() const { return mParent; }
+
+bool HitTestingTreeNode::IsAncestorOf(const HitTestingTreeNode* aOther) const {
+  for (const HitTestingTreeNode* cur = aOther; cur; cur = cur->GetParent()) {
+    if (cur == this) {
+      return true;
+    }
+  }
+  return false;
+}
+
+AsyncPanZoomController* HitTestingTreeNode::GetApzc() const { return mApzc; }
+
+AsyncPanZoomController* HitTestingTreeNode::GetNearestContainingApzc() const {
+  for (const HitTestingTreeNode* n = this; n; n = n->GetParent()) {
+    if (n->GetApzc()) {
+      return n->GetApzc();
+    }
+  }
+  return nullptr;
+}
+
+bool HitTestingTreeNode::IsPrimaryHolder() const {
+  return mIsPrimaryApzcHolder;
+}
+
+LayersId HitTestingTreeNode::GetLayersId() const { return mLayersId; }
+
+void HitTestingTreeNode::SetHitTestData(
+    const LayerIntRect& aVisibleRect, const LayerIntSize& aRemoteDocumentSize,
+    const CSSTransformMatrix& aTransform, const EventRegionsOverride& aOverride,
+    const Maybe<ScrollableLayerGuid::ViewID>& aAsyncZoomContainerId) {
+  mVisibleRect = aVisibleRect;
+  mRemoteDocumentSize = aRemoteDocumentSize;
+  mTransform = aTransform;
+  mOverride = aOverride;
+  mAsyncZoomContainerId = aAsyncZoomContainerId;
+}
+
+EventRegionsOverride HitTestingTreeNode::GetEventRegionsOverride() const {
+  return mOverride;
+}
+
+const CSSTransformMatrix& HitTestingTreeNode::GetTransform() const {
+  return mTransform;
+}
+
+LayerToScreenMatrix4x4 HitTestingTreeNode::GetTransformToGecko(
+    LayersId aRemoteLayersId) const {
+  if (mParent) {
+    LayerToParentLayerMatrix4x4 thisToParent =
+        mTransform * AsyncTransformMatrix();
+    if (mApzc) {
+      thisToParent =
+          thisToParent * ViewAs<AsyncTransformComponentMatrix>(
+                             mApzc->GetTransformToLastDispatchedPaint(
+                                 LayoutAndVisual, aRemoteLayersId));
+    }
+    ParentLayerToScreenMatrix4x4 parentToRoot =
+        ViewAs<ParentLayerToScreenMatrix4x4>(
+            mParent->GetTransformToGecko(aRemoteLayersId),
+            PixelCastJustification::MovingDownToChildren);
+    return thisToParent * parentToRoot;
+  }
+
+  return ViewAs<LayerToScreenMatrix4x4>(
+      mTransform * AsyncTransformMatrix(),
+      PixelCastJustification::ScreenIsParentLayerForRoot);
+}
+
+const LayerIntRect& HitTestingTreeNode::GetVisibleRect() const {
+  return mVisibleRect;
+}
+
+ScreenRect HitTestingTreeNode::GetRemoteDocumentScreenRect(
+    LayersId aRemoteDocumentLayersId) const {
+  ScreenRect result = TransformBy(
+      GetTransformToGecko(aRemoteDocumentLayersId),
+      IntRectToRect(LayerIntRect(LayerIntPoint(), mRemoteDocumentSize)));
+
+  for (const HitTestingTreeNode* node = this; node; node = node->GetParent()) {
+    if (!node->GetApzc()) {
+      continue;
+    }
+
+    ParentLayerRect compositionBounds = node->GetApzc()->GetCompositionBounds();
+    if (compositionBounds.IsEmpty()) {
+      return ScreenRect();
+    }
+
+    ScreenRect scrollPortOnScreenCoordinate = TransformBy(
+        node->GetParent()
+            ? node->GetParent()->GetTransformToGecko(node->GetLayersId())
+            : LayerToScreenMatrix4x4(),
+        ViewAs<LayerPixel>(compositionBounds,
+                           PixelCastJustification::MovingDownToChildren));
+    if (scrollPortOnScreenCoordinate.IsEmpty()) {
+      return ScreenRect();
+    }
+
+    result = result.Intersect(scrollPortOnScreenCoordinate);
+    if (result.IsEmpty()) {
+      return ScreenRect();
+    }
+  }
+  return result;
+}
+
+Maybe<ScrollableLayerGuid::ViewID> HitTestingTreeNode::GetAsyncZoomContainerId()
+    const {
+  return mAsyncZoomContainerId;
+}
+
+void HitTestingTreeNode::Dump(const char* aPrefix) const {
+  MOZ_LOG(
+      sApzMgrLog, LogLevel::Debug,
+      ("%sHitTestingTreeNode (%p) APZC (%p) g=(%s) %s%s%s t=(%s) "
+       "%s%s\n",
+       aPrefix, this, mApzc.get(),
+       mApzc ? ToString(mApzc->GetGuid()).c_str()
+             : nsPrintfCString("l=0x%" PRIx64, uint64_t(mLayersId)).get(),
+       (mOverride & EventRegionsOverride::ForceDispatchToContent) ? "fdtc "
+                                                                  : "",
+       (mOverride & EventRegionsOverride::ForceEmptyHitRegion) ? "fehr " : "",
+       (mFixedPosTarget != ScrollableLayerGuid::NULL_SCROLL_ID)
+           ? nsPrintfCString("fixed=%" PRIu64 " ", mFixedPosTarget).get()
+           : "",
+       ToString(mTransform).c_str(),
+       mScrollbarData.mDirection.isSome() ? " scrollbar" : "",
+       IsScrollThumbNode() ? " scrollthumb" : ""));
+
+  if (!mLastChild) {
+    return;
+  }
+
+  // Dump the children in order from first child to last child
+  std::stack<HitTestingTreeNode*> children;
+  for (HitTestingTreeNode* child = mLastChild.get(); child;
+       child = child->mPrevSibling) {
+    children.push(child);
+  }
+  nsPrintfCString childPrefix("%s  ", aPrefix);
+  while (!children.empty()) {
+    children.top()->Dump(childPrefix.get());
+    children.pop();
+  }
+}
+
+void HitTestingTreeNode::SetApzcParent(AsyncPanZoomController* aParent) {
+  // precondition: GetApzc() is non-null
+  MOZ_ASSERT(GetApzc() != nullptr);
+  if (IsPrimaryHolder()) {
+    GetApzc()->SetParent(aParent);
+  } else {
+    MOZ_ASSERT(GetApzc()->GetParent() == aParent);
+  }
+}
+
+void HitTestingTreeNode::Lock(const RecursiveMutexAutoLock& aProofOfTreeLock) {
+  mLockCount++;
+}
+
+void HitTestingTreeNode::Unlock(
+    const RecursiveMutexAutoLock& aProofOfTreeLock) {
+  MOZ_ASSERT(mLockCount > 0);
+  mLockCount--;
+}
+
+HitTestingTreeNodeAutoLock::HitTestingTreeNodeAutoLock()
+    : mTreeMutex(nullptr) {}
+
+HitTestingTreeNodeAutoLock::~HitTestingTreeNodeAutoLock() { Clear(); }
+
+void HitTestingTreeNodeAutoLock::Initialize(
+    const RecursiveMutexAutoLock& aProofOfTreeLock,
+    already_AddRefed<HitTestingTreeNode> aNode, RecursiveMutex& aTreeMutex) {
+  MOZ_ASSERT(!mNode);
+
+  mNode = aNode;
+  mTreeMutex = &aTreeMutex;
+
+  mNode->Lock(aProofOfTreeLock);
+}
+
+void HitTestingTreeNodeAutoLock::Clear() {
+  if (!mNode) {
+    return;
+  }
+  MOZ_ASSERT(mTreeMutex);
+
+  {  // scope lock
+    RecursiveMutexAutoLock lock(*mTreeMutex);
+    mNode->Unlock(lock);
+  }
+  mNode = nullptr;
+  mTreeMutex = nullptr;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/HitTestingTreeNode.h b/gfx/layers/apz/src/HitTestingTreeNode.h
new file mode 100644
index 0000000000..a4958b1af5
--- /dev/null
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -0,0 +1,272 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_HitTestingTreeNode_h
+#define mozilla_layers_HitTestingTreeNode_h
+
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/gfx/Matrix.h"                  // for Matrix4x4
+#include "mozilla/layers/LayersTypes.h"          // for EventRegions
+#include "mozilla/layers/ScrollableLayerGuid.h"  // for ScrollableLayerGuid
+#include "mozilla/layers/ScrollbarData.h"        // for ScrollbarData
+#include "mozilla/Maybe.h"                       // for Maybe
+#include "mozilla/RecursiveMutex.h"              // for RecursiveMutexAutoLock
+#include "mozilla/RefPtr.h"                      // for nsRefPtr
+namespace mozilla {
+namespace layers {
+
+class AsyncDragMetrics;
+class AsyncPanZoomController;
+
+/**
+ * This class represents a node in a tree that is used by the APZCTreeManager
+ * to do hit testing. The tree is roughly a copy of the layer tree, but will
+ * contain multiple nodes in cases where the layer has multiple FrameMetrics.
+ * In other words, the structure of this tree should be identical to the
+ * WebRenderScrollDataWrapper tree (see documentation in
+ * WebRenderScrollDataWrapper.h).
+ *
+ * Not all HitTestingTreeNode instances will have an APZC associated with them;
+ * only HitTestingTreeNodes that correspond to layers with scrollable metrics
+ * have APZCs.
+ * Multiple HitTestingTreeNode instances may share the same underlying APZC
+ * instance if the layers they represent share the same scrollable metrics (i.e.
+ * are part of the same animated geometry root). If this happens, exactly one of
+ * the HitTestingTreeNode instances will be designated as the "primary holder"
+ * of the APZC. When this primary holder is destroyed, it will destroy the APZC
+ * along with it; in contrast, destroying non-primary-holder nodes will not
+ * destroy the APZC.
+ * Code should not make assumptions about which of the nodes will be the
+ * primary holder, only that that there will be exactly one for each APZC in
+ * the tree.
+ *
+ * The reason this tree exists at all is so that we can do hit-testing on the
+ * thread that we receive input on (referred to the as the controller thread in
+ * APZ terminology), which may be different from the compositor thread.
+ * Accessing the compositor layer tree can only be done on the compositor
+ * thread, and so it is simpler to make a copy of the hit-testing related
+ * properties into a separate tree.
+ *
+ * The tree pointers on the node (mLastChild, etc.) can only be manipulated
+ * while holding the APZ tree lock. Any code that wishes to use a
+ * HitTestingTreeNode outside of holding the tree lock should do so by using
+ * the HitTestingTreeNodeAutoLock wrapper, which prevents the node from
+ * being recycled (and also holds a RefPtr to the node to prevent it from
+ * getting freed).
+ */
+class HitTestingTreeNode {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(HitTestingTreeNode);
+
+ private:
+  ~HitTestingTreeNode();
+
+ public:
+  HitTestingTreeNode(AsyncPanZoomController* aApzc, bool aIsPrimaryHolder,
+                     LayersId aLayersId);
+  void RecycleWith(const RecursiveMutexAutoLock& aProofOfTreeLock,
+                   AsyncPanZoomController* aApzc, LayersId aLayersId);
+  // Clears the tree pointers on the node, thereby breaking RefPtr cycles. This
+  // can trigger free'ing of this and other HitTestingTreeNode instances.
+  void Destroy();
+
+  // Returns true if and only if the node is available for recycling as part
+  // of a hit-testing tree update. Note that this node can have Destroy() called
+  // on it whether or not it is recyclable.
+  bool IsRecyclable(const RecursiveMutexAutoLock& aProofOfTreeLock);
+
+  /* Tree construction methods */
+
+  void SetLastChild(HitTestingTreeNode* aChild);
+  void SetPrevSibling(HitTestingTreeNode* aSibling);
+  void MakeRoot();
+
+  /* Tree walking methods. GetFirstChild is O(n) in the number of children. The
+   * other tree walking methods are all O(1). */
+
+  HitTestingTreeNode* GetFirstChild() const;
+  HitTestingTreeNode* GetLastChild() const;
+  HitTestingTreeNode* GetPrevSibling() const;
+  HitTestingTreeNode* GetParent() const;
+
+  bool IsAncestorOf(const HitTestingTreeNode* aOther) const;
+
+  /* APZC related methods */
+
+  AsyncPanZoomController* GetApzc() const;
+  AsyncPanZoomController* GetNearestContainingApzc() const;
+  bool IsPrimaryHolder() const;
+  LayersId GetLayersId() const;
+
+  /* Hit test related methods */
+
+  void SetHitTestData(
+      const LayerIntRect& aVisibleRect, const LayerIntSize& aRemoteDocumentSize,
+      const CSSTransformMatrix& aTransform,
+      const EventRegionsOverride& aOverride,
+      const Maybe<ScrollableLayerGuid::ViewID>& aAsyncZoomContainerId);
+
+  /* Scrollbar info */
+
+  void SetScrollbarData(const Maybe<uint64_t>& aScrollbarAnimationId,
+                        const ScrollbarData& aScrollbarData);
+  bool MatchesScrollDragMetrics(const AsyncDragMetrics& aDragMetrics,
+                                LayersId aLayersId) const;
+  bool IsScrollbarNode() const;  // Scroll thumb or scrollbar container layer.
+  bool IsScrollbarContainerNode() const;  // Scrollbar container layer.
+  // This can only be called if IsScrollbarNode() is true
+  ScrollDirection GetScrollbarDirection() const;
+  bool IsScrollThumbNode() const;  // Scroll thumb container layer.
+  ScrollableLayerGuid::ViewID GetScrollTargetId() const;
+  const ScrollbarData& GetScrollbarData() const;
+  Maybe<uint64_t> GetScrollbarAnimationId() const;
+
+  /* Fixed pos info */
+
+  void SetFixedPosData(ScrollableLayerGuid::ViewID aFixedPosTarget,
+                       SideBits aFixedPosSides,
+                       const Maybe<uint64_t>& aFixedPositionAnimationId);
+  ScrollableLayerGuid::ViewID GetFixedPosTarget() const;
+  SideBits GetFixedPosSides() const;
+  Maybe<uint64_t> GetFixedPositionAnimationId() const;
+
+  /* Sticky pos info */
+  void SetStickyPosData(ScrollableLayerGuid::ViewID aStickyPosTarget,
+                        const LayerRectAbsolute& aScrollRangeOuter,
+                        const LayerRectAbsolute& aScrollRangeInner,
+                        const Maybe<uint64_t>& aStickyPositionAnimationId);
+  ScrollableLayerGuid::ViewID GetStickyPosTarget() const;
+  const LayerRectAbsolute& GetStickyScrollRangeOuter() const;
+  const LayerRectAbsolute& GetStickyScrollRangeInner() const;
+  Maybe<uint64_t> GetStickyPositionAnimationId() const;
+
+  /* Returns the mOverride flag. */
+  EventRegionsOverride GetEventRegionsOverride() const;
+  const CSSTransformMatrix& GetTransform() const;
+  /* This is similar to APZCTreeManager::GetApzcToGeckoTransform but without
+   * the async bits. It's used on the main-thread for transforming coordinates
+   * across a BrowserParent/BrowserChild interface.
+   * |aRemoteLayersId| is the LayersId of the remote subtree for which this
+   * transform will be used. */
+  LayerToScreenMatrix4x4 GetTransformToGecko(LayersId aRemoteLayersId) const;
+  const LayerIntRect& GetVisibleRect() const;
+
+  /* Returns the screen coordinate rectangle of remote iframe corresponding to
+   * this node. The rectangle is the result of clipped by ancestor async
+   * scrolling. */
+  ScreenRect GetRemoteDocumentScreenRect(
+      LayersId aRemoteDocumentLayersId) const;
+
+  Maybe<ScrollableLayerGuid::ViewID> GetAsyncZoomContainerId() const;
+
+  /* Debug helpers */
+  void Dump(const char* aPrefix = "") const;
+
+ private:
+  friend class HitTestingTreeNodeAutoLock;
+  // Functions that are private but called from HitTestingTreeNodeAutoLock
+  void Lock(const RecursiveMutexAutoLock& aProofOfTreeLock);
+  void Unlock(const RecursiveMutexAutoLock& aProofOfTreeLock);
+
+  void SetApzcParent(AsyncPanZoomController* aApzc);
+
+  RefPtr<HitTestingTreeNode> mLastChild;
+  RefPtr<HitTestingTreeNode> mPrevSibling;
+  RefPtr<HitTestingTreeNode> mParent;
+
+  RefPtr<AsyncPanZoomController> mApzc;
+  bool mIsPrimaryApzcHolder;
+  int mLockCount;
+
+  LayersId mLayersId;
+
+  // This is only set if WebRender is enabled, and only for HTTNs
+  // where IsScrollThumbNode() returns true. It holds the animation id that we
+  // use to move the thumb node to reflect async scrolling.
+  Maybe<uint64_t> mScrollbarAnimationId;
+
+  // This is set for scrollbar Container and Thumb layers.
+  ScrollbarData mScrollbarData;
+
+  // This is only set if WebRender is enabled. It holds the animation id that
+  // we use to adjust fixed position content for the toolbar.
+  Maybe<uint64_t> mFixedPositionAnimationId;
+
+  ScrollableLayerGuid::ViewID mFixedPosTarget;
+  SideBits mFixedPosSides;
+
+  ScrollableLayerGuid::ViewID mStickyPosTarget;
+  LayerRectAbsolute mStickyScrollRangeOuter;
+  LayerRectAbsolute mStickyScrollRangeInner;
+  // This is only set if WebRender is enabled. It holds the animation id that
+  // we use to adjust sticky position content for the toolbar.
+  Maybe<uint64_t> mStickyPositionAnimationId;
+
+  LayerIntRect mVisibleRect;
+
+  /* The size of remote iframe on the corresponding layer coordinate.
+   * It's empty if this node is not for remote iframe. */
+  LayerIntSize mRemoteDocumentSize;
+
+  /* This is the transform from layer L. This does NOT include any async
+   * transforms. */
+  CSSTransformMatrix mTransform;
+
+  /* If the layer is the async zoom container layer then this will hold the id.
+   */
+  Maybe<ScrollableLayerGuid::ViewID> mAsyncZoomContainerId;
+
+  /* Indicates whether or not the event regions on this node need to be
+   * overridden in a certain way. */
+  EventRegionsOverride mOverride;
+};
+
+/**
+ * A class that allows safe usage of a HitTestingTreeNode outside of the APZ
+ * tree lock. In general, this class should be Initialize()'d inside the tree
+ * lock (enforced by the proof-of-lock to Initialize), and then can be returned
+ * to a scope outside the tree lock and used safely. Upon destruction or
+ * Clear() being called, it unlocks the underlying node at which point it can
+ * be recycled or freed.
+ */
+class HitTestingTreeNodeAutoLock final {
+ public:
+  HitTestingTreeNodeAutoLock();
+  ~HitTestingTreeNodeAutoLock();
+  // Make it move-only. Note that the default implementations of the move
+  // constructor and assignment operator are correct: they'll call the
+  // move constructor of mNode, which will null out mNode on the moved-from
+  // object, and Clear() will early-exit when the moved-from object's
+  // destructor is called.
+  HitTestingTreeNodeAutoLock(HitTestingTreeNodeAutoLock&&) = default;
+  HitTestingTreeNodeAutoLock& operator=(HitTestingTreeNodeAutoLock&&) = default;
+
+  void Initialize(const RecursiveMutexAutoLock& aProofOfTreeLock,
+                  already_AddRefed<HitTestingTreeNode> aNode,
+                  RecursiveMutex& aTreeMutex);
+  void Clear();
+
+  // Convenience operators to simplify the using code.
+  explicit operator bool() const { return !!mNode; }
+  bool operator!() const { return !mNode; }
+  HitTestingTreeNode* operator->() const { return mNode.get(); }
+
+  // Allow getting back a raw pointer to the node, but only inside the scope
+  // of the tree lock. The caller is responsible for ensuring that they do not
+  // use the raw pointer outside that scope.
+  HitTestingTreeNode* Get(
+      mozilla::RecursiveMutexAutoLock& aProofOfTreeLock) const {
+    return mNode.get();
+  }
+
+ private:
+  RefPtr<HitTestingTreeNode> mNode;
+  RecursiveMutex* mTreeMutex;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_HitTestingTreeNode_h
diff --git a/gfx/layers/apz/src/IAPZHitTester.cpp b/gfx/layers/apz/src/IAPZHitTester.cpp
new file mode 100644
index 0000000000..75a6076cd4
--- /dev/null
+++ b/gfx/layers/apz/src/IAPZHitTester.cpp
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "IAPZHitTester.h"
+#include "APZCTreeManager.h"
+#include "AsyncPanZoomController.h"
+
+namespace mozilla {
+namespace layers {
+
+IAPZHitTester::HitTestResult IAPZHitTester::CloneHitTestResult(
+    RecursiveMutexAutoLock& aProofOfTreeLock,
+    const IAPZHitTester::HitTestResult& aHitTestResult) const {
+  HitTestResult result;
+
+  result.mTargetApzc = aHitTestResult.mTargetApzc;
+  result.mHitResult = aHitTestResult.mHitResult;
+  result.mLayersId = aHitTestResult.mLayersId;
+  result.mFixedPosSides = aHitTestResult.mFixedPosSides;
+  result.mHitOverscrollGutter = aHitTestResult.mHitOverscrollGutter;
+
+  RefPtr<HitTestingTreeNode> scrollbarNode =
+      aHitTestResult.mScrollbarNode.Get(aProofOfTreeLock);
+  RefPtr<HitTestingTreeNode> node = aHitTestResult.mNode.Get(aProofOfTreeLock);
+
+  if (aHitTestResult.mScrollbarNode) {
+    InitializeHitTestingTreeNodeAutoLock(result.mScrollbarNode,
+                                         aProofOfTreeLock, scrollbarNode);
+  }
+  if (aHitTestResult.mNode) {
+    InitializeHitTestingTreeNodeAutoLock(result.mNode, aProofOfTreeLock, node);
+  }
+
+  return result;
+}
+
+RecursiveMutex& IAPZHitTester::GetTreeLock() { return mTreeManager->mTreeLock; }
+
+LayersId IAPZHitTester::GetRootLayersId() const {
+  return mTreeManager->mRootLayersId;
+}
+
+HitTestingTreeNode* IAPZHitTester::GetRootNode() const {
+  mTreeManager->mTreeLock.AssertCurrentThreadIn();
+  return mTreeManager->mRootNode;
+}
+
+HitTestingTreeNode* IAPZHitTester::FindRootNodeForLayersId(
+    LayersId aLayersId) const {
+  return mTreeManager->FindRootNodeForLayersId(aLayersId);
+}
+
+AsyncPanZoomController* IAPZHitTester::FindRootApzcForLayersId(
+    LayersId aLayersId) const {
+  HitTestingTreeNode* resultNode = FindRootNodeForLayersId(aLayersId);
+  return resultNode ? resultNode->GetApzc() : nullptr;
+}
+
+already_AddRefed<HitTestingTreeNode> IAPZHitTester::GetTargetNode(
+    const ScrollableLayerGuid& aGuid,
+    ScrollableLayerGuid::Comparator aComparator) {
+  // Acquire the tree lock so that derived classes can call this from
+  // methods other than GetAPZCAtPoint().
+  RecursiveMutexAutoLock lock(mTreeManager->mTreeLock);
+  return mTreeManager->GetTargetNode(aGuid, aComparator);
+}
+
+void IAPZHitTester::InitializeHitTestingTreeNodeAutoLock(
+    HitTestingTreeNodeAutoLock& aAutoLock,
+    const RecursiveMutexAutoLock& aProofOfTreeLock,
+    RefPtr<HitTestingTreeNode>& aNode) const {
+  aAutoLock.Initialize(aProofOfTreeLock, aNode.forget(),
+                       mTreeManager->mTreeLock);
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/IAPZHitTester.h b/gfx/layers/apz/src/IAPZHitTester.h
new file mode 100644
index 0000000000..10bf23f28e
--- /dev/null
+++ b/gfx/layers/apz/src/IAPZHitTester.h
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_IAPZHitTester_h
+#define mozilla_layers_IAPZHitTester_h
+
+#include "HitTestingTreeNode.h"  // for HitTestingTreeNodeAutoLock
+#include "mozilla/RefPtr.h"
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/layers/LayersTypes.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+class APZCTreeManager;
+
+class IAPZHitTester {
+ public:
+  virtual ~IAPZHitTester() = default;
+
+  // Not a constructor because we want external code to be able to pass a hit
+  // tester to the APZCTreeManager constructor, which will then initialize it.
+  void Initialize(APZCTreeManager* aTreeManager) {
+    mTreeManager = aTreeManager;
+  }
+
+  // Represents the results of an APZ hit test.
+  struct HitTestResult {
+    // The APZC targeted by the hit test.
+    RefPtr<AsyncPanZoomController> mTargetApzc;
+    // The applicable hit test flags.
+    gfx::CompositorHitTestInfo mHitResult;
+    // The layers id of the content that was hit.
+    // This effectively identifiers the process that was hit for
+    // Fission purposes.
+    LayersId mLayersId;
+    // If a scrollbar was hit, this will be populated with the
+    // scrollbar node. The AutoLock allows accessing the scrollbar
+    // node without having to hold the tree lock.
+    HitTestingTreeNodeAutoLock mScrollbarNode;
+    // If content that is fixed to the root-content APZC was hit,
+    // the sides of the viewport to which the content is fixed.
+    SideBits mFixedPosSides = SideBits::eNone;
+    // If a fixed/sticky position element was hit, this will be populated with
+    // the hit-testing tree node. The AutoLock allows accessing the node
+    // without having to hold the tree lock.
+    HitTestingTreeNodeAutoLock mNode;
+    // This is set to true If mTargetApzc is overscrolled and the
+    // event targeted the gap space ("gutter") created by the overscroll.
+    bool mHitOverscrollGutter = false;
+
+    HitTestResult() = default;
+    // Make it move-only.
+    HitTestResult(HitTestResult&&) = default;
+    HitTestResult& operator=(HitTestResult&&) = default;
+  };
+
+  virtual HitTestResult GetAPZCAtPoint(
+      const ScreenPoint& aHitTestPoint,
+      const RecursiveMutexAutoLock& aProofOfTreeLock) = 0;
+
+  HitTestResult CloneHitTestResult(RecursiveMutexAutoLock& aProofOfTreeLock,
+                                   const HitTestResult& aHitTestResult) const;
+
+ protected:
+  APZCTreeManager* mTreeManager = nullptr;
+
+  // We are a friend of APZCTreeManager but our derived classes
+  // are not. Wrap a few private members of APZCTreeManager for
+  // use by derived classes.
+  RecursiveMutex& GetTreeLock();
+  LayersId GetRootLayersId() const;
+  HitTestingTreeNode* GetRootNode() const;
+  HitTestingTreeNode* FindRootNodeForLayersId(LayersId aLayersId) const;
+  AsyncPanZoomController* FindRootApzcForLayersId(LayersId aLayersId) const;
+  already_AddRefed<HitTestingTreeNode> GetTargetNode(
+      const ScrollableLayerGuid& aGuid,
+      ScrollableLayerGuid::Comparator aComparator);
+  void InitializeHitTestingTreeNodeAutoLock(
+      HitTestingTreeNodeAutoLock& aAutoLock,
+      const RecursiveMutexAutoLock& aProofOfTreeLock,
+      RefPtr<HitTestingTreeNode>& aNode) const;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_IAPZHitTester_h
diff --git a/gfx/layers/apz/src/InputBlockState.cpp b/gfx/layers/apz/src/InputBlockState.cpp
new file mode 100644
index 0000000000..367fbf9e90
--- /dev/null
+++ b/gfx/layers/apz/src/InputBlockState.cpp
@@ -0,0 +1,848 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "InputBlockState.h"
+
+#include "APZUtils.h"
+#include "AsyncPanZoomController.h"  // for AsyncPanZoomController
+
+#include "mozilla/MouseEvents.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_mousewheel.h"
+#include "mozilla/StaticPrefs_test.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+#include "mozilla/ToString.h"
+#include "mozilla/layers/IAPZCTreeManager.h"  // for AllowedTouchBehavior
+#include "OverscrollHandoffState.h"
+#include "QueuedInput.h"
+
+static mozilla::LazyLogModule sApzIbsLog("apz.inputstate");
+#define TBS_LOG(...) MOZ_LOG(sApzIbsLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+static uint64_t sBlockCounter = InputBlockState::NO_BLOCK_ID + 1;
+
+InputBlockState::InputBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags)
+    : mTargetApzc(aTargetApzc),
+      mRequiresTargetConfirmation(aFlags.mRequiresTargetConfirmation),
+      mBlockId(sBlockCounter++),
+      mTransformToApzc(aTargetApzc->GetTransformToThis()) {
+  // We should never be constructed with a nullptr target.
+  MOZ_ASSERT(mTargetApzc);
+  mOverscrollHandoffChain = mTargetApzc->BuildOverscrollHandoffChain();
+  // If a new block starts on a scrollthumb and we have APZ scrollbar
+  // dragging enabled, defer confirmation until we get the drag metrics
+  // for the thumb.
+  bool startingDrag = StaticPrefs::apz_drag_enabled() && aFlags.mHitScrollThumb;
+  mTargetConfirmed = aFlags.mTargetConfirmed && !startingDrag
+                         ? TargetConfirmationState::eConfirmed
+                         : TargetConfirmationState::eUnconfirmed;
+}
+
+bool InputBlockState::SetConfirmedTargetApzc(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationState aState, InputData* aFirstInput,
+    bool aForScrollbarDrag) {
+  MOZ_ASSERT(aState == TargetConfirmationState::eConfirmed ||
+             aState == TargetConfirmationState::eTimedOut);
+
+  // Sometimes, bugs in compositor hit testing can lead to APZ confirming
+  // a different target than the main thread. If this happens for a drag
+  // block created for a scrollbar drag, the consequences can be fairly
+  // user-unfriendly, such as the scrollbar not being draggable at all,
+  // or it scrolling the contents of the wrong scrollframe. In debug
+  // builds, we assert in this situation, so that the
+  // underlying compositor hit testing bug can be fixed. In release builds,
+  // however, we just silently accept the main thread's confirmed target,
+  // which will produce the expected behaviour (apart from drag events
+  // received so far being dropped).
+  if (AsDragBlock() && aForScrollbarDrag &&
+      mTargetConfirmed == TargetConfirmationState::eConfirmed &&
+      aState == TargetConfirmationState::eConfirmed && mTargetApzc &&
+      aTargetApzc && mTargetApzc->GetGuid() != aTargetApzc->GetGuid()) {
+    MOZ_ASSERT(false,
+               "APZ and main thread confirmed scrollbar drag block with "
+               "different targets");
+    UpdateTargetApzc(aTargetApzc);
+    return true;
+  }
+
+  if (mTargetConfirmed != TargetConfirmationState::eUnconfirmed) {
+    return false;
+  }
+  mTargetConfirmed = aState;
+
+  TBS_LOG("%p got confirmed target APZC %p\n", this, mTargetApzc.get());
+  if (mTargetApzc == aTargetApzc) {
+    // The confirmed target is the same as the tentative one, so we're done.
+    return true;
+  }
+
+  TBS_LOG("%p replacing unconfirmed target %p with real target %p\n", this,
+          mTargetApzc.get(), aTargetApzc.get());
+
+  UpdateTargetApzc(aTargetApzc);
+  return true;
+}
+
+void InputBlockState::UpdateTargetApzc(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc) {
+  if (mTargetApzc == aTargetApzc) {
+    MOZ_ASSERT_UNREACHABLE(
+        "The new target APZC should be different from the old one");
+    return;
+  }
+
+  if (mTargetApzc) {
+    // Restore overscroll state on the previous target APZC and ancestor APZCs
+    // in the scroll handoff chain other than the new one.
+    mTargetApzc->SnapBackIfOverscrolled();
+
+    uint32_t i = mOverscrollHandoffChain->IndexOf(mTargetApzc) + 1;
+    for (; i < mOverscrollHandoffChain->Length(); i++) {
+      AsyncPanZoomController* apzc = mOverscrollHandoffChain->GetApzcAtIndex(i);
+      if (apzc != aTargetApzc) {
+        MOZ_ASSERT(!apzc->IsOverscrolled() ||
+                   apzc->IsOverscrollAnimationRunning());
+        apzc->SnapBackIfOverscrolled();
+      }
+    }
+  }
+
+  // note that aTargetApzc MAY be null here.
+  mTargetApzc = aTargetApzc;
+  mTransformToApzc = aTargetApzc ? aTargetApzc->GetTransformToThis()
+                                 : ScreenToParentLayerMatrix4x4();
+  mOverscrollHandoffChain =
+      (mTargetApzc ? mTargetApzc->BuildOverscrollHandoffChain() : nullptr);
+}
+
+const RefPtr<AsyncPanZoomController>& InputBlockState::GetTargetApzc() const {
+  return mTargetApzc;
+}
+
+const RefPtr<const OverscrollHandoffChain>&
+InputBlockState::GetOverscrollHandoffChain() const {
+  return mOverscrollHandoffChain;
+}
+
+uint64_t InputBlockState::GetBlockId() const { return mBlockId; }
+
+bool InputBlockState::IsTargetConfirmed() const {
+  return mTargetConfirmed != TargetConfirmationState::eUnconfirmed;
+}
+
+bool InputBlockState::ShouldDropEvents() const {
+  return mRequiresTargetConfirmation &&
+         (mTargetConfirmed != TargetConfirmationState::eConfirmed);
+}
+
+bool InputBlockState::IsDownchainOf(AsyncPanZoomController* aA,
+                                    AsyncPanZoomController* aB) const {
+  if (aA == aB) {
+    return true;
+  }
+
+  bool seenA = false;
+  for (size_t i = 0; i < mOverscrollHandoffChain->Length(); ++i) {
+    AsyncPanZoomController* apzc = mOverscrollHandoffChain->GetApzcAtIndex(i);
+    if (apzc == aB) {
+      return seenA;
+    }
+    if (apzc == aA) {
+      seenA = true;
+    }
+  }
+  return false;
+}
+
+void InputBlockState::SetScrolledApzc(AsyncPanZoomController* aApzc) {
+  // An input block should only have one scrolled APZC.
+  MOZ_ASSERT(!mScrolledApzc || (StaticPrefs::apz_allow_immediate_handoff()
+                                    ? IsDownchainOf(mScrolledApzc, aApzc)
+                                    : mScrolledApzc == aApzc));
+
+  mScrolledApzc = aApzc;
+}
+
+AsyncPanZoomController* InputBlockState::GetScrolledApzc() const {
+  return mScrolledApzc;
+}
+
+bool InputBlockState::IsDownchainOfScrolledApzc(
+    AsyncPanZoomController* aApzc) const {
+  MOZ_ASSERT(aApzc && mScrolledApzc);
+
+  return IsDownchainOf(mScrolledApzc, aApzc);
+}
+
+void InputBlockState::DispatchEvent(const InputData& aEvent) const {
+  GetTargetApzc()->HandleInputEvent(aEvent, mTransformToApzc);
+}
+
+CancelableBlockState::CancelableBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags)
+    : InputBlockState(aTargetApzc, aFlags),
+      mPreventDefault(false),
+      mContentResponded(false),
+      mContentResponseTimerExpired(false) {}
+
+bool CancelableBlockState::SetContentResponse(bool aPreventDefault) {
+  if (mContentResponded) {
+    return false;
+  }
+  TBS_LOG("%p got content response %d with timer expired %d\n", this,
+          aPreventDefault, mContentResponseTimerExpired);
+  mPreventDefault = aPreventDefault;
+  mContentResponded = true;
+  return true;
+}
+
+bool CancelableBlockState::TimeoutContentResponse() {
+  if (mContentResponseTimerExpired) {
+    return false;
+  }
+  TBS_LOG("%p got content timer expired with response received %d\n", this,
+          mContentResponded);
+  if (!mContentResponded) {
+    mPreventDefault = false;
+  }
+  mContentResponseTimerExpired = true;
+  return true;
+}
+
+bool CancelableBlockState::IsContentResponseTimerExpired() const {
+  return mContentResponseTimerExpired;
+}
+
+bool CancelableBlockState::IsDefaultPrevented() const {
+  MOZ_ASSERT(mContentResponded || mContentResponseTimerExpired);
+  return mPreventDefault;
+}
+
+bool CancelableBlockState::IsReadyForHandling() const {
+  if (!IsTargetConfirmed()) {
+    return false;
+  }
+  return mContentResponded || mContentResponseTimerExpired;
+}
+
+bool CancelableBlockState::ShouldDropEvents() const {
+  return InputBlockState::ShouldDropEvents() || IsDefaultPrevented();
+}
+
+DragBlockState::DragBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags, const MouseInput& aInitialEvent)
+    : CancelableBlockState(aTargetApzc, aFlags), mReceivedMouseUp(false) {}
+
+bool DragBlockState::HasReceivedMouseUp() { return mReceivedMouseUp; }
+
+void DragBlockState::MarkMouseUpReceived() { mReceivedMouseUp = true; }
+
+void DragBlockState::SetInitialThumbPos(OuterCSSCoord aThumbPos) {
+  mInitialThumbPos = aThumbPos;
+}
+
+void DragBlockState::SetDragMetrics(const AsyncDragMetrics& aDragMetrics,
+                                    const CSSRect& aScrollableRect) {
+  mDragMetrics = aDragMetrics;
+  mInitialScrollableRect = aScrollableRect;
+}
+
+void DragBlockState::DispatchEvent(const InputData& aEvent) const {
+  MouseInput mouseInput = aEvent.AsMouseInput();
+  if (!mouseInput.TransformToLocal(mTransformToApzc)) {
+    return;
+  }
+
+  GetTargetApzc()->HandleDragEvent(mouseInput, mDragMetrics, mInitialThumbPos,
+                                   mInitialScrollableRect);
+}
+
+bool DragBlockState::MustStayActive() { return !mReceivedMouseUp; }
+
+const char* DragBlockState::Type() { return "drag"; }
+// This is used to track the current wheel transaction.
+static uint64_t sLastWheelBlockId = InputBlockState::NO_BLOCK_ID;
+
+WheelBlockState::WheelBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags, const ScrollWheelInput& aInitialEvent)
+    : CancelableBlockState(aTargetApzc, aFlags),
+      mScrollSeriesCounter(0),
+      mTransactionEnded(false) {
+  sLastWheelBlockId = GetBlockId();
+
+  if (aFlags.mTargetConfirmed) {
+    // Find the nearest APZC in the overscroll handoff chain that is scrollable.
+    // If we get a content confirmation later that the apzc is different, then
+    // content should have found a scrollable apzc, so we don't need to handle
+    // that case.
+    RefPtr<AsyncPanZoomController> apzc =
+        mOverscrollHandoffChain->FindFirstScrollable(aInitialEvent,
+                                                     &mAllowedScrollDirections);
+
+    if (apzc) {
+      if (apzc != GetTargetApzc()) {
+        UpdateTargetApzc(apzc);
+      }
+    } else if (!mOverscrollHandoffChain->CanBePanned(
+                   mOverscrollHandoffChain->GetApzcAtIndex(0))) {
+      // If there's absolutely nothing scrollable start a transaction and mark
+      // this as such to we know to store our EventTime.
+      mIsScrollable = false;
+    } else {
+      // Scrollable, but not in this direction.
+      EndTransaction();
+    }
+  }
+}
+
+bool WheelBlockState::SetContentResponse(bool aPreventDefault) {
+  if (aPreventDefault) {
+    EndTransaction();
+  }
+  return CancelableBlockState::SetContentResponse(aPreventDefault);
+}
+
+bool WheelBlockState::SetConfirmedTargetApzc(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationState aState, InputData* aFirstInput,
+    bool aForScrollbarDrag) {
+  // The APZC that we find via APZCCallbackHelpers may not be the same APZC
+  // ESM or OverscrollHandoff would have computed. Make sure we get the right
+  // one by looking for the first apzc the next pending event can scroll.
+  RefPtr<AsyncPanZoomController> apzc = aTargetApzc;
+  if (apzc && aFirstInput) {
+    apzc = apzc->BuildOverscrollHandoffChain()->FindFirstScrollable(
+        *aFirstInput, &mAllowedScrollDirections);
+  }
+
+  InputBlockState::SetConfirmedTargetApzc(apzc, aState, aFirstInput,
+                                          aForScrollbarDrag);
+  return true;
+}
+
+void WheelBlockState::Update(ScrollWheelInput& aEvent) {
+  // We might not be in a transaction if the block never started in a
+  // transaction - for example, if nothing was scrollable.
+  if (!InTransaction()) {
+    return;
+  }
+
+  // The current "scroll series" is a like a sub-transaction. It has a separate
+  // timeout of 80ms. Since we need to compute wheel deltas at different phases
+  // of a transaction (for example, when it is updated, and later when the
+  // event action is taken), we affix the scroll series counter to the event.
+  // This makes GetScrollWheelDelta() consistent.
+  if (!mLastEventTime.IsNull() &&
+      (aEvent.mTimeStamp - mLastEventTime).ToMilliseconds() >
+          StaticPrefs::mousewheel_scroll_series_timeout()) {
+    mScrollSeriesCounter = 0;
+  }
+  aEvent.mScrollSeriesNumber = ++mScrollSeriesCounter;
+
+  // If we can't scroll in the direction of the wheel event, we don't update
+  // the last move time. This allows us to timeout a transaction even if the
+  // mouse isn't moving.
+  //
+  // We skip this check if the target is not yet confirmed, so that when it is
+  // confirmed, we don't timeout the transaction.
+  RefPtr<AsyncPanZoomController> apzc = GetTargetApzc();
+  if (mIsScrollable && IsTargetConfirmed() && !apzc->CanScroll(aEvent)) {
+    return;
+  }
+
+  // Update the time of the last known good event, and reset the mouse move
+  // time to null. This will reset the delays on both the general transaction
+  // timeout and the mouse-move-in-frame timeout.
+  mLastEventTime = aEvent.mTimeStamp;
+  mLastMouseMove = TimeStamp();
+}
+
+bool WheelBlockState::MustStayActive() { return !mTransactionEnded; }
+
+const char* WheelBlockState::Type() { return "scroll wheel"; }
+
+bool WheelBlockState::ShouldAcceptNewEvent() const {
+  if (!InTransaction()) {
+    // If we're not in a transaction, start a new one.
+    return false;
+  }
+
+  RefPtr<AsyncPanZoomController> apzc = GetTargetApzc();
+  if (apzc->IsDestroyed()) {
+    return false;
+  }
+
+  return true;
+}
+
+bool WheelBlockState::MaybeTimeout(const ScrollWheelInput& aEvent) {
+  MOZ_ASSERT(InTransaction());
+
+  if (MaybeTimeout(aEvent.mTimeStamp)) {
+    return true;
+  }
+
+  if (!mLastMouseMove.IsNull()) {
+    // If there's a recent mouse movement, we can time out the transaction
+    // early.
+    TimeDuration duration = TimeStamp::Now() - mLastMouseMove;
+    if (duration.ToMilliseconds() >=
+        StaticPrefs::mousewheel_transaction_ignoremovedelay()) {
+      TBS_LOG("%p wheel transaction timed out after mouse move\n", this);
+      EndTransaction();
+      return true;
+    }
+  }
+
+  return false;
+}
+
+bool WheelBlockState::MaybeTimeout(const TimeStamp& aTimeStamp) {
+  MOZ_ASSERT(InTransaction());
+
+  // End the transaction if the event occurred > 1.5s after the most recently
+  // seen wheel event.
+  TimeDuration duration = aTimeStamp - mLastEventTime;
+  if (duration.ToMilliseconds() <
+      StaticPrefs::mousewheel_transaction_timeout()) {
+    return false;
+  }
+
+  TBS_LOG("%p wheel transaction timed out\n", this);
+
+  if (StaticPrefs::test_mousescroll()) {
+    RefPtr<AsyncPanZoomController> apzc = GetTargetApzc();
+    apzc->NotifyMozMouseScrollEvent(u"MozMouseScrollTransactionTimeout"_ns);
+  }
+
+  EndTransaction();
+  return true;
+}
+
+void WheelBlockState::OnMouseMove(
+    const ScreenIntPoint& aPoint,
+    const Maybe<ScrollableLayerGuid>& aTargetGuid) {
+  MOZ_ASSERT(InTransaction());
+
+  if (!GetTargetApzc()->Contains(aPoint) ||
+      // If the mouse moved over to a different APZC, `mIsScrollable`
+      // may no longer be false and needs to be recomputed.
+      (!mIsScrollable && aTargetGuid.isSome() &&
+       aTargetGuid.value() != GetTargetApzc()->GetGuid())) {
+    EndTransaction();
+    return;
+  }
+
+  if (mLastMouseMove.IsNull()) {
+    // If the cursor is moving inside the frame, and it is more than the
+    // ignoremovedelay time since the last scroll operation, we record
+    // this as the most recent mouse movement.
+    TimeStamp now = TimeStamp::Now();
+    TimeDuration duration = now - mLastEventTime;
+    if (duration.ToMilliseconds() >=
+        StaticPrefs::mousewheel_transaction_ignoremovedelay()) {
+      mLastMouseMove = now;
+    }
+  }
+}
+
+void WheelBlockState::UpdateTargetApzc(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc) {
+  InputBlockState::UpdateTargetApzc(aTargetApzc);
+
+  // If we found there was no target apzc, then we end the transaction.
+  if (!GetTargetApzc()) {
+    EndTransaction();
+  }
+}
+
+bool WheelBlockState::InTransaction() const {
+  // We consider a wheel block to be in a transaction if it has a confirmed
+  // target and is the most recent wheel input block to be created.
+  if (GetBlockId() != sLastWheelBlockId) {
+    return false;
+  }
+
+  if (mTransactionEnded) {
+    return false;
+  }
+
+  MOZ_ASSERT(GetTargetApzc());
+  return true;
+}
+
+bool WheelBlockState::AllowScrollHandoff() const {
+  // If we're in a wheel transaction, we do not allow overscroll handoff until
+  // a new event ends the wheel transaction.
+  return !IsTargetConfirmed() || !InTransaction();
+}
+
+void WheelBlockState::EndTransaction() {
+  TBS_LOG("%p ending wheel transaction\n", this);
+  mTransactionEnded = true;
+}
+
+PanGestureBlockState::PanGestureBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags, const PanGestureInput& aInitialEvent)
+    : CancelableBlockState(aTargetApzc, aFlags),
+      mInterrupted(false),
+      mWaitingForContentResponse(false),
+      mWaitingForBrowserGestureResponse(false),
+      mStartedBrowserGesture(false) {
+  if (aFlags.mTargetConfirmed) {
+    // Find the nearest APZC in the overscroll handoff chain that is scrollable.
+    // If we get a content confirmation later that the apzc is different, then
+    // content should have found a scrollable apzc, so we don't need to handle
+    // that case.
+    RefPtr<AsyncPanZoomController> apzc =
+        mOverscrollHandoffChain->FindFirstScrollable(aInitialEvent,
+                                                     &mAllowedScrollDirections);
+
+    if (apzc && apzc != GetTargetApzc()) {
+      UpdateTargetApzc(apzc);
+    }
+  }
+}
+
+bool PanGestureBlockState::SetConfirmedTargetApzc(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationState aState, InputData* aFirstInput,
+    bool aForScrollbarDrag) {
+  // The APZC that we find via APZCCallbackHelpers may not be the same APZC
+  // ESM or OverscrollHandoff would have computed. Make sure we get the right
+  // one by looking for the first apzc the next pending event can scroll.
+  RefPtr<AsyncPanZoomController> apzc = aTargetApzc;
+  if (apzc && aFirstInput) {
+    RefPtr<AsyncPanZoomController> scrollableApzc =
+        apzc->BuildOverscrollHandoffChain()->FindFirstScrollable(
+            *aFirstInput, &mAllowedScrollDirections);
+    if (scrollableApzc) {
+      apzc = scrollableApzc;
+    }
+  }
+
+  InputBlockState::SetConfirmedTargetApzc(apzc, aState, aFirstInput,
+                                          aForScrollbarDrag);
+  return true;
+}
+
+bool PanGestureBlockState::MustStayActive() { return !mInterrupted; }
+
+const char* PanGestureBlockState::Type() { return "pan gesture"; }
+
+bool PanGestureBlockState::SetContentResponse(bool aPreventDefault) {
+  if (aPreventDefault) {
+    TBS_LOG("%p setting interrupted flag\n", this);
+    mInterrupted = true;
+  }
+  bool stateChanged = CancelableBlockState::SetContentResponse(aPreventDefault);
+  if (mWaitingForContentResponse) {
+    mWaitingForContentResponse = false;
+    stateChanged = true;
+  }
+  return stateChanged;
+}
+
+bool PanGestureBlockState::IsReadyForHandling() const {
+  if (!CancelableBlockState::IsReadyForHandling()) {
+    return false;
+  }
+  return !mWaitingForBrowserGestureResponse &&
+         (!mWaitingForContentResponse || IsContentResponseTimerExpired());
+}
+
+bool PanGestureBlockState::ShouldDropEvents() const {
+  return CancelableBlockState::ShouldDropEvents() || mStartedBrowserGesture;
+}
+
+bool PanGestureBlockState::TimeoutContentResponse() {
+  // Reset mWaitingForBrowserGestureResponse here so that we will not wait for
+  // the response forever.
+  mWaitingForBrowserGestureResponse = false;
+  return CancelableBlockState::TimeoutContentResponse();
+}
+
+bool PanGestureBlockState::AllowScrollHandoff() const { return false; }
+
+void PanGestureBlockState::SetNeedsToWaitForContentResponse(
+    bool aWaitForContentResponse) {
+  mWaitingForContentResponse = aWaitForContentResponse;
+}
+
+void PanGestureBlockState::SetNeedsToWaitForBrowserGestureResponse(
+    bool aWaitForBrowserGestureResponse) {
+  mWaitingForBrowserGestureResponse = aWaitForBrowserGestureResponse;
+}
+
+void PanGestureBlockState::SetBrowserGestureResponse(
+    BrowserGestureResponse aResponse) {
+  mWaitingForBrowserGestureResponse = false;
+  mStartedBrowserGesture = bool(aResponse);
+}
+
+PinchGestureBlockState::PinchGestureBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags)
+    : CancelableBlockState(aTargetApzc, aFlags),
+      mInterrupted(false),
+      mWaitingForContentResponse(false) {}
+
+bool PinchGestureBlockState::MustStayActive() { return true; }
+
+const char* PinchGestureBlockState::Type() { return "pinch gesture"; }
+
+bool PinchGestureBlockState::SetContentResponse(bool aPreventDefault) {
+  if (aPreventDefault) {
+    TBS_LOG("%p setting interrupted flag\n", this);
+    mInterrupted = true;
+  }
+  bool stateChanged = CancelableBlockState::SetContentResponse(aPreventDefault);
+  if (mWaitingForContentResponse) {
+    mWaitingForContentResponse = false;
+    stateChanged = true;
+  }
+  return stateChanged;
+}
+
+bool PinchGestureBlockState::IsReadyForHandling() const {
+  if (!CancelableBlockState::IsReadyForHandling()) {
+    return false;
+  }
+  return !mWaitingForContentResponse || IsContentResponseTimerExpired();
+}
+
+void PinchGestureBlockState::SetNeedsToWaitForContentResponse(
+    bool aWaitForContentResponse) {
+  mWaitingForContentResponse = aWaitForContentResponse;
+}
+
+TouchBlockState::TouchBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    TargetConfirmationFlags aFlags, TouchCounter& aCounter)
+    : CancelableBlockState(aTargetApzc, aFlags),
+      mAllowedTouchBehaviorSet(false),
+      mDuringFastFling(false),
+      mSingleTapOccurred(false),
+      mInSlop(false),
+      mForLongTap(false),
+      mLongTapWasProcessed(false),
+      mIsWaitingLongTapResult(false),
+      mNeedsWaitTouchMove(false),
+      mTouchCounter(aCounter),
+      mStartTime(GetTargetApzc()->GetFrameTime().Time()) {
+  mOriginalTargetConfirmedState = mTargetConfirmed;
+  TBS_LOG("Creating %p\n", this);
+}
+
+bool TouchBlockState::SetAllowedTouchBehaviors(
+    const nsTArray<TouchBehaviorFlags>& aBehaviors) {
+  if (mAllowedTouchBehaviorSet) {
+    return false;
+  }
+  TBS_LOG("%p got allowed touch behaviours for %zu points\n", this,
+          aBehaviors.Length());
+  mAllowedTouchBehaviors.AppendElements(aBehaviors);
+  mAllowedTouchBehaviorSet = true;
+  return true;
+}
+
+bool TouchBlockState::GetAllowedTouchBehaviors(
+    nsTArray<TouchBehaviorFlags>& aOutBehaviors) const {
+  if (!mAllowedTouchBehaviorSet) {
+    return false;
+  }
+  aOutBehaviors.AppendElements(mAllowedTouchBehaviors);
+  return true;
+}
+
+bool TouchBlockState::HasAllowedTouchBehaviors() const {
+  return mAllowedTouchBehaviorSet;
+}
+
+void TouchBlockState::CopyPropertiesFrom(const TouchBlockState& aOther) {
+  TBS_LOG("%p copying properties from %p\n", this, &aOther);
+  MOZ_ASSERT(aOther.mAllowedTouchBehaviorSet ||
+             aOther.IsContentResponseTimerExpired());
+  SetAllowedTouchBehaviors(aOther.mAllowedTouchBehaviors);
+  mTransformToApzc = aOther.mTransformToApzc;
+}
+
+bool TouchBlockState::IsReadyForHandling() const {
+  if (!CancelableBlockState::IsReadyForHandling()) {
+    return false;
+  }
+
+  if (mIsWaitingLongTapResult) {
+    return false;
+  }
+
+  return mAllowedTouchBehaviorSet || IsContentResponseTimerExpired();
+}
+
+void TouchBlockState::SetDuringFastFling() {
+  TBS_LOG("%p setting fast-motion flag\n", this);
+  mDuringFastFling = true;
+}
+
+bool TouchBlockState::IsDuringFastFling() const { return mDuringFastFling; }
+
+void TouchBlockState::SetSingleTapOccurred() {
+  TBS_LOG("%p setting single-tap-occurred flag\n", this);
+  mSingleTapOccurred = true;
+}
+
+bool TouchBlockState::SingleTapOccurred() const { return mSingleTapOccurred; }
+
+bool TouchBlockState::MustStayActive() {
+  // If this touch block is for long-tap, it doesn't need to be active after the
+  // block was processed, it will be taken over by the original touch block
+  // which will stay active.
+  return !mForLongTap || !IsReadyForHandling();
+}
+
+const char* TouchBlockState::Type() { return "touch"; }
+
+TimeDuration TouchBlockState::GetTimeSinceBlockStart() const {
+  return GetTargetApzc()->GetFrameTime().Time() - mStartTime;
+}
+
+void TouchBlockState::DispatchEvent(const InputData& aEvent) const {
+  MOZ_ASSERT(aEvent.mInputType == MULTITOUCH_INPUT);
+  mTouchCounter.Update(aEvent.AsMultiTouchInput());
+  CancelableBlockState::DispatchEvent(aEvent);
+}
+
+bool TouchBlockState::TouchActionAllowsPinchZoom() const {
+  // Pointer events specification requires that all touch points allow zoom.
+  for (auto& behavior : mAllowedTouchBehaviors) {
+    if (!(behavior & AllowedTouchBehavior::PINCH_ZOOM)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool TouchBlockState::TouchActionAllowsDoubleTapZoom() const {
+  for (auto& behavior : mAllowedTouchBehaviors) {
+    if (!(behavior & AllowedTouchBehavior::ANIMATING_ZOOM)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool TouchBlockState::TouchActionAllowsPanningX() const {
+  if (mAllowedTouchBehaviors.IsEmpty()) {
+    // Default to allowed
+    return true;
+  }
+  TouchBehaviorFlags flags = mAllowedTouchBehaviors[0];
+  return (flags & AllowedTouchBehavior::HORIZONTAL_PAN);
+}
+
+bool TouchBlockState::TouchActionAllowsPanningY() const {
+  if (mAllowedTouchBehaviors.IsEmpty()) {
+    // Default to allowed
+    return true;
+  }
+  TouchBehaviorFlags flags = mAllowedTouchBehaviors[0];
+  return (flags & AllowedTouchBehavior::VERTICAL_PAN);
+}
+
+bool TouchBlockState::TouchActionAllowsPanningXY() const {
+  if (mAllowedTouchBehaviors.IsEmpty()) {
+    // Default to allowed
+    return true;
+  }
+  TouchBehaviorFlags flags = mAllowedTouchBehaviors[0];
+  return (flags & AllowedTouchBehavior::HORIZONTAL_PAN) &&
+         (flags & AllowedTouchBehavior::VERTICAL_PAN);
+}
+
+bool TouchBlockState::UpdateSlopState(const MultiTouchInput& aInput,
+                                      bool aApzcCanConsumeEvents) {
+  if (aInput.mType == MultiTouchInput::MULTITOUCH_START) {
+    // this is by definition the first event in this block. If it's the first
+    // touch, then we enter a slop state.
+    mInSlop = (aInput.mTouches.Length() == 1);
+    if (mInSlop) {
+      mSlopOrigin = aInput.mTouches[0].mScreenPoint;
+      TBS_LOG("%p entering slop with origin %s\n", this,
+              ToString(mSlopOrigin).c_str());
+    }
+    return false;
+  }
+  if (mInSlop) {
+    ScreenCoord threshold = 0;
+    // If the target was confirmed to null then the threshold doesn't
+    // matter anyway since the events will never be processed.
+    if (const RefPtr<AsyncPanZoomController>& apzc = GetTargetApzc()) {
+      threshold = aApzcCanConsumeEvents ? apzc->GetTouchStartTolerance()
+                                        : apzc->GetTouchMoveTolerance();
+    }
+    bool stayInSlop =
+        (aInput.mType == MultiTouchInput::MULTITOUCH_MOVE) &&
+        (aInput.mTouches.Length() == 1) &&
+        ((aInput.mTouches[0].mScreenPoint - mSlopOrigin).Length() < threshold);
+    if (!stayInSlop) {
+      // we're out of the slop zone, and will stay out for the remainder of
+      // this block
+      TBS_LOG("%p exiting slop\n", this);
+      mInSlop = false;
+    }
+  }
+  return mInSlop;
+}
+
+bool TouchBlockState::IsInSlop() const { return mInSlop; }
+
+Maybe<ScrollDirection> TouchBlockState::GetBestGuessPanDirection(
+    const MultiTouchInput& aInput) {
+  if (aInput.mType != MultiTouchInput::MULTITOUCH_MOVE ||
+      aInput.mTouches.Length() != 1) {
+    return Nothing();
+  }
+  ScreenPoint vector = aInput.mTouches[0].mScreenPoint - mSlopOrigin;
+  double angle = atan2(vector.y, vector.x);  // range [-pi, pi]
+  angle = fabs(angle);                       // range [0, pi]
+
+  double angleThreshold = TouchActionAllowsPanningXY()
+                              ? StaticPrefs::apz_axis_lock_lock_angle()
+                              : StaticPrefs::apz_axis_lock_direct_pan_angle();
+  if (apz::IsCloseToHorizontal(angle, angleThreshold)) {
+    return Some(ScrollDirection::eHorizontal);
+  }
+  if (apz::IsCloseToVertical(angle, angleThreshold)) {
+    return Some(ScrollDirection::eVertical);
+  }
+  return Nothing();
+}
+
+uint32_t TouchBlockState::GetActiveTouchCount() const {
+  return mTouchCounter.GetActiveTouchCount();
+}
+
+bool TouchBlockState::IsTargetOriginallyConfirmed() const {
+  return mOriginalTargetConfirmedState != TargetConfirmationState::eUnconfirmed;
+}
+
+KeyboardBlockState::KeyboardBlockState(
+    const RefPtr<AsyncPanZoomController>& aTargetApzc)
+    : InputBlockState(aTargetApzc, TargetConfirmationFlags{true}) {}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/InputBlockState.h b/gfx/layers/apz/src/InputBlockState.h
new file mode 100644
index 0000000000..f20a4a5901
--- /dev/null
+++ b/gfx/layers/apz/src/InputBlockState.h
@@ -0,0 +1,592 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_InputBlockState_h
+#define mozilla_layers_InputBlockState_h
+
+#include "InputData.h"           // for MultiTouchInput
+#include "mozilla/RefCounted.h"  // for RefCounted
+#include "mozilla/RefPtr.h"      // for RefPtr
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/gfx/Matrix.h"  // for Matrix4x4
+#include "mozilla/layers/APZUtils.h"
+#include "mozilla/layers/LayersTypes.h"  // for TouchBehaviorFlags
+#include "mozilla/layers/AsyncDragMetrics.h"
+#include "mozilla/layers/TouchCounter.h"
+#include "mozilla/TimeStamp.h"  // for TimeStamp
+#include "nsTArray.h"           // for nsTArray
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+class OverscrollHandoffChain;
+class CancelableBlockState;
+class TouchBlockState;
+class WheelBlockState;
+class DragBlockState;
+class PanGestureBlockState;
+class PinchGestureBlockState;
+class KeyboardBlockState;
+enum class BrowserGestureResponse : bool;
+
+/**
+ * A base class that stores state common to various input blocks.
+ * Note that the InputBlockState constructor acquires the tree lock, so callers
+ * from inside AsyncPanZoomController should ensure that the APZC lock is not
+ * held.
+ */
+class InputBlockState : public RefCounted<InputBlockState> {
+ public:
+  MOZ_DECLARE_REFCOUNTED_TYPENAME(InputBlockState)
+
+  static const uint64_t NO_BLOCK_ID = 0;
+
+  enum class TargetConfirmationState : uint8_t {
+    eUnconfirmed,
+    eTimedOut,
+    eConfirmed
+  };
+
+  InputBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                  TargetConfirmationFlags aFlags);
+  virtual ~InputBlockState() = default;
+
+  virtual CancelableBlockState* AsCancelableBlock() { return nullptr; }
+  virtual TouchBlockState* AsTouchBlock() { return nullptr; }
+  virtual WheelBlockState* AsWheelBlock() { return nullptr; }
+  virtual DragBlockState* AsDragBlock() { return nullptr; }
+  virtual PanGestureBlockState* AsPanGestureBlock() { return nullptr; }
+  virtual PinchGestureBlockState* AsPinchGestureBlock() { return nullptr; }
+  virtual KeyboardBlockState* AsKeyboardBlock() { return nullptr; }
+
+  virtual bool SetConfirmedTargetApzc(
+      const RefPtr<AsyncPanZoomController>& aTargetApzc,
+      TargetConfirmationState aState, InputData* aFirstInput,
+      bool aForScrollbarDrag);
+  const RefPtr<AsyncPanZoomController>& GetTargetApzc() const;
+  const RefPtr<const OverscrollHandoffChain>& GetOverscrollHandoffChain() const;
+  uint64_t GetBlockId() const;
+
+  bool IsTargetConfirmed() const;
+
+  virtual bool ShouldDropEvents() const;
+
+  void SetScrolledApzc(AsyncPanZoomController* aApzc);
+  AsyncPanZoomController* GetScrolledApzc() const;
+  bool IsDownchainOfScrolledApzc(AsyncPanZoomController* aApzc) const;
+
+  /**
+   * Dispatch the event to the target APZC. Mostly this is a hook for
+   * subclasses to do any per-event processing they need to.
+   */
+  virtual void DispatchEvent(const InputData& aEvent) const;
+
+  /**
+   * Return true if this input block must stay active if it would otherwise
+   * be removed as the last item in the pending queue.
+   */
+  virtual bool MustStayActive() = 0;
+
+ protected:
+  virtual void UpdateTargetApzc(
+      const RefPtr<AsyncPanZoomController>& aTargetApzc);
+
+ private:
+  // Checks whether |aA| is an ancestor of |aB| (or the same as |aB|) in
+  // |mOverscrollHandoffChain|.
+  bool IsDownchainOf(AsyncPanZoomController* aA,
+                     AsyncPanZoomController* aB) const;
+
+ private:
+  RefPtr<AsyncPanZoomController> mTargetApzc;
+  bool mRequiresTargetConfirmation;
+  const uint64_t mBlockId;
+
+  // The APZC that was actually scrolled by events in this input block.
+  // This is used in configurations where a single input block is only
+  // allowed to scroll a single APZC (configurations where
+  // StaticPrefs::apz_allow_immediate_handoff() is false). Set the first time an
+  // input event in this block scrolls an APZC.
+  RefPtr<AsyncPanZoomController> mScrolledApzc;
+
+ protected:
+  TargetConfirmationState mTargetConfirmed;
+  RefPtr<const OverscrollHandoffChain> mOverscrollHandoffChain;
+
+  // Used to transform events from global screen space to |mTargetApzc|'s
+  // screen space. It's cached at the beginning of the input block so that
+  // all events in the block are in the same coordinate space.
+  ScreenToParentLayerMatrix4x4 mTransformToApzc;
+};
+
+/**
+ * This class represents a set of events that can be cancelled by web content
+ * via event listeners.
+ *
+ * Each cancelable input block can be cancelled by web content, and
+ * this information is stored in the mPreventDefault flag. Because web
+ * content runs on the Gecko main thread, we cannot always wait for web
+ * content's response. Instead, there is a timeout that sets this flag in the
+ * case where web content doesn't respond in time. The mContentResponded and
+ * mContentResponseTimerExpired flags indicate which of these scenarios
+ * occurred.
+ */
+class CancelableBlockState : public InputBlockState {
+ public:
+  CancelableBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                       TargetConfirmationFlags aFlags);
+
+  CancelableBlockState* AsCancelableBlock() override { return this; }
+
+  /**
+   * Record whether or not content cancelled this block of events.
+   * @param aPreventDefault true iff the block is cancelled.
+   * @return false if this block has already received a response from
+   *         web content, true if not.
+   */
+  virtual bool SetContentResponse(bool aPreventDefault);
+
+  /**
+   * Record that content didn't respond in time.
+   * @return false if this block already timed out, true if not.
+   */
+  virtual bool TimeoutContentResponse();
+
+  /**
+   * Checks if the content response timer has already expired.
+   */
+  bool IsContentResponseTimerExpired() const;
+
+  /**
+   * @return true iff web content cancelled this block of events.
+   */
+  bool IsDefaultPrevented() const;
+
+  /**
+   * @return true iff this block has received all the information needed
+   *         to properly dispatch the events in the block.
+   */
+  virtual bool IsReadyForHandling() const;
+
+  /**
+   * Return a descriptive name for the block kind.
+   */
+  virtual const char* Type() = 0;
+
+  bool ShouldDropEvents() const override;
+
+  void ResetContentResponseTimerExpired() {
+    mContentResponseTimerExpired = false;
+    mContentResponded = false;
+  }
+
+ private:
+  bool mPreventDefault;
+  bool mContentResponded;
+  bool mContentResponseTimerExpired;
+};
+
+/**
+ * A single block of wheel events.
+ */
+class WheelBlockState : public CancelableBlockState {
+ public:
+  WheelBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                  TargetConfirmationFlags aFlags,
+                  const ScrollWheelInput& aEvent);
+
+  bool SetContentResponse(bool aPreventDefault) override;
+  bool MustStayActive() override;
+  const char* Type() override;
+  bool SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                              TargetConfirmationState aState,
+                              InputData* aFirstInput,
+                              bool aForScrollbarDrag) override;
+
+  WheelBlockState* AsWheelBlock() override { return this; }
+
+  /**
+   * Determine whether this wheel block is accepting new events.
+   */
+  bool ShouldAcceptNewEvent() const;
+
+  /**
+   * Call to check whether a wheel event will cause the current transaction to
+   * timeout.
+   */
+  bool MaybeTimeout(const ScrollWheelInput& aEvent);
+
+  /**
+   * Called from APZCTM when a mouse move or drag+drop event occurs, before
+   * the event has been processed.
+   */
+  void OnMouseMove(const ScreenIntPoint& aPoint,
+                   const Maybe<ScrollableLayerGuid>& aTargetGuid);
+
+  /**
+   * Returns whether or not the block is participating in a wheel transaction.
+   * This means that the block is the most recent input block to be created,
+   * and no events have occurred that would require scrolling a different
+   * frame.
+   *
+   * @return True if in a transaction, false otherwise.
+   */
+  bool InTransaction() const;
+
+  /**
+   * Mark the block as no longer participating in a wheel transaction. This
+   * will force future wheel events to begin a new input block.
+   */
+  void EndTransaction();
+
+  /**
+   * @return Whether or not overscrolling is prevented for this wheel block.
+   */
+  bool AllowScrollHandoff() const;
+
+  /**
+   * Called to check and possibly end the transaction due to a timeout.
+   *
+   * @return True if the transaction ended, false otherwise.
+   */
+  bool MaybeTimeout(const TimeStamp& aTimeStamp);
+
+  /**
+   * Update the wheel transaction state for a new event.
+   */
+  void Update(ScrollWheelInput& aEvent);
+
+  ScrollDirections GetAllowedScrollDirections() const {
+    return mAllowedScrollDirections;
+  }
+
+ protected:
+  void UpdateTargetApzc(
+      const RefPtr<AsyncPanZoomController>& aTargetApzc) override;
+
+ private:
+  TimeStamp mLastEventTime;
+  TimeStamp mLastMouseMove;
+  uint32_t mScrollSeriesCounter;
+  bool mTransactionEnded;
+  bool mIsScrollable = true;
+  ScrollDirections mAllowedScrollDirections;
+};
+
+/**
+ * A block of mouse events that are part of a drag
+ */
+class DragBlockState : public CancelableBlockState {
+ public:
+  DragBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                 TargetConfirmationFlags aFlags, const MouseInput& aEvent);
+
+  bool MustStayActive() override;
+  const char* Type() override;
+
+  bool HasReceivedMouseUp();
+  void MarkMouseUpReceived();
+
+  DragBlockState* AsDragBlock() override { return this; }
+
+  void SetInitialThumbPos(OuterCSSCoord aThumbPos);
+  void SetDragMetrics(const AsyncDragMetrics& aDragMetrics,
+                      const CSSRect& aScrollableRect);
+
+  void DispatchEvent(const InputData& aEvent) const override;
+
+ private:
+  AsyncDragMetrics mDragMetrics;
+  OuterCSSCoord mInitialThumbPos;
+  CSSRect mInitialScrollableRect;
+  bool mReceivedMouseUp;
+};
+
+/**
+ * A single block of pan gesture events.
+ */
+class PanGestureBlockState : public CancelableBlockState {
+ public:
+  PanGestureBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                       TargetConfirmationFlags aFlags,
+                       const PanGestureInput& aEvent);
+
+  bool SetContentResponse(bool aPreventDefault) override;
+  bool IsReadyForHandling() const override;
+  bool MustStayActive() override;
+  const char* Type() override;
+  bool SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                              TargetConfirmationState aState,
+                              InputData* aFirstInput,
+                              bool aForScrollbarDrag) override;
+
+  PanGestureBlockState* AsPanGestureBlock() override { return this; }
+
+  bool ShouldDropEvents() const override;
+
+  bool TimeoutContentResponse() override;
+
+  /**
+   * @return Whether or not overscrolling is prevented for this block.
+   */
+  bool AllowScrollHandoff() const;
+
+  bool WasInterrupted() const { return mInterrupted; }
+
+  void SetNeedsToWaitForContentResponse(bool aWaitForContentResponse);
+  void SetNeedsToWaitForBrowserGestureResponse(
+      bool aWaitForBrowserGestureResponse);
+  void SetBrowserGestureResponse(BrowserGestureResponse aResponse);
+
+  ScrollDirections GetAllowedScrollDirections() const {
+    return mAllowedScrollDirections;
+  }
+
+ private:
+  bool mInterrupted;
+  bool mWaitingForContentResponse;
+  // A pan gesture may be used for browser's swipe gestures so APZ needs to wait
+  // for the response from the browser whether the gesture has been used for
+  // swipe or not. This `mWaitingForBrowserGestureResponse` flag represents the
+  // waiting state. And below `mStartedBrowserGesture` represents the response
+  // from the browser.
+  bool mWaitingForBrowserGestureResponse;
+  bool mStartedBrowserGesture;
+  ScrollDirections mAllowedScrollDirections;
+};
+
+/**
+ * A single block of pinch gesture events.
+ */
+class PinchGestureBlockState : public CancelableBlockState {
+ public:
+  PinchGestureBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                         TargetConfirmationFlags aFlags);
+
+  bool SetContentResponse(bool aPreventDefault) override;
+  bool IsReadyForHandling() const override;
+  bool MustStayActive() override;
+  const char* Type() override;
+
+  PinchGestureBlockState* AsPinchGestureBlock() override { return this; }
+
+  bool WasInterrupted() const { return mInterrupted; }
+
+  void SetNeedsToWaitForContentResponse(bool aWaitForContentResponse);
+
+ private:
+  bool mInterrupted;
+  bool mWaitingForContentResponse;
+};
+
+/**
+ * This class represents a single touch block. A touch block is
+ * a set of touch events that can be cancelled by web content via
+ * touch event listeners.
+ *
+ * Every touch-start event creates a new touch block. In this case, the
+ * touch block consists of the touch-start, followed by all touch events
+ * up to but not including the next touch-start (except in the case where
+ * a long-tap happens, see below). Note that in particular we cannot know
+ * when a touch block ends until the next one is started. Most touch
+ * blocks are created by receipt of a touch-start event.
+ *
+ * Every long-tap event also creates a new touch block, since it can also
+ * be consumed by web content. In this case, when the long-tap event is
+ * dispatched to web content, a new touch block is started to hold the remaining
+ * touch events, up to but not including the next touch start (or long-tap).
+ *
+ * Additionally, if touch-action is enabled, each touch block should
+ * have a set of allowed touch behavior flags; one for each touch point.
+ * This also requires running code on the Gecko main thread, and so may
+ * be populated with some latency. The mAllowedTouchBehaviorSet and
+ * mAllowedTouchBehaviors variables track this information.
+ */
+class TouchBlockState : public CancelableBlockState {
+ public:
+  explicit TouchBlockState(const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                           TargetConfirmationFlags aFlags,
+                           TouchCounter& aTouchCounter);
+
+  TouchBlockState* AsTouchBlock() override { return this; }
+
+  /**
+   * Set the allowed touch behavior flags for this block.
+   * @return false if this block already has these flags set, true if not.
+   */
+  bool SetAllowedTouchBehaviors(const nsTArray<TouchBehaviorFlags>& aBehaviors);
+  /**
+   * If the allowed touch behaviors have been set, populate them into
+   * |aOutBehaviors| and return true. Else, return false.
+   */
+  bool GetAllowedTouchBehaviors(
+      nsTArray<TouchBehaviorFlags>& aOutBehaviors) const;
+
+  /**
+   * Returns true if the allowed touch behaviours have been set, or if touch
+   * action is disabled.
+   */
+  bool HasAllowedTouchBehaviors() const;
+
+  /**
+   * Copy various properties from another block.
+   */
+  void CopyPropertiesFrom(const TouchBlockState& aOther);
+
+  /**
+   * @return true iff this block has received all the information needed
+   *         to properly dispatch the events in the block.
+   */
+  bool IsReadyForHandling() const override;
+
+  /**
+   * Sets a flag that indicates this input block occurred while the APZ was
+   * in a state of fast flinging. This affects gestures that may be produced
+   * from input events in this block.
+   */
+  void SetDuringFastFling();
+  /**
+   * @return true iff SetDuringFastFling was called on this block.
+   */
+  bool IsDuringFastFling() const;
+  /**
+   * Set the single-tap-occurred flag that indicates that this touch block
+   * triggered a single tap event.
+   */
+  void SetSingleTapOccurred();
+  /**
+   * @return true iff the single-tap-occurred flag is set on this block.
+   */
+  bool SingleTapOccurred() const;
+
+  /**
+   * @return false iff touch-action is enabled and the allowed touch behaviors
+   * for this touch block do not allow pinch-zooming.
+   */
+  bool TouchActionAllowsPinchZoom() const;
+  /**
+   * @return false iff touch-action is enabled and the allowed touch behaviors
+   * for this touch block do not allow double-tap zooming.
+   */
+  bool TouchActionAllowsDoubleTapZoom() const;
+  /**
+   * @return false iff touch-action is enabled and the allowed touch behaviors
+   *         for the first touch point do not allow panning in the specified
+   *         direction(s).
+   */
+  bool TouchActionAllowsPanningX() const;
+  bool TouchActionAllowsPanningY() const;
+  bool TouchActionAllowsPanningXY() const;
+
+  /**
+   * Notifies the input block of an incoming touch event so that the block can
+   * update its internal slop state. "Slop" refers to the area around the
+   * initial touchstart where we drop touchmove events so that content doesn't
+   * see them. The |aApzcCanConsumeEvents| parameter is factored into how large
+   * the slop area is - if this is true the slop area is larger.
+   * @return true iff the provided event is a touchmove in the slop area and
+   *         so should not be sent to content.
+   */
+  bool UpdateSlopState(const MultiTouchInput& aInput,
+                       bool aApzcCanConsumeEvents);
+  bool IsInSlop() const;
+  bool ForLongTap() const { return mForLongTap; }
+  void SetForLongTap() { mForLongTap = true; }
+  bool WasLongTapProcessed() const { return mLongTapWasProcessed; }
+  void SetLongTapProcessed() {
+    MOZ_ASSERT(!mForLongTap);
+    mLongTapWasProcessed = true;
+    mIsWaitingLongTapResult = false;
+  }
+
+  void SetWaitingLongTapResult() {
+    MOZ_ASSERT(!mForLongTap);
+    mIsWaitingLongTapResult = true;
+  }
+  bool IsWaitingLongTapResult() const { return mIsWaitingLongTapResult; }
+
+  void SetNeedsToWaitTouchMove(bool aNeedsWaitTouchMove) {
+    mNeedsWaitTouchMove = aNeedsWaitTouchMove;
+  }
+  bool IsReadyForCallback() const { return !mNeedsWaitTouchMove; };
+
+  /**
+   * Based on the slop origin and the given input event, return a best guess
+   * as to the pan direction of this touch block. Returns Nothing() if no guess
+   * can be made.
+   */
+  Maybe<ScrollDirection> GetBestGuessPanDirection(
+      const MultiTouchInput& aInput);
+
+  /**
+   * Returns the number of touch points currently active.
+   */
+  uint32_t GetActiveTouchCount() const;
+
+  void DispatchEvent(const InputData& aEvent) const override;
+  bool MustStayActive() override;
+  const char* Type() override;
+  TimeDuration GetTimeSinceBlockStart() const;
+  bool IsTargetOriginallyConfirmed() const;
+
+ private:
+  nsTArray<TouchBehaviorFlags> mAllowedTouchBehaviors;
+  bool mAllowedTouchBehaviorSet;
+  bool mDuringFastFling;
+  bool mSingleTapOccurred;
+  bool mInSlop;
+  // A long tap involves two touch blocks: the original touch
+  // block containing the `touchstart`, and a second one
+  // specifically for the long tap. `mForLongTap` is set on the
+  // second touch block. `mLongTapWasProcessed` is set
+  // on the first touch block after the long tap was processed.
+  bool mForLongTap;
+  bool mLongTapWasProcessed;
+
+  // A flag representing a state while we are waiting for a content response for
+  // the long tap.
+  // The reason why we have this flag separately from `mLongTapWasProcessed` is
+  // the block is not ready to be processed during the wait, and is ready once
+  // after `mLongTapWasProcessed` became true.
+  bool mIsWaitingLongTapResult;
+  // A flag representing a state that this block still needs to wait for a
+  // content response for a touch move event. It will be set just before
+  // triggering a long-press event.
+  bool mNeedsWaitTouchMove;
+  ScreenIntPoint mSlopOrigin;
+  // A reference to the InputQueue's touch counter
+  TouchCounter& mTouchCounter;
+  TimeStamp mStartTime;
+  // The original `mTargetConfirmed`. This is necessary to tell whether there's
+  // any APZ-aware event listener in the content after we've got a content
+  // response, because in the case of a long-tap event we need to wait a content
+  // response again.
+  TargetConfirmationState mOriginalTargetConfirmedState;
+};
+
+/**
+ * This class represents a set of keyboard inputs targeted at the same Apzc.
+ */
+class KeyboardBlockState : public InputBlockState {
+ public:
+  explicit KeyboardBlockState(
+      const RefPtr<AsyncPanZoomController>& aTargetApzc);
+
+  KeyboardBlockState* AsKeyboardBlock() override { return this; }
+
+  bool MustStayActive() override { return false; }
+
+  /**
+   * @return Whether or not overscrolling is prevented for this keyboard block.
+   */
+  bool AllowScrollHandoff() const { return false; }
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_InputBlockState_h
diff --git a/gfx/layers/apz/src/InputQueue.cpp b/gfx/layers/apz/src/InputQueue.cpp
new file mode 100644
index 0000000000..8ad75a9794
--- /dev/null
+++ b/gfx/layers/apz/src/InputQueue.cpp
@@ -0,0 +1,1211 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "InputQueue.h"
+
+#include <inttypes.h>
+
+#include "AsyncPanZoomController.h"
+
+#include "GestureEventListener.h"
+#include "InputBlockState.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/EventForwards.h"
+#include "mozilla/layers/APZInputBridge.h"
+#include "mozilla/layers/APZThreadUtils.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ToString.h"
+#include "OverscrollHandoffState.h"
+#include "QueuedInput.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_ui.h"
+
+static mozilla::LazyLogModule sApzInpLog("apz.inputqueue");
+#define INPQ_LOG(...) MOZ_LOG(sApzInpLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+InputQueue::InputQueue() = default;
+
+InputQueue::~InputQueue() { mQueuedInputs.Clear(); }
+
+APZEventResult InputQueue::ReceiveInputEvent(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, InputData& aEvent,
+    const Maybe<nsTArray<TouchBehaviorFlags>>& aTouchBehaviors) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  AutoRunImmediateTimeout timeoutRunner{this};
+
+  switch (aEvent.mInputType) {
+    case MULTITOUCH_INPUT: {
+      const MultiTouchInput& event = aEvent.AsMultiTouchInput();
+      return ReceiveTouchInput(aTarget, aFlags, event, aTouchBehaviors);
+    }
+
+    case SCROLLWHEEL_INPUT: {
+      const ScrollWheelInput& event = aEvent.AsScrollWheelInput();
+      return ReceiveScrollWheelInput(aTarget, aFlags, event);
+    }
+
+    case PANGESTURE_INPUT: {
+      const PanGestureInput& event = aEvent.AsPanGestureInput();
+      return ReceivePanGestureInput(aTarget, aFlags, event);
+    }
+
+    case PINCHGESTURE_INPUT: {
+      const PinchGestureInput& event = aEvent.AsPinchGestureInput();
+      return ReceivePinchGestureInput(aTarget, aFlags, event);
+    }
+
+    case MOUSE_INPUT: {
+      MouseInput& event = aEvent.AsMouseInput();
+      return ReceiveMouseInput(aTarget, aFlags, event);
+    }
+
+    case KEYBOARD_INPUT: {
+      // Every keyboard input must have a confirmed target
+      MOZ_ASSERT(aTarget && aFlags.mTargetConfirmed);
+
+      const KeyboardInput& event = aEvent.AsKeyboardInput();
+      return ReceiveKeyboardInput(aTarget, aFlags, event);
+    }
+
+    default: {
+      // The `mStatus` for other input type is only used by tests, so just
+      // pass through the return value of HandleInputEvent() for now.
+      APZEventResult result(aTarget, aFlags);
+      nsEventStatus status =
+          aTarget->HandleInputEvent(aEvent, aTarget->GetTransformToThis());
+      switch (status) {
+        case nsEventStatus_eIgnore:
+          result.SetStatusAsIgnore();
+          break;
+        case nsEventStatus_eConsumeNoDefault:
+          result.SetStatusAsConsumeNoDefault();
+          break;
+        case nsEventStatus_eConsumeDoDefault:
+          result.SetStatusAsConsumeDoDefault(aTarget);
+          break;
+        default:
+          MOZ_ASSERT_UNREACHABLE("An invalid status");
+          break;
+      }
+      return result;
+    }
+  }
+}
+
+APZEventResult InputQueue::ReceiveTouchInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, const MultiTouchInput& aEvent,
+    const Maybe<nsTArray<TouchBehaviorFlags>>& aTouchBehaviors) {
+  APZEventResult result(aTarget, aFlags);
+
+  RefPtr<TouchBlockState> block;
+  bool waitingForContentResponse = false;
+  if (aEvent.mType == MultiTouchInput::MULTITOUCH_START) {
+    nsTArray<TouchBehaviorFlags> currentBehaviors;
+    bool haveBehaviors = false;
+    if (mActiveTouchBlock) {
+      haveBehaviors =
+          mActiveTouchBlock->GetAllowedTouchBehaviors(currentBehaviors);
+      // If the behaviours aren't set, but the main-thread response timer on
+      // the block is expired we still treat it as though it has behaviors,
+      // because in that case we still want to interrupt the fast-fling and
+      // use the default behaviours.
+      haveBehaviors |= mActiveTouchBlock->IsContentResponseTimerExpired();
+    }
+
+    block = StartNewTouchBlock(aTarget, aFlags);
+    INPQ_LOG("started new touch block %p id %" PRIu64 " for target %p\n",
+             block.get(), block->GetBlockId(), aTarget.get());
+
+    // XXX using the chain from |block| here may be wrong in cases where the
+    // target isn't confirmed and the real target turns out to be something
+    // else. For now assume this is rare enough that it's not an issue.
+    if (mQueuedInputs.IsEmpty() && aEvent.mTouches.Length() == 1 &&
+        block->GetOverscrollHandoffChain()->HasFastFlungApzc() &&
+        haveBehaviors) {
+      // If we're already in a fast fling, and a single finger goes down, then
+      // we want special handling for the touch event, because it shouldn't get
+      // delivered to content. Note that we don't set this flag when going
+      // from a fast fling to a pinch state (i.e. second finger goes down while
+      // the first finger is moving).
+      block->SetDuringFastFling();
+      block->SetConfirmedTargetApzc(
+          aTarget, InputBlockState::TargetConfirmationState::eConfirmed,
+          nullptr /* the block was just created so it has no events */,
+          false /* not a scrollbar drag */);
+      block->SetAllowedTouchBehaviors(currentBehaviors);
+      INPQ_LOG("block %p tagged as fast-motion\n", block.get());
+    } else if (aTouchBehaviors) {
+      // If this block isn't started during a fast-fling, and APZCTM has
+      // provided touch behavior information, then put it on the block so
+      // that the ArePointerEventsConsumable call below can use it.
+      block->SetAllowedTouchBehaviors(*aTouchBehaviors);
+    }
+
+    CancelAnimationsForNewBlock(block);
+
+    waitingForContentResponse = MaybeRequestContentResponse(aTarget, block);
+  } else {
+    // for touch inputs that don't start a block, APZCTM shouldn't be giving
+    // us any touch behaviors.
+    MOZ_ASSERT(aTouchBehaviors.isNothing());
+
+    // If the active touch block is for a long tap, add new touch events into
+    // the original touch block, to ensure that they're only processed if the
+    // original touch block is not prevented.
+    block = mActiveTouchBlock && mActiveTouchBlock->ForLongTap()
+                ? mPrevActiveTouchBlock.get()
+                : mActiveTouchBlock.get();
+    if (!block) {
+      NS_WARNING(
+          "Received a non-start touch event while no touch blocks active!");
+      return result;
+    }
+
+    INPQ_LOG("received new touch event (type=%d) in block %p\n", aEvent.mType,
+             block.get());
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  // Note that the |aTarget| the APZCTM sent us may contradict the confirmed
+  // target set on the block. In this case the confirmed target (which may be
+  // null) should take priority. This is equivalent to just always using the
+  // target (confirmed or not) from the block.
+  RefPtr<AsyncPanZoomController> target = block->GetTargetApzc();
+
+  // XXX calling ArePointerEventsConsumable on |target| may be wrong here if
+  // the target isn't confirmed and the real target turns out to be something
+  // else. For now assume this is rare enough that it's not an issue.
+  PointerEventsConsumableFlags consumableFlags;
+  if (target) {
+    consumableFlags = target->ArePointerEventsConsumable(block, aEvent);
+  }
+  if (block->IsDuringFastFling()) {
+    INPQ_LOG("dropping event due to block %p being in fast motion\n",
+             block.get());
+    result.SetStatusForFastFling(*block, aFlags, consumableFlags, target);
+  } else {  // handling depends on ArePointerEventsConsumable()
+    bool consumable = consumableFlags.IsConsumable();
+    const bool wasInSlop = block->IsInSlop();
+    if (block->UpdateSlopState(aEvent, consumable)) {
+      INPQ_LOG("dropping event due to block %p being in %sslop\n", block.get(),
+               consumable ? "" : "mini-");
+      result.SetStatusAsConsumeNoDefault();
+    } else {
+      // If all following conditions are met, we need to wait for a content
+      // response (again);
+      //  1) this is the first event bailing out from in-slop state after a
+      //     long-tap event has been fired
+      //  2) there's any APZ-aware event listeners
+      //  3) the event block hasn't yet been prevented
+      //
+      // An example scenario;
+      //  in the content there are two event listeners for `touchstart` and
+      //  `touchmove` respectively, and doing `preventDefault()` in the
+      //  `touchmove` event listener. Then if the user kept touching at a point
+      //  until a long-tap event happens, then if the user started moving their
+      // finger, we have to wait for a content response twice, one is for
+      // `touchstart` and one is for `touchmove`.
+      if (wasInSlop &&
+          (block->WasLongTapProcessed() || block->IsWaitingLongTapResult()) &&
+          !block->IsTargetOriginallyConfirmed() && !block->ShouldDropEvents()) {
+        INPQ_LOG(
+            "bailing out from in-stop state in block %p after a long-tap "
+            "happened\n",
+            block.get());
+        block->ResetContentResponseTimerExpired();
+        ScheduleMainThreadTimeout(aTarget, block);
+      }
+      block->SetNeedsToWaitTouchMove(false);
+      result.SetStatusForTouchEvent(*block, aFlags, consumableFlags, target);
+    }
+  }
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+  ProcessQueue();
+
+  // If this block just started and is waiting for a content response, but
+  // also in a slop state (i.e. touchstart gets delivered to content but
+  // not any touchmoves), then we might end up in a situation where we don't
+  // get the content response until the timeout is hit because we never exit
+  // the slop state. But if that timeout is longer than the long-press timeout,
+  // then the long-press gets delayed too. Avoid that by scheduling a callback
+  // with the long-press timeout that will force the block to get processed.
+  int32_t longTapTimeout = StaticPrefs::ui_click_hold_context_menus_delay();
+  int32_t contentTimeout = StaticPrefs::apz_content_response_timeout();
+  if (waitingForContentResponse && longTapTimeout < contentTimeout &&
+      block->IsInSlop() && GestureEventListener::IsLongTapEnabled()) {
+    MOZ_ASSERT(aEvent.mType == MultiTouchInput::MULTITOUCH_START);
+    MOZ_ASSERT(!block->IsDuringFastFling());
+    RefPtr<Runnable> maybeLongTap = NewRunnableMethod<uint64_t>(
+        "layers::InputQueue::MaybeLongTapTimeout", this,
+        &InputQueue::MaybeLongTapTimeout, block->GetBlockId());
+    INPQ_LOG("scheduling maybe-long-tap timeout for target %p\n",
+             aTarget.get());
+    aTarget->PostDelayedTask(maybeLongTap.forget(), longTapTimeout);
+  }
+
+  return result;
+}
+
+APZEventResult InputQueue::ReceiveMouseInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, MouseInput& aEvent) {
+  APZEventResult result(aTarget, aFlags);
+
+  // On a new mouse down we can have a new target so we must force a new block
+  // with a new target.
+  bool newBlock = DragTracker::StartsDrag(aEvent);
+
+  RefPtr<DragBlockState> block = newBlock ? nullptr : mActiveDragBlock.get();
+  if (block && block->HasReceivedMouseUp()) {
+    block = nullptr;
+  }
+
+  if (!block && mDragTracker.InDrag()) {
+    // If there's no current drag block, but we're getting a move with a button
+    // down, we need to start a new drag block because we're obviously already
+    // in the middle of a drag (it probably got interrupted by something else).
+    INPQ_LOG(
+        "got a drag event outside a drag block, need to create a block to hold "
+        "it\n");
+    newBlock = true;
+  }
+
+  mDragTracker.Update(aEvent);
+
+  if (!newBlock && !block) {
+    // This input event is not in a drag block, so we're not doing anything
+    // with it, return eIgnore.
+    return result;
+  }
+
+  if (!block) {
+    MOZ_ASSERT(newBlock);
+    block = new DragBlockState(aTarget, aFlags, aEvent);
+
+    INPQ_LOG(
+        "started new drag block %p id %" PRIu64
+        "for %sconfirmed target %p; on scrollbar: %d; on scrollthumb: %d\n",
+        block.get(), block->GetBlockId(), aFlags.mTargetConfirmed ? "" : "un",
+        aTarget.get(), aFlags.mHitScrollbar, aFlags.mHitScrollThumb);
+
+    mActiveDragBlock = block;
+
+    if (aFlags.mHitScrollThumb || !aFlags.mHitScrollbar) {
+      // If we're running autoscroll, we'll always cancel it during the
+      // following call of CancelAnimationsForNewBlock.  At this time,
+      // we don't want to fire `click` event on the web content for web-compat
+      // with Chrome.  Therefore, we notify widget of it with the flag.
+      if ((aEvent.mType == MouseInput::MOUSE_DOWN ||
+           aEvent.mType == MouseInput::MOUSE_UP) &&
+          block->GetOverscrollHandoffChain()->HasAutoscrollApzc()) {
+        aEvent.mPreventClickEvent = true;
+      }
+      CancelAnimationsForNewBlock(block);
+    }
+    MaybeRequestContentResponse(aTarget, block);
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+  ProcessQueue();
+
+  if (DragTracker::EndsDrag(aEvent)) {
+    block->MarkMouseUpReceived();
+  }
+
+  // The event is part of a drag block and could potentially cause
+  // scrolling, so return DoDefault.
+  result.SetStatusAsConsumeDoDefault(*block);
+  return result;
+}
+
+APZEventResult InputQueue::ReceiveScrollWheelInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, const ScrollWheelInput& aEvent) {
+  APZEventResult result(aTarget, aFlags);
+
+  RefPtr<WheelBlockState> block = mActiveWheelBlock.get();
+  // If the block is not accepting new events we'll create a new input block
+  // (and therefore a new wheel transaction).
+  if (block &&
+      (!block->ShouldAcceptNewEvent() || block->MaybeTimeout(aEvent))) {
+    block = nullptr;
+  }
+
+  MOZ_ASSERT(!block || block->InTransaction());
+
+  if (!block) {
+    block = new WheelBlockState(aTarget, aFlags, aEvent);
+    INPQ_LOG("started new scroll wheel block %p id %" PRIu64
+             " for %starget %p\n",
+             block.get(), block->GetBlockId(),
+             aFlags.mTargetConfirmed ? "confirmed " : "", aTarget.get());
+
+    mActiveWheelBlock = block;
+
+    CancelAnimationsForNewBlock(block, ExcludeWheel);
+    MaybeRequestContentResponse(aTarget, block);
+  } else {
+    INPQ_LOG("received new wheel event in block %p\n", block.get());
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  // Note that the |aTarget| the APZCTM sent us may contradict the confirmed
+  // target set on the block. In this case the confirmed target (which may be
+  // null) should take priority. This is equivalent to just always using the
+  // target (confirmed or not) from the block, which is what
+  // ProcessQueue() does.
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+
+  // The WheelBlockState needs to affix a counter to the event before we process
+  // it. Note that the counter is affixed to the copy in the queue rather than
+  // |aEvent|.
+  block->Update(mQueuedInputs.LastElement()->Input()->AsScrollWheelInput());
+
+  ProcessQueue();
+
+  result.SetStatusAsConsumeDoDefault(*block);
+  return result;
+}
+
+APZEventResult InputQueue::ReceiveKeyboardInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, const KeyboardInput& aEvent) {
+  APZEventResult result(aTarget, aFlags);
+
+  RefPtr<KeyboardBlockState> block = mActiveKeyboardBlock.get();
+
+  // If the block is targeting a different Apzc than this keyboard event then
+  // we'll create a new input block
+  if (block && block->GetTargetApzc() != aTarget) {
+    block = nullptr;
+  }
+
+  if (!block) {
+    block = new KeyboardBlockState(aTarget);
+    INPQ_LOG("started new keyboard block %p id %" PRIu64 " for target %p\n",
+             block.get(), block->GetBlockId(), aTarget.get());
+
+    mActiveKeyboardBlock = block;
+  } else {
+    INPQ_LOG("received new keyboard event in block %p\n", block.get());
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+
+  ProcessQueue();
+
+  // If APZ is allowing passive listeners then we must dispatch the event to
+  // content, otherwise we can consume the event.
+  if (StaticPrefs::apz_keyboard_passive_listeners()) {
+    result.SetStatusAsConsumeDoDefault(*block);
+  } else {
+    result.SetStatusAsConsumeNoDefault();
+  }
+  return result;
+}
+
+static bool CanScrollTargetHorizontally(const PanGestureInput& aInitialEvent,
+                                        PanGestureBlockState* aBlock) {
+  PanGestureInput horizontalComponent = aInitialEvent;
+  horizontalComponent.mPanDisplacement.y = 0;
+  ScrollDirections allowedScrollDirections;
+  RefPtr<AsyncPanZoomController> horizontallyScrollableAPZC =
+      aBlock->GetOverscrollHandoffChain()->FindFirstScrollable(
+          horizontalComponent, &allowedScrollDirections,
+          OverscrollHandoffChain::IncludeOverscroll::No);
+  return horizontallyScrollableAPZC &&
+         horizontallyScrollableAPZC == aBlock->GetTargetApzc() &&
+         allowedScrollDirections.contains(ScrollDirection::eHorizontal);
+}
+
+APZEventResult InputQueue::ReceivePanGestureInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, const PanGestureInput& aEvent) {
+  APZEventResult result(aTarget, aFlags);
+
+  if (aEvent.mType == PanGestureInput::PANGESTURE_MAYSTART ||
+      aEvent.mType == PanGestureInput::PANGESTURE_CANCELLED) {
+    // Ignore these events for now.
+    result.SetStatusAsConsumeDoDefault(aTarget);
+    return result;
+  }
+
+  if (aEvent.mType == PanGestureInput::PANGESTURE_INTERRUPTED) {
+    if (RefPtr<PanGestureBlockState> block = mActivePanGestureBlock.get()) {
+      mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+      ProcessQueue();
+    }
+    result.SetStatusAsIgnore();
+    return result;
+  }
+
+  RefPtr<PanGestureBlockState> block;
+  if (aEvent.mType != PanGestureInput::PANGESTURE_START) {
+    block = mActivePanGestureBlock.get();
+  }
+
+  PanGestureInput event = aEvent;
+
+  // Below `SetStatusAsConsumeDoDefault()` preserves `mHandledResult` of
+  // `result` which was set in the ctor of APZEventResult at the top of this
+  // function based on `aFlag` so that the `mHandledResult` value is reliable to
+  // tell whether the event will be handled by the root content APZC at least
+  // for swipe-navigation stuff. E.g. if a pan-start event scrolled the root
+  // scroll container, we don't need to anything for swipe-navigation.
+  result.SetStatusAsConsumeDoDefault();
+
+  if (!block || block->WasInterrupted()) {
+    if (event.mType == PanGestureInput::PANGESTURE_MOMENTUMSTART ||
+        event.mType == PanGestureInput::PANGESTURE_MOMENTUMPAN ||
+        event.mType == PanGestureInput::PANGESTURE_MOMENTUMEND) {
+      // If there are momentum events after an interruption, discard them.
+      // However, if there is a non-momentum event (indicating the user
+      // continued scrolling on the touchpad), a new input block is started
+      // by turning the event into a pan-start below.
+      return result;
+    }
+    if (event.mType != PanGestureInput::PANGESTURE_START) {
+      // Only PANGESTURE_START events are allowed to start a new pan gesture
+      // block, but we really want to start a new block here, so we magically
+      // turn this input into a PANGESTURE_START.
+      INPQ_LOG(
+          "transmogrifying pan input %d to PANGESTURE_START for new block\n",
+          event.mType);
+      event.mType = PanGestureInput::PANGESTURE_START;
+    }
+    block = new PanGestureBlockState(aTarget, aFlags, event);
+    INPQ_LOG("started new pan gesture block %p id %" PRIu64 " for target %p\n",
+             block.get(), block->GetBlockId(), aTarget.get());
+
+    mActivePanGestureBlock = block;
+
+    CancelAnimationsForNewBlock(block);
+    const bool waitingForContentResponse =
+        MaybeRequestContentResponse(aTarget, block);
+
+    if (event.AllowsSwipe() && !CanScrollTargetHorizontally(event, block)) {
+      // We will ask the browser whether this pan event is going to be used for
+      // swipe or not, so we need to wait the response.
+      block->SetNeedsToWaitForBrowserGestureResponse(true);
+      if (!waitingForContentResponse) {
+        ScheduleMainThreadTimeout(aTarget, block);
+      }
+      if (aFlags.mTargetConfirmed) {
+        // This event may trigger a swipe gesture, depending on what our caller
+        // wants to do it. We need to suspend handling of this block until we
+        // get a content response which will tell us whether to proceed or abort
+        // the block.
+        block->SetNeedsToWaitForContentResponse(true);
+
+        // Inform our caller that we haven't scrolled in response to the event
+        // and that a swipe can be started from this event if desired.
+        result.SetStatusAsIgnore();
+      }
+    }
+  } else {
+    INPQ_LOG("received new pan event (type=%d) in block %p\n", aEvent.mType,
+             block.get());
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  // Note that the |aTarget| the APZCTM sent us may contradict the confirmed
+  // target set on the block. In this case the confirmed target (which may be
+  // null) should take priority. This is equivalent to just always using the
+  // target (confirmed or not) from the block, which is what
+  // ProcessQueue() does.
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(event, *block));
+  ProcessQueue();
+
+  return result;
+}
+
+APZEventResult InputQueue::ReceivePinchGestureInput(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags, const PinchGestureInput& aEvent) {
+  APZEventResult result(aTarget, aFlags);
+
+  RefPtr<PinchGestureBlockState> block;
+  if (aEvent.mType != PinchGestureInput::PINCHGESTURE_START) {
+    block = mActivePinchGestureBlock.get();
+  }
+
+  result.SetStatusAsConsumeDoDefault(aTarget);
+
+  if (!block || block->WasInterrupted()) {
+    if (aEvent.mType != PinchGestureInput::PINCHGESTURE_START) {
+      // Only PINCHGESTURE_START events are allowed to start a new pinch gesture
+      // block.
+      INPQ_LOG("pinchgesture block %p was interrupted %d\n", block.get(),
+               block ? block->WasInterrupted() : 0);
+      return result;
+    }
+    block = new PinchGestureBlockState(aTarget, aFlags);
+    INPQ_LOG("started new pinch gesture block %p id %" PRIu64
+             " for target %p\n",
+             block.get(), block->GetBlockId(), aTarget.get());
+
+    mActivePinchGestureBlock = block;
+    block->SetNeedsToWaitForContentResponse(true);
+
+    CancelAnimationsForNewBlock(block);
+    MaybeRequestContentResponse(aTarget, block);
+  } else {
+    INPQ_LOG("received new pinch event (type=%d) in block %p\n", aEvent.mType,
+             block.get());
+  }
+
+  result.mInputBlockId = block->GetBlockId();
+
+  // Note that the |aTarget| the APZCTM sent us may contradict the confirmed
+  // target set on the block. In this case the confirmed target (which may be
+  // null) should take priority. This is equivalent to just always using the
+  // target (confirmed or not) from the block, which is what
+  // ProcessQueue() does.
+  mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent, *block));
+  ProcessQueue();
+
+  return result;
+}
+
+void InputQueue::CancelAnimationsForNewBlock(InputBlockState* aBlock,
+                                             CancelAnimationFlags aExtraFlags) {
+  // We want to cancel animations here as soon as possible (i.e. without waiting
+  // for content responses) because a finger has gone down and we don't want to
+  // keep moving the content under the finger. However, to prevent "future"
+  // touchstart events from interfering with "past" animations (i.e. from a
+  // previous touch block that is still being processed) we only do this
+  // animation-cancellation if there are no older touch blocks still in the
+  // queue.
+  if (mQueuedInputs.IsEmpty()) {
+    aBlock->GetOverscrollHandoffChain()->CancelAnimations(
+        aExtraFlags | ExcludeOverscroll | ScrollSnap);
+  }
+}
+
+bool InputQueue::MaybeRequestContentResponse(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    CancelableBlockState* aBlock) {
+  bool waitForMainThread = false;
+  if (aBlock->IsTargetConfirmed()) {
+    // Content won't prevent-default this, so we can just set the flag directly.
+    INPQ_LOG("not waiting for content response on block %p\n", aBlock);
+    aBlock->SetContentResponse(false);
+  } else {
+    waitForMainThread = true;
+  }
+  if (aBlock->AsTouchBlock() &&
+      !aBlock->AsTouchBlock()->HasAllowedTouchBehaviors()) {
+    INPQ_LOG("waiting for main thread touch-action info on block %p\n", aBlock);
+    waitForMainThread = true;
+  }
+  if (waitForMainThread) {
+    // We either don't know for sure if aTarget is the right APZC, or we may
+    // need to wait to give content the opportunity to prevent-default the
+    // touch events. Either way we schedule a timeout so the main thread stuff
+    // can run.
+    ScheduleMainThreadTimeout(aTarget, aBlock);
+  }
+  return waitForMainThread;
+}
+
+uint64_t InputQueue::InjectNewTouchBlock(AsyncPanZoomController* aTarget) {
+  AutoRunImmediateTimeout timeoutRunner{this};
+  TouchBlockState* block = StartNewTouchBlockForLongTap(aTarget);
+  INPQ_LOG("injecting new touch block %p with id %" PRIu64 " and target %p\n",
+           block, block->GetBlockId(), aTarget);
+  ScheduleMainThreadTimeout(aTarget, block);
+  return block->GetBlockId();
+}
+
+TouchBlockState* InputQueue::StartNewTouchBlock(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    TargetConfirmationFlags aFlags) {
+  TouchBlockState* newBlock =
+      new TouchBlockState(aTarget, aFlags, mTouchCounter);
+
+  mActiveTouchBlock = newBlock;
+  return newBlock;
+}
+
+TouchBlockState* InputQueue::StartNewTouchBlockForLongTap(
+    const RefPtr<AsyncPanZoomController>& aTarget) {
+  TouchBlockState* newBlock = new TouchBlockState(
+      aTarget, TargetConfirmationFlags{true}, mTouchCounter);
+
+  TouchBlockState* currentBlock = GetCurrentTouchBlock();
+  // We should never enter here without a current touch block, because this
+  // codepath is invoked from the OnLongPress handler in
+  // AsyncPanZoomController, which should bail out if there is no current
+  // touch block.
+  MOZ_ASSERT(currentBlock);
+  newBlock->CopyPropertiesFrom(*currentBlock);
+  newBlock->SetForLongTap();
+
+  // Tell the original touch block that we are going to fire a long tap event.
+  // NOTE: If we get a new touch-move event while we are waiting for a response
+  // of the long-tap event, we need to wait it before processing the original
+  // touch block because if the long-tap event response prevents us from
+  // scrolling we must stop processing any subsequent touch-move events in the
+  // same block.
+  currentBlock->SetWaitingLongTapResult();
+
+  // We need to keep the current block alive, it will be used once after this
+  // new touch block for long-tap was processed.
+  mPrevActiveTouchBlock = currentBlock;
+  mActiveTouchBlock = newBlock;
+  return newBlock;
+}
+
+InputBlockState* InputQueue::GetCurrentBlock() const {
+  APZThreadUtils::AssertOnControllerThread();
+  return mQueuedInputs.IsEmpty() ? nullptr : mQueuedInputs[0]->Block();
+}
+
+TouchBlockState* InputQueue::GetCurrentTouchBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsTouchBlock() : mActiveTouchBlock.get();
+}
+
+WheelBlockState* InputQueue::GetCurrentWheelBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsWheelBlock() : mActiveWheelBlock.get();
+}
+
+DragBlockState* InputQueue::GetCurrentDragBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsDragBlock() : mActiveDragBlock.get();
+}
+
+PanGestureBlockState* InputQueue::GetCurrentPanGestureBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsPanGestureBlock() : mActivePanGestureBlock.get();
+}
+
+PinchGestureBlockState* InputQueue::GetCurrentPinchGestureBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsPinchGestureBlock() : mActivePinchGestureBlock.get();
+}
+
+KeyboardBlockState* InputQueue::GetCurrentKeyboardBlock() const {
+  InputBlockState* block = GetCurrentBlock();
+  return block ? block->AsKeyboardBlock() : mActiveKeyboardBlock.get();
+}
+
+WheelBlockState* InputQueue::GetActiveWheelTransaction() const {
+  WheelBlockState* block = mActiveWheelBlock.get();
+  if (!block || !block->InTransaction()) {
+    return nullptr;
+  }
+  return block;
+}
+
+bool InputQueue::HasReadyTouchBlock() const {
+  return !mQueuedInputs.IsEmpty() &&
+         mQueuedInputs[0]->Block()->AsTouchBlock() &&
+         mQueuedInputs[0]->Block()->AsTouchBlock()->IsReadyForHandling();
+}
+
+bool InputQueue::AllowScrollHandoff() const {
+  if (GetCurrentWheelBlock()) {
+    return GetCurrentWheelBlock()->AllowScrollHandoff();
+  }
+  if (GetCurrentPanGestureBlock()) {
+    return GetCurrentPanGestureBlock()->AllowScrollHandoff();
+  }
+  if (GetCurrentKeyboardBlock()) {
+    return GetCurrentKeyboardBlock()->AllowScrollHandoff();
+  }
+  return true;
+}
+
+bool InputQueue::IsDragOnScrollbar(bool aHitScrollbar) {
+  if (!mDragTracker.InDrag()) {
+    return false;
+  }
+  // Now that we know we are in a drag, get the info from the drag tracker.
+  // We keep it in the tracker rather than the block because the block can get
+  // interrupted by something else (like a wheel event) and then a new block
+  // will get created without the info we want. The tracker will persist though.
+  return mDragTracker.IsOnScrollbar(aHitScrollbar);
+}
+
+void InputQueue::ScheduleMainThreadTimeout(
+    const RefPtr<AsyncPanZoomController>& aTarget,
+    CancelableBlockState* aBlock) {
+  INPQ_LOG("scheduling main thread timeout for target %p\n", aTarget.get());
+  RefPtr<Runnable> timeoutTask = NewRunnableMethod<uint64_t>(
+      "layers::InputQueue::MainThreadTimeout", this,
+      &InputQueue::MainThreadTimeout, aBlock->GetBlockId());
+  int32_t timeout = StaticPrefs::apz_content_response_timeout();
+  if (timeout == 0) {
+    // If the timeout is zero, treat it as a request to ignore any main
+    // thread confirmation and unconditionally use fallback behaviour for
+    // when a timeout is reached. This codepath is used by tests that
+    // want to exercise the fallback behaviour.
+    // To ensure the fallback behaviour is used unconditionally, the timeout
+    // is run right away instead of using PostDelayedTask(). However,
+    // we can't run it right here, because MainThreadTimeout() expects that
+    // the input block has at least one input event in mQueuedInputs, and
+    // the event that triggered this call may not have been added to
+    // mQueuedInputs yet.
+    mImmediateTimeout = std::move(timeoutTask);
+  } else {
+    aTarget->PostDelayedTask(timeoutTask.forget(), timeout);
+  }
+}
+
+InputBlockState* InputQueue::GetBlockForId(uint64_t aInputBlockId) {
+  return FindBlockForId(aInputBlockId, nullptr);
+}
+
+void InputQueue::AddInputBlockCallback(uint64_t aInputBlockId,
+                                       InputBlockCallbackInfo&& aCallbackInfo) {
+  mInputBlockCallbacks.insert(InputBlockCallbackMap::value_type(
+      aInputBlockId, std::move(aCallbackInfo)));
+}
+
+InputBlockState* InputQueue::FindBlockForId(uint64_t aInputBlockId,
+                                            InputData** aOutFirstInput) {
+  for (const auto& queuedInput : mQueuedInputs) {
+    if (queuedInput->Block()->GetBlockId() == aInputBlockId) {
+      if (aOutFirstInput) {
+        *aOutFirstInput = queuedInput->Input();
+      }
+      return queuedInput->Block();
+    }
+  }
+
+  InputBlockState* block = nullptr;
+  if (mActiveTouchBlock && mActiveTouchBlock->GetBlockId() == aInputBlockId) {
+    block = mActiveTouchBlock.get();
+  } else if (mPrevActiveTouchBlock &&
+             mPrevActiveTouchBlock->GetBlockId() == aInputBlockId) {
+    block = mPrevActiveTouchBlock.get();
+  } else if (mActiveWheelBlock &&
+             mActiveWheelBlock->GetBlockId() == aInputBlockId) {
+    block = mActiveWheelBlock.get();
+  } else if (mActiveDragBlock &&
+             mActiveDragBlock->GetBlockId() == aInputBlockId) {
+    block = mActiveDragBlock.get();
+  } else if (mActivePanGestureBlock &&
+             mActivePanGestureBlock->GetBlockId() == aInputBlockId) {
+    block = mActivePanGestureBlock.get();
+  } else if (mActivePinchGestureBlock &&
+             mActivePinchGestureBlock->GetBlockId() == aInputBlockId) {
+    block = mActivePinchGestureBlock.get();
+  } else if (mActiveKeyboardBlock &&
+             mActiveKeyboardBlock->GetBlockId() == aInputBlockId) {
+    block = mActiveKeyboardBlock.get();
+  }
+  // Since we didn't encounter this block while iterating through mQueuedInputs,
+  // it must have no events associated with it at the moment.
+  if (aOutFirstInput) {
+    *aOutFirstInput = nullptr;
+  }
+  return block;
+}
+
+void InputQueue::MainThreadTimeout(uint64_t aInputBlockId) {
+  // It's possible that this function gets called after the controller thread
+  // was discarded during shutdown.
+  if (!APZThreadUtils::IsControllerThreadAlive()) {
+    return;
+  }
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got a main thread timeout; block=%" PRIu64 "\n", aInputBlockId);
+  bool success = false;
+  InputData* firstInput = nullptr;
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, &firstInput);
+  if (inputBlock && inputBlock->AsCancelableBlock()) {
+    CancelableBlockState* block = inputBlock->AsCancelableBlock();
+    // time out the touch-listener response and also confirm the existing
+    // target apzc in the case where the main thread doesn't get back to us
+    // fast enough.
+    success = block->TimeoutContentResponse();
+    success |= block->SetConfirmedTargetApzc(
+        block->GetTargetApzc(),
+        InputBlockState::TargetConfirmationState::eTimedOut, firstInput,
+        // This actually could be a scrollbar drag, but we pass
+        // aForScrollbarDrag=false because for scrollbar drags,
+        // SetConfirmedTargetApzc() will also be called by ConfirmDragBlock(),
+        // and we pass aForScrollbarDrag=true there.
+        false);
+  } else if (inputBlock) {
+    NS_WARNING("input block is not a cancelable block");
+  }
+  if (success) {
+    if (inputBlock->AsTouchBlock() && inputBlock->AsTouchBlock()->IsInSlop()) {
+      // If the touch block is still in slop, it's still possible this block
+      // needs to send a touchmove to content after the long-press gesture
+      // since preventDefault() in a touchmove event handler should stop
+      // handling the block at all.
+      inputBlock->AsTouchBlock()->SetNeedsToWaitTouchMove(true);
+    }
+    ProcessQueue();
+  }
+}
+
+void InputQueue::MaybeLongTapTimeout(uint64_t aInputBlockId) {
+  // It's possible that this function gets called after the controller thread
+  // was discarded during shutdown.
+  if (!APZThreadUtils::IsControllerThreadAlive()) {
+    return;
+  }
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got a maybe-long-tap timeout; block=%" PRIu64 "\n", aInputBlockId);
+
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, nullptr);
+  MOZ_ASSERT(!inputBlock || inputBlock->AsTouchBlock());
+  if (inputBlock && inputBlock->AsTouchBlock()->IsInSlop()) {
+    // If the block is still in slop, it won't have sent a touchmove to content
+    // and so content will not have sent a content response. But also it means
+    // the touchstart should trigger a long-press gesture so let's force the
+    // block to get processed now.
+    MainThreadTimeout(aInputBlockId);
+  }
+}
+
+void InputQueue::ContentReceivedInputBlock(uint64_t aInputBlockId,
+                                           bool aPreventDefault) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got a content response; block=%" PRIu64 " preventDefault=%d\n",
+           aInputBlockId, aPreventDefault);
+  bool success = false;
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, nullptr);
+  if (inputBlock && inputBlock->AsCancelableBlock()) {
+    CancelableBlockState* block = inputBlock->AsCancelableBlock();
+    success = block->SetContentResponse(aPreventDefault);
+  } else if (inputBlock) {
+    NS_WARNING("input block is not a cancelable block");
+  } else {
+    INPQ_LOG("couldn't find block=%" PRIu64 "\n", aInputBlockId);
+  }
+  if (success) {
+    if (ProcessQueue()) {
+      // If we've switched the active touch block back to the original touch
+      // block from the block for long-tap, run ProcessQueue again.
+      // If we haven't yet received new touch-move events which need to be
+      // processed (e.g. we are waiting for a content response for a touch-move
+      // event), below ProcessQueue call is mostly no-op.
+      ProcessQueue();
+    }
+  }
+}
+
+void InputQueue::SetConfirmedTargetApzc(
+    uint64_t aInputBlockId, const RefPtr<AsyncPanZoomController>& aTargetApzc) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got a target apzc; block=%" PRIu64 " guid=%s\n", aInputBlockId,
+           aTargetApzc ? ToString(aTargetApzc->GetGuid()).c_str() : "");
+  bool success = false;
+  InputData* firstInput = nullptr;
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, &firstInput);
+  if (inputBlock && inputBlock->AsCancelableBlock()) {
+    CancelableBlockState* block = inputBlock->AsCancelableBlock();
+    success = block->SetConfirmedTargetApzc(
+        aTargetApzc, InputBlockState::TargetConfirmationState::eConfirmed,
+        firstInput,
+        // This actually could be a scrollbar drag, but we pass
+        // aForScrollbarDrag=false because for scrollbar drags,
+        // SetConfirmedTargetApzc() will also be called by ConfirmDragBlock(),
+        // and we pass aForScrollbarDrag=true there.
+        false);
+  } else if (inputBlock) {
+    NS_WARNING("input block is not a cancelable block");
+  }
+  if (success) {
+    ProcessQueue();
+  }
+}
+
+void InputQueue::ConfirmDragBlock(
+    uint64_t aInputBlockId, const RefPtr<AsyncPanZoomController>& aTargetApzc,
+    const AsyncDragMetrics& aDragMetrics) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got a target apzc; block=%" PRIu64 " guid=%s dragtarget=%" PRIu64
+           "\n",
+           aInputBlockId,
+           aTargetApzc ? ToString(aTargetApzc->GetGuid()).c_str() : "",
+           aDragMetrics.mViewId);
+  bool success = false;
+  InputData* firstInput = nullptr;
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, &firstInput);
+  if (inputBlock && inputBlock->AsDragBlock()) {
+    DragBlockState* block = inputBlock->AsDragBlock();
+
+    // We use the target initial scrollable rect for updating the thumb position
+    // during dragging the thumb even if the scrollable rect got expanded during
+    // the drag.
+    block->SetDragMetrics(aDragMetrics, aTargetApzc->GetScrollableRect());
+    success = block->SetConfirmedTargetApzc(
+        aTargetApzc, InputBlockState::TargetConfirmationState::eConfirmed,
+        firstInput,
+        /* aForScrollbarDrag = */ true);
+  }
+  if (success) {
+    ProcessQueue();
+  }
+}
+
+void InputQueue::SetAllowedTouchBehavior(
+    uint64_t aInputBlockId, const nsTArray<TouchBehaviorFlags>& aBehaviors) {
+  APZThreadUtils::AssertOnControllerThread();
+
+  INPQ_LOG("got allowed touch behaviours; block=%" PRIu64 "\n", aInputBlockId);
+  bool success = false;
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, nullptr);
+  if (inputBlock && inputBlock->AsTouchBlock()) {
+    TouchBlockState* block = inputBlock->AsTouchBlock();
+    success = block->SetAllowedTouchBehaviors(aBehaviors);
+  } else if (inputBlock) {
+    NS_WARNING("input block is not a touch block");
+  }
+  if (success) {
+    ProcessQueue();
+  }
+}
+
+void InputQueue::SetBrowserGestureResponse(uint64_t aInputBlockId,
+                                           BrowserGestureResponse aResponse) {
+  InputBlockState* inputBlock = FindBlockForId(aInputBlockId, nullptr);
+
+  if (inputBlock && inputBlock->AsPanGestureBlock()) {
+    PanGestureBlockState* block = inputBlock->AsPanGestureBlock();
+    block->SetBrowserGestureResponse(aResponse);
+  } else if (inputBlock) {
+    NS_WARNING("input block is not a pan gesture block");
+  }
+  ProcessQueue();
+}
+
+static APZHandledResult GetHandledResultFor(
+    const AsyncPanZoomController* aApzc,
+    const InputBlockState& aCurrentInputBlock, nsEventStatus aEagerStatus) {
+  if (aCurrentInputBlock.ShouldDropEvents()) {
+    return APZHandledResult{APZHandledPlace::HandledByContent, aApzc};
+  }
+
+  if (!aApzc) {
+    return APZHandledResult{APZHandledPlace::HandledByContent, aApzc};
+  }
+
+  if (aApzc->IsRootContent()) {
+    // If the eager status was eIgnore, we would have returned an eager result
+    // of Unhandled if there had been no event handler. Now that we know the
+    // event handler did not preventDefault() the input block, return Unhandled
+    // as the delayed result.
+    // FIXME: A more accurate implementation would be to re-do the entire
+    // computation that determines the status (i.e. calling
+    // ArePointerEventsConsumable()) with the confirmed target APZC.
+    return (aEagerStatus == nsEventStatus_eConsumeDoDefault &&
+            aApzc->CanVerticalScrollWithDynamicToolbar())
+               ? APZHandledResult{APZHandledPlace::HandledByRoot, aApzc}
+               : APZHandledResult{APZHandledPlace::Unhandled, aApzc, true};
+  }
+
+  bool mayTriggerPullToRefresh =
+      aCurrentInputBlock.GetOverscrollHandoffChain()
+          ->ScrollingUpWillTriggerPullToRefresh(aApzc);
+  if (mayTriggerPullToRefresh) {
+    return APZHandledResult{APZHandledPlace::Unhandled, aApzc, true};
+  }
+
+  auto [willMoveDynamicToolbar, rootApzc] =
+      aCurrentInputBlock.GetOverscrollHandoffChain()
+          ->ScrollingDownWillMoveDynamicToolbar(aApzc);
+  if (!willMoveDynamicToolbar) {
+    return APZHandledResult{APZHandledPlace::HandledByContent, aApzc};
+  }
+
+  // Return `HandledByRoot` if scroll positions in all relevant APZC are at the
+  // bottom edge and if there are contents covered by the dynamic toolbar.
+  MOZ_ASSERT(rootApzc && rootApzc->IsRootContent());
+  return APZHandledResult{APZHandledPlace::HandledByRoot, rootApzc};
+}
+
+bool InputQueue::ProcessQueue() {
+  APZThreadUtils::AssertOnControllerThread();
+
+  while (!mQueuedInputs.IsEmpty()) {
+    InputBlockState* curBlock = mQueuedInputs[0]->Block();
+    CancelableBlockState* cancelable = curBlock->AsCancelableBlock();
+    if (cancelable && !cancelable->IsReadyForHandling()) {
+      break;
+    }
+
+    INPQ_LOG(
+        "processing input from block %p; preventDefault %d shouldDropEvents %d "
+        "target %p\n",
+        curBlock, cancelable && cancelable->IsDefaultPrevented(),
+        curBlock->ShouldDropEvents(), curBlock->GetTargetApzc().get());
+    RefPtr<AsyncPanZoomController> target = curBlock->GetTargetApzc();
+
+    // If there is an input block callback registered for this
+    // input block, invoke it.
+    //
+    // NOTE: In the case where the block is a touch block and the block is not
+    // ready to invoke the callback because of waiting a touch move response
+    // from content, we skip the block.
+    if (!curBlock->AsTouchBlock() ||
+        curBlock->AsTouchBlock()->IsReadyForCallback()) {
+      auto it = mInputBlockCallbacks.find(curBlock->GetBlockId());
+      if (it != mInputBlockCallbacks.end()) {
+        INPQ_LOG("invoking the callback for input from block %p id %" PRIu64
+                 "\n",
+                 curBlock, curBlock->GetBlockId());
+        APZHandledResult handledResult =
+            GetHandledResultFor(target, *curBlock, it->second.mEagerStatus);
+        it->second.mCallback(curBlock->GetBlockId(), handledResult);
+        // The callback is one-shot; discard it after calling it.
+        mInputBlockCallbacks.erase(it);
+      }
+    }
+
+    // target may be null here if the initial target was unconfirmed and then
+    // we later got a confirmed null target. in that case drop the events.
+    if (target) {
+      // If the event is targeting a different APZC than the previous one,
+      // we want to clear the previous APZC's gesture state regardless of
+      // whether we're actually dispatching the event or not.
+      if (mLastActiveApzc && mLastActiveApzc != target &&
+          mTouchCounter.GetActiveTouchCount() > 0) {
+        mLastActiveApzc->ResetTouchInputState();
+      }
+      if (curBlock->ShouldDropEvents()) {
+        if (curBlock->AsTouchBlock()) {
+          target->ResetTouchInputState();
+        } else if (curBlock->AsPanGestureBlock()) {
+          target->ResetPanGestureInputState();
+        }
+      } else {
+        UpdateActiveApzc(target);
+        curBlock->DispatchEvent(*(mQueuedInputs[0]->Input()));
+      }
+    }
+    mQueuedInputs.RemoveElementAt(0);
+  }
+
+  bool processQueueAgain = false;
+  if (CanDiscardBlock(mActiveTouchBlock)) {
+    const bool forLongTap = mActiveTouchBlock->ForLongTap();
+    const bool wasDefaultPrevented = mActiveTouchBlock->IsDefaultPrevented();
+    INPQ_LOG("discarding a touch block %p id %" PRIu64 "\n",
+             mActiveTouchBlock.get(), mActiveTouchBlock->GetBlockId());
+    mActiveTouchBlock = nullptr;
+    MOZ_ASSERT_IF(forLongTap, mPrevActiveTouchBlock);
+    if (forLongTap) {
+      INPQ_LOG("switching back to the original touch block %p id %" PRIu64 "\n",
+               mPrevActiveTouchBlock.get(),
+               mPrevActiveTouchBlock->GetBlockId());
+
+      mPrevActiveTouchBlock->SetLongTapProcessed();
+      if (wasDefaultPrevented && !mPrevActiveTouchBlock->IsDefaultPrevented()) {
+        // Take over the preventDefaulted info for the long-tap event (i.e. for
+        // the contextmenu event) to the original touch block so that the
+        // original touch block will never process incoming touch events.
+        mPrevActiveTouchBlock->ResetContentResponseTimerExpired();
+        mPrevActiveTouchBlock->SetContentResponse(true);
+      }
+      mActiveTouchBlock = mPrevActiveTouchBlock;
+      mPrevActiveTouchBlock = nullptr;
+      processQueueAgain = true;
+    }
+  }
+  if (CanDiscardBlock(mActiveWheelBlock)) {
+    mActiveWheelBlock = nullptr;
+  }
+  if (CanDiscardBlock(mActiveDragBlock)) {
+    mActiveDragBlock = nullptr;
+  }
+  if (CanDiscardBlock(mActivePanGestureBlock)) {
+    mActivePanGestureBlock = nullptr;
+  }
+  if (CanDiscardBlock(mActivePinchGestureBlock)) {
+    mActivePinchGestureBlock = nullptr;
+  }
+  if (CanDiscardBlock(mActiveKeyboardBlock)) {
+    mActiveKeyboardBlock = nullptr;
+  }
+
+  return processQueueAgain;
+}
+
+bool InputQueue::CanDiscardBlock(InputBlockState* aBlock) {
+  if (!aBlock ||
+      (aBlock->AsCancelableBlock() &&
+       !aBlock->AsCancelableBlock()->IsReadyForHandling()) ||
+      aBlock->MustStayActive()) {
+    return false;
+  }
+  InputData* firstInput = nullptr;
+  FindBlockForId(aBlock->GetBlockId(), &firstInput);
+  if (firstInput) {
+    // The block has at least one input event still in the queue, so it's
+    // not depleted
+    return false;
+  }
+  return true;
+}
+
+void InputQueue::UpdateActiveApzc(
+    const RefPtr<AsyncPanZoomController>& aNewActive) {
+  mLastActiveApzc = aNewActive;
+}
+
+void InputQueue::Clear() {
+  // On Android, where the controller thread is the Android UI thread,
+  // it's possible for this to be called after the main thread has
+  // already run the shutdown task that clears the state used to
+  // implement APZThreadUtils::AssertOnControllerThread().
+  // In such cases, we still want to perform the cleanup.
+  if (APZThreadUtils::IsControllerThreadAlive()) {
+    APZThreadUtils::AssertOnControllerThread();
+  }
+
+  mQueuedInputs.Clear();
+  mActiveTouchBlock = nullptr;
+  mPrevActiveTouchBlock = nullptr;
+  mActiveWheelBlock = nullptr;
+  mActiveDragBlock = nullptr;
+  mActivePanGestureBlock = nullptr;
+  mActivePinchGestureBlock = nullptr;
+  mActiveKeyboardBlock = nullptr;
+  mLastActiveApzc = nullptr;
+}
+
+InputQueue::AutoRunImmediateTimeout::AutoRunImmediateTimeout(InputQueue* aQueue)
+    : mQueue(aQueue) {
+  MOZ_ASSERT(!mQueue->mImmediateTimeout);
+}
+
+InputQueue::AutoRunImmediateTimeout::~AutoRunImmediateTimeout() {
+  if (mQueue->mImmediateTimeout) {
+    mQueue->mImmediateTimeout->Run();
+    mQueue->mImmediateTimeout = nullptr;
+  }
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/InputQueue.h b/gfx/layers/apz/src/InputQueue.h
new file mode 100644
index 0000000000..62515fc49d
--- /dev/null
+++ b/gfx/layers/apz/src/InputQueue.h
@@ -0,0 +1,290 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_InputQueue_h
+#define mozilla_layers_InputQueue_h
+
+#include "APZUtils.h"
+#include "DragTracker.h"
+#include "InputData.h"
+#include "mozilla/EventForwards.h"
+#include "mozilla/layers/TouchCounter.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "nsTArray.h"
+
+#include <unordered_map>
+
+namespace mozilla {
+
+class InputData;
+class MultiTouchInput;
+class ScrollWheelInput;
+
+namespace layers {
+
+class AsyncPanZoomController;
+class InputBlockState;
+class CancelableBlockState;
+class TouchBlockState;
+class WheelBlockState;
+class DragBlockState;
+class PanGestureBlockState;
+class PinchGestureBlockState;
+class KeyboardBlockState;
+class AsyncDragMetrics;
+class QueuedInput;
+struct APZEventResult;
+struct APZHandledResult;
+enum class BrowserGestureResponse : bool;
+
+using InputBlockCallback = std::function<void(uint64_t aInputBlockId,
+                                              APZHandledResult aHandledResult)>;
+
+struct InputBlockCallbackInfo {
+  nsEventStatus mEagerStatus;
+  InputBlockCallback mCallback;
+};
+
+/**
+ * This class stores incoming input events, associated with "input blocks",
+ * until they are ready for handling.
+ */
+class InputQueue {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(InputQueue)
+
+ public:
+  InputQueue();
+
+  /**
+   * Notifies the InputQueue of a new incoming input event. The APZC that the
+   * input event was targeted to should be provided in the |aTarget| parameter.
+   * See the documentation on APZCTreeManager::ReceiveInputEvent for info on
+   * return values from this function.
+   */
+  APZEventResult ReceiveInputEvent(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, InputData& aEvent,
+      const Maybe<nsTArray<TouchBehaviorFlags>>& aTouchBehaviors = Nothing());
+  /**
+   * This function should be invoked to notify the InputQueue when web content
+   * decides whether or not it wants to cancel a block of events. The block
+   * id to which this applies should be provided in |aInputBlockId|.
+   */
+  void ContentReceivedInputBlock(uint64_t aInputBlockId, bool aPreventDefault);
+  /**
+   * This function should be invoked to notify the InputQueue once the target
+   * APZC to handle an input block has been confirmed. In practice this should
+   * generally be decidable upon receipt of the input event, but in some cases
+   * we may need to query the layout engine to know for sure. The input block
+   * this applies to should be specified via the |aInputBlockId| parameter.
+   */
+  void SetConfirmedTargetApzc(
+      uint64_t aInputBlockId,
+      const RefPtr<AsyncPanZoomController>& aTargetApzc);
+  /**
+   * This function is invoked to confirm that the drag block should be handled
+   * by the APZ.
+   */
+  void ConfirmDragBlock(uint64_t aInputBlockId,
+                        const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                        const AsyncDragMetrics& aDragMetrics);
+  /**
+   * This function should be invoked to notify the InputQueue of the touch-
+   * action properties for the different touch points in an input block. The
+   * input block this applies to should be specified by the |aInputBlockId|
+   * parameter. If touch-action is not enabled on the platform, this function
+   * does nothing and need not be called.
+   */
+  void SetAllowedTouchBehavior(uint64_t aInputBlockId,
+                               const nsTArray<TouchBehaviorFlags>& aBehaviors);
+  /**
+   * Adds a new touch block at the end of the input queue that has the same
+   * allowed touch behaviour flags as the the touch block currently being
+   * processed. This should only be called when processing of a touch block
+   * triggers the creation of a new touch block. Returns the input block id
+   * of the the newly-created block.
+   */
+  uint64_t InjectNewTouchBlock(AsyncPanZoomController* aTarget);
+  /**
+   * Returns the pending input block at the head of the queue, if there is one.
+   * This may return null if there all input events have been processed.
+   */
+  InputBlockState* GetCurrentBlock() const;
+  /*
+   * Returns the current pending input block as a specific kind of block. If
+   * GetCurrentBlock() returns null, these functions additionally check the
+   * mActiveXXXBlock field of the corresponding input type to see if there is
+   * a depleted but still active input block, and returns that if found. These
+   * functions may return null if no block is found.
+   */
+  TouchBlockState* GetCurrentTouchBlock() const;
+  WheelBlockState* GetCurrentWheelBlock() const;
+  DragBlockState* GetCurrentDragBlock() const;
+  PanGestureBlockState* GetCurrentPanGestureBlock() const;
+  PinchGestureBlockState* GetCurrentPinchGestureBlock() const;
+  KeyboardBlockState* GetCurrentKeyboardBlock() const;
+  /**
+   * Returns true iff the pending block at the head of the queue is a touch
+   * block and is ready for handling.
+   */
+  bool HasReadyTouchBlock() const;
+  /**
+   * If there is an active wheel transaction, returns the WheelBlockState
+   * representing the transaction. Otherwise, returns null. "Active" in this
+   * function name is the same kind of "active" as in mActiveWheelBlock - that
+   * is, new incoming wheel events will go into the "active" block.
+   */
+  WheelBlockState* GetActiveWheelTransaction() const;
+  /**
+   * Remove all input blocks from the input queue.
+   */
+  void Clear();
+  /**
+   * Whether the current pending block allows scroll handoff.
+   */
+  bool AllowScrollHandoff() const;
+  /**
+   * If there is currently a drag in progress, return whether or not it was
+   * targeted at a scrollbar. If the drag was newly-created and doesn't know,
+   * use the provided |aOnScrollbar| to populate that information.
+   */
+  bool IsDragOnScrollbar(bool aOnScrollbar);
+
+  InputBlockState* GetBlockForId(uint64_t aInputBlockId);
+
+  void AddInputBlockCallback(uint64_t aInputBlockId,
+                             InputBlockCallbackInfo&& aCallback);
+
+  void SetBrowserGestureResponse(uint64_t aInputBlockId,
+                                 BrowserGestureResponse aResponse);
+
+ private:
+  ~InputQueue();
+
+  // RAII class for automatically running a timeout task that may
+  // need to be run immediately after an event has been queued.
+  class AutoRunImmediateTimeout final {
+   public:
+    explicit AutoRunImmediateTimeout(InputQueue* aQueue);
+    ~AutoRunImmediateTimeout();
+
+   private:
+    InputQueue* mQueue;
+  };
+
+  TouchBlockState* StartNewTouchBlock(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags);
+
+  TouchBlockState* StartNewTouchBlockForLongTap(
+      const RefPtr<AsyncPanZoomController>& aTarget);
+
+  /**
+   * If animations are present for the current pending input block, cancel
+   * them as soon as possible.
+   */
+  void CancelAnimationsForNewBlock(InputBlockState* aBlock,
+                                   CancelAnimationFlags aExtraFlags = Default);
+
+  /**
+   * If we need to wait for a content response, schedule that now. Returns true
+   * if the timeout was scheduled, false otherwise.
+   */
+  bool MaybeRequestContentResponse(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      CancelableBlockState* aBlock);
+
+  APZEventResult ReceiveTouchInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, const MultiTouchInput& aEvent,
+      const Maybe<nsTArray<TouchBehaviorFlags>>& aTouchBehaviors);
+  APZEventResult ReceiveMouseInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, MouseInput& aEvent);
+  APZEventResult ReceiveScrollWheelInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, const ScrollWheelInput& aEvent);
+  APZEventResult ReceivePanGestureInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, const PanGestureInput& aEvent);
+  APZEventResult ReceivePinchGestureInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, const PinchGestureInput& aEvent);
+  APZEventResult ReceiveKeyboardInput(
+      const RefPtr<AsyncPanZoomController>& aTarget,
+      TargetConfirmationFlags aFlags, const KeyboardInput& aEvent);
+
+  /**
+   * Helper function that searches mQueuedInputs for the first block matching
+   * the given id, and returns it. If |aOutFirstInput| is non-null, it is
+   * populated with a pointer to the first input in mQueuedInputs that
+   * corresponds to the block, or null if no such input was found. Note that
+   * even if there are no inputs in mQueuedInputs, this function can return
+   * non-null if the block id provided matches one of the depleted-but-still-
+   * active blocks (mActiveTouchBlock, mActiveWheelBlock, etc.).
+   */
+  InputBlockState* FindBlockForId(uint64_t aInputBlockId,
+                                  InputData** aOutFirstInput);
+  void ScheduleMainThreadTimeout(const RefPtr<AsyncPanZoomController>& aTarget,
+                                 CancelableBlockState* aBlock);
+  void MainThreadTimeout(uint64_t aInputBlockId);
+  void MaybeLongTapTimeout(uint64_t aInputBlockId);
+
+  // Returns true if there's one more queued event we need to process as a
+  // result of switching the active block back to the original touch block from
+  // the touch block for long-tap.
+  bool ProcessQueue();
+  bool CanDiscardBlock(InputBlockState* aBlock);
+  void UpdateActiveApzc(const RefPtr<AsyncPanZoomController>& aNewActive);
+
+ private:
+  // The queue of input events that have not yet been fully processed.
+  // This member must only be accessed on the controller/UI thread.
+  nsTArray<UniquePtr<QueuedInput>> mQueuedInputs;
+
+  // These are the most recently created blocks of each input type. They are
+  // "active" in the sense that new inputs of that type are associated with
+  // them. Note that these pointers may be null if no inputs of the type have
+  // arrived, or if the inputs for the type formed a complete block that was
+  // then discarded.
+  RefPtr<TouchBlockState> mActiveTouchBlock;
+  RefPtr<WheelBlockState> mActiveWheelBlock;
+  RefPtr<DragBlockState> mActiveDragBlock;
+  RefPtr<PanGestureBlockState> mActivePanGestureBlock;
+  RefPtr<PinchGestureBlockState> mActivePinchGestureBlock;
+  RefPtr<KeyboardBlockState> mActiveKeyboardBlock;
+
+  // In the case where a long-tap event triggered by keeping touching happens
+  // we need to keep both the touch block for the long-tap and the original
+  // touch block started with `touch-start`. This value holds the original block
+  // until the long-tap block is processed.
+  RefPtr<TouchBlockState> mPrevActiveTouchBlock;
+
+  // The APZC to which the last event was delivered
+  RefPtr<AsyncPanZoomController> mLastActiveApzc;
+
+  // Track touches so we know when to clear mLastActiveApzc
+  TouchCounter mTouchCounter;
+
+  // Track mouse inputs so we know if we're in a drag or not
+  DragTracker mDragTracker;
+
+  // Temporarily stores a timeout task that needs to be run as soon as
+  // as the event that triggered it has been queued.
+  RefPtr<Runnable> mImmediateTimeout;
+
+  // Maps input block ids to callbacks that will be invoked when the input block
+  // is ready for handling.
+  using InputBlockCallbackMap =
+      std::unordered_map<uint64_t, InputBlockCallbackInfo>;
+  InputBlockCallbackMap mInputBlockCallbacks;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_InputQueue_h
diff --git a/gfx/layers/apz/src/KeyboardMap.cpp b/gfx/layers/apz/src/KeyboardMap.cpp
new file mode 100644
index 0000000000..00b1363b38
--- /dev/null
+++ b/gfx/layers/apz/src/KeyboardMap.cpp
@@ -0,0 +1,172 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/KeyboardMap.h"
+
+#include "mozilla/TextEvents.h"  // for IgnoreModifierState, ShortcutKeyCandidate
+
+namespace mozilla {
+namespace layers {
+
+KeyboardShortcut::KeyboardShortcut()
+    : mKeyCode(0),
+      mCharCode(0),
+      mModifiers(0),
+      mModifiersMask(0),
+      mEventType(KeyboardInput::KeyboardEventType::KEY_OTHER),
+      mDispatchToContent(false) {}
+
+KeyboardShortcut::KeyboardShortcut(KeyboardInput::KeyboardEventType aEventType,
+                                   uint32_t aKeyCode, uint32_t aCharCode,
+                                   Modifiers aModifiers,
+                                   Modifiers aModifiersMask,
+                                   const KeyboardScrollAction& aAction)
+    : mAction(aAction),
+      mKeyCode(aKeyCode),
+      mCharCode(aCharCode),
+      mModifiers(aModifiers),
+      mModifiersMask(aModifiersMask),
+      mEventType(aEventType),
+      mDispatchToContent(false) {}
+
+KeyboardShortcut::KeyboardShortcut(KeyboardInput::KeyboardEventType aEventType,
+                                   uint32_t aKeyCode, uint32_t aCharCode,
+                                   Modifiers aModifiers,
+                                   Modifiers aModifiersMask)
+    : mKeyCode(aKeyCode),
+      mCharCode(aCharCode),
+      mModifiers(aModifiers),
+      mModifiersMask(aModifiersMask),
+      mEventType(aEventType),
+      mDispatchToContent(true) {}
+
+/* static */
+void KeyboardShortcut::AppendHardcodedShortcuts(
+    nsTArray<KeyboardShortcut>& aShortcuts) {
+  // Tab
+  KeyboardShortcut tab1;
+  tab1.mDispatchToContent = true;
+  tab1.mKeyCode = NS_VK_TAB;
+  tab1.mCharCode = 0;
+  tab1.mModifiers = 0;
+  tab1.mModifiersMask = 0;
+  tab1.mEventType = KeyboardInput::KEY_PRESS;
+  aShortcuts.AppendElement(tab1);
+
+  // F6
+  KeyboardShortcut tab2;
+  tab2.mDispatchToContent = true;
+  tab2.mKeyCode = NS_VK_F6;
+  tab2.mCharCode = 0;
+  tab2.mModifiers = 0;
+  tab2.mModifiersMask = 0;
+  tab2.mEventType = KeyboardInput::KEY_PRESS;
+  aShortcuts.AppendElement(tab2);
+}
+
+bool KeyboardShortcut::Matches(const KeyboardInput& aInput,
+                               const IgnoreModifierState& aIgnore,
+                               uint32_t aOverrideCharCode) const {
+  return mEventType == aInput.mType && MatchesKey(aInput, aOverrideCharCode) &&
+         MatchesModifiers(aInput, aIgnore);
+}
+
+bool KeyboardShortcut::MatchesKey(const KeyboardInput& aInput,
+                                  uint32_t aOverrideCharCode) const {
+  // Compare by the key code if we have one
+  if (!mCharCode) {
+    return mKeyCode == aInput.mKeyCode;
+  }
+
+  // We are comparing by char code
+  uint32_t charCode;
+
+  // If we are comparing against a shortcut candidate then we might
+  // have an override char code
+  if (aOverrideCharCode) {
+    charCode = aOverrideCharCode;
+  } else {
+    charCode = aInput.mCharCode;
+  }
+
+  // Both char codes must be in lowercase to compare correctly
+  if (IS_IN_BMP(charCode)) {
+    charCode = ToLowerCase(static_cast<char16_t>(charCode));
+  }
+
+  return mCharCode == charCode;
+}
+
+bool KeyboardShortcut::MatchesModifiers(
+    const KeyboardInput& aInput, const IgnoreModifierState& aIgnore) const {
+  Modifiers modifiersMask = mModifiersMask;
+
+  // If we are ignoring Shift or Meta (Windows key), then unset that part of the
+  // mask
+  if (aIgnore.mMeta) {
+    modifiersMask &= ~MODIFIER_META;
+  }
+  if (aIgnore.mShift) {
+    modifiersMask &= ~MODIFIER_SHIFT;
+  }
+
+  // Mask off the modifiers we are ignoring from the keyboard input
+  return (aInput.modifiers & modifiersMask) == mModifiers;
+}
+
+KeyboardMap::KeyboardMap(nsTArray<KeyboardShortcut>&& aShortcuts)
+    : mShortcuts(aShortcuts) {}
+
+KeyboardMap::KeyboardMap() = default;
+
+Maybe<KeyboardShortcut> KeyboardMap::FindMatch(
+    const KeyboardInput& aEvent) const {
+  // If there are no shortcut candidates, then just search with with the
+  // keyboard input
+  if (aEvent.mShortcutCandidates.IsEmpty()) {
+    return FindMatchInternal(aEvent, IgnoreModifierState());
+  }
+
+  // Otherwise do a search with each shortcut candidate in order
+  for (const auto& key : aEvent.mShortcutCandidates) {
+    IgnoreModifierState ignoreModifierState;
+    ignoreModifierState.mShift =
+        key.mShiftState == ShortcutKeyCandidate::ShiftState::Ignorable;
+
+    auto match = FindMatchInternal(aEvent, ignoreModifierState, key.mCharCode);
+    if (match) {
+      return match;
+    }
+  }
+  return Nothing();
+}
+
+Maybe<KeyboardShortcut> KeyboardMap::FindMatchInternal(
+    const KeyboardInput& aEvent, const IgnoreModifierState& aIgnore,
+    uint32_t aOverrideCharCode) const {
+  for (auto& shortcut : mShortcuts) {
+    if (shortcut.Matches(aEvent, aIgnore, aOverrideCharCode)) {
+      return Some(shortcut);
+    }
+  }
+
+#ifdef XP_WIN
+  // Windows native applications ignore Windows-Logo key state when checking
+  // shortcut keys even if the key is pressed.  Therefore, if there is no
+  // shortcut key which exactly matches current modifier state, we should
+  // retry to look for a shortcut key without the Windows-Logo key press.
+  if (!aIgnore.mMeta && (aEvent.modifiers & MODIFIER_META)) {
+    IgnoreModifierState ignoreModifierState(aIgnore);
+    ignoreModifierState.mMeta = true;
+    return FindMatchInternal(aEvent, ignoreModifierState, aOverrideCharCode);
+  }
+#endif
+
+  return Nothing();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/KeyboardMap.h b/gfx/layers/apz/src/KeyboardMap.h
new file mode 100644
index 0000000000..32ec8ea61d
--- /dev/null
+++ b/gfx/layers/apz/src/KeyboardMap.h
@@ -0,0 +1,118 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_KeyboardMap_h
+#define mozilla_layers_KeyboardMap_h
+
+#include <stdint.h>  // for uint32_t
+
+#include "InputData.h"             // for KeyboardInput
+#include "nsIScrollableFrame.h"    // for nsIScrollableFrame::ScrollUnit
+#include "nsTArray.h"              // for nsTArray
+#include "mozilla/Maybe.h"         // for mozilla::Maybe
+#include "KeyboardScrollAction.h"  // for KeyboardScrollAction
+
+namespace mozilla {
+
+struct IgnoreModifierState;
+
+namespace layers {
+
+class KeyboardMap;
+
+/**
+ * This class is an off main-thread <xul:handler> for scrolling commands.
+ */
+class KeyboardShortcut final {
+ public:
+  KeyboardShortcut();
+
+  /**
+   * Create a keyboard shortcut that when matched can be handled by executing
+   * the specified keyboard action.
+   */
+  KeyboardShortcut(KeyboardInput::KeyboardEventType aEventType,
+                   uint32_t aKeyCode, uint32_t aCharCode, Modifiers aModifiers,
+                   Modifiers aModifiersMask,
+                   const KeyboardScrollAction& aAction);
+
+  /**
+   * Create a keyboard shortcut that when matched should be handled by ignoring
+   * the keyboard event and dispatching it to content.
+   */
+  KeyboardShortcut(KeyboardInput::KeyboardEventType aEventType,
+                   uint32_t aKeyCode, uint32_t aCharCode, Modifiers aModifiers,
+                   Modifiers aModifiersMask);
+
+  /**
+   * There are some default actions for keyboard inputs that are hardcoded in
+   * EventStateManager instead of being represented as XBL handlers. This adds
+   * keyboard shortcuts to match these inputs and dispatch them to content.
+   */
+  static void AppendHardcodedShortcuts(nsTArray<KeyboardShortcut>& aShortcuts);
+
+ protected:
+  friend mozilla::layers::KeyboardMap;
+
+  bool Matches(const KeyboardInput& aInput, const IgnoreModifierState& aIgnore,
+               uint32_t aOverrideCharCode = 0) const;
+
+ private:
+  bool MatchesKey(const KeyboardInput& aInput,
+                  uint32_t aOverrideCharCode) const;
+  bool MatchesModifiers(const KeyboardInput& aInput,
+                        const IgnoreModifierState& aIgnore) const;
+
+ public:
+  // The action to perform when this shortcut is matched,
+  // and not flagged to be dispatched to content
+  KeyboardScrollAction mAction;
+
+  // Only one of mKeyCode or mCharCode may be non-zero
+  // whichever one is non-zero is the one to compare when matching
+  uint32_t mKeyCode;
+  uint32_t mCharCode;
+
+  // The modifiers that must be active for this shortcut
+  Modifiers mModifiers;
+  // The modifiers to compare when matching this shortcut
+  Modifiers mModifiersMask;
+
+  // The type of keyboard event to match against
+  KeyboardInput::KeyboardEventType mEventType;
+
+  // Whether events matched by this must be dispatched to content
+  bool mDispatchToContent;
+};
+
+/**
+ * A keyboard map is an off main-thread <xul:binding> for scrolling commands.
+ */
+class KeyboardMap final {
+ public:
+  KeyboardMap();
+  explicit KeyboardMap(nsTArray<KeyboardShortcut>&& aShortcuts);
+
+  const nsTArray<KeyboardShortcut>& Shortcuts() const { return mShortcuts; }
+
+  /**
+   * Search through the internal list of shortcuts for a match for the input
+   * event
+   */
+  Maybe<KeyboardShortcut> FindMatch(const KeyboardInput& aEvent) const;
+
+ private:
+  Maybe<KeyboardShortcut> FindMatchInternal(
+      const KeyboardInput& aEvent, const IgnoreModifierState& aIgnore,
+      uint32_t aOverrideCharCode = 0) const;
+
+  CopyableTArray<KeyboardShortcut> mShortcuts;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_KeyboardMap_h
diff --git a/gfx/layers/apz/src/KeyboardScrollAction.cpp b/gfx/layers/apz/src/KeyboardScrollAction.cpp
new file mode 100644
index 0000000000..42d9a8bff2
--- /dev/null
+++ b/gfx/layers/apz/src/KeyboardScrollAction.cpp
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/layers/KeyboardScrollAction.h"
+
+namespace mozilla {
+namespace layers {
+
+/* static */ ScrollUnit KeyboardScrollAction::GetScrollUnit(
+    KeyboardScrollAction::KeyboardScrollActionType aDeltaType) {
+  switch (aDeltaType) {
+    case KeyboardScrollAction::eScrollCharacter:
+      return ScrollUnit::LINES;
+    case KeyboardScrollAction::eScrollLine:
+      return ScrollUnit::LINES;
+    case KeyboardScrollAction::eScrollPage:
+      return ScrollUnit::PAGES;
+    case KeyboardScrollAction::eScrollComplete:
+      return ScrollUnit::WHOLE;
+  }
+
+  // Silence an overzealous warning
+  return ScrollUnit::WHOLE;
+}
+
+KeyboardScrollAction::KeyboardScrollAction()
+    : mType(KeyboardScrollAction::eScrollCharacter), mForward(false) {}
+
+KeyboardScrollAction::KeyboardScrollAction(KeyboardScrollActionType aType,
+                                           bool aForward)
+    : mType(aType), mForward(aForward) {}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/KeyboardScrollAction.h b/gfx/layers/apz/src/KeyboardScrollAction.h
new file mode 100644
index 0000000000..780006c1b3
--- /dev/null
+++ b/gfx/layers/apz/src/KeyboardScrollAction.h
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_KeyboardScrollAction_h
+#define mozilla_layers_KeyboardScrollAction_h
+
+#include <cstdint>  // for uint8_t
+
+#include "mozilla/ScrollTypes.h"
+#include "mozilla/DefineEnum.h"  // for MOZ_DEFINE_ENUM
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * This class represents a scrolling action to be performed on a scrollable
+ * layer.
+ */
+struct KeyboardScrollAction final {
+ public:
+  // clang-format off
+  MOZ_DEFINE_ENUM_WITH_BASE_AT_CLASS_SCOPE(
+    KeyboardScrollActionType, uint8_t, (
+      eScrollCharacter,
+      eScrollLine,
+      eScrollPage,
+      eScrollComplete
+  ));
+  // clang-format on
+
+  static ScrollUnit GetScrollUnit(KeyboardScrollActionType aDeltaType);
+
+  KeyboardScrollAction();
+  KeyboardScrollAction(KeyboardScrollActionType aType, bool aForward);
+
+  // The type of scroll to perform for this action
+  KeyboardScrollActionType mType;
+  // Whether to scroll forward or backward along the axis of this action type
+  bool mForward;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_KeyboardScrollAction_h
diff --git a/gfx/layers/apz/src/Overscroll.h b/gfx/layers/apz/src/Overscroll.h
new file mode 100644
index 0000000000..1fb7c3e487
--- /dev/null
+++ b/gfx/layers/apz/src/Overscroll.h
@@ -0,0 +1,250 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_Overscroll_h
+#define mozilla_layers_Overscroll_h
+
+#include "AsyncPanZoomAnimation.h"
+#include "AsyncPanZoomController.h"
+#include "mozilla/TimeStamp.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+namespace layers {
+
+// Animation used by GenericOverscrollEffect.
+class OverscrollAnimation : public AsyncPanZoomAnimation {
+ public:
+  OverscrollAnimation(AsyncPanZoomController& aApzc,
+                      const ParentLayerPoint& aVelocity,
+                      SideBits aOverscrollSideBits)
+      : mApzc(aApzc), mOverscrollSideBits(aOverscrollSideBits) {
+    MOZ_ASSERT(
+        (mOverscrollSideBits & SideBits::eTopBottom) != SideBits::eTopBottom &&
+            (mOverscrollSideBits & SideBits::eLeftRight) !=
+                SideBits::eLeftRight,
+        "Don't allow overscrolling on both sides at the same time");
+    if ((aOverscrollSideBits & SideBits::eLeftRight) != SideBits::eNone) {
+      mApzc.mX.StartOverscrollAnimation(aVelocity.x);
+    }
+    if ((aOverscrollSideBits & SideBits::eTopBottom) != SideBits::eNone) {
+      mApzc.mY.StartOverscrollAnimation(aVelocity.y);
+    }
+  }
+  virtual ~OverscrollAnimation() {
+    mApzc.mX.EndOverscrollAnimation();
+    mApzc.mY.EndOverscrollAnimation();
+  }
+
+  virtual bool DoSample(FrameMetrics& aFrameMetrics,
+                        const TimeDuration& aDelta) override {
+    // Can't inline these variables due to short-circuit evaluation.
+    bool continueX = mApzc.mX.IsOverscrollAnimationAlive() &&
+                     mApzc.mX.SampleOverscrollAnimation(
+                         aDelta, mOverscrollSideBits & SideBits::eLeftRight);
+    bool continueY = mApzc.mY.IsOverscrollAnimationAlive() &&
+                     mApzc.mY.SampleOverscrollAnimation(
+                         aDelta, mOverscrollSideBits & SideBits::eTopBottom);
+    if (!continueX && !continueY) {
+      // If we got into overscroll from a fling, that fling did not request a
+      // fling snap to avoid a resulting scrollTo from cancelling the overscroll
+      // animation too early. We do still want to request a fling snap, though,
+      // in case the end of the axis at which we're overscrolled is not a valid
+      // snap point, so we request one now. If there are no snap points, this
+      // will do nothing. If there are snap points, we'll get a scrollTo that
+      // snaps us back to the nearest valid snap point. The scroll snapping is
+      // done in a deferred task, otherwise the state change to NOTHING caused
+      // by the overscroll animation ending would clobber a possible state
+      // change to SMOOTH_SCROLL in ScrollSnap().
+      mDeferredTasks.AppendElement(NewRunnableMethod<ScrollSnapFlags>(
+          "layers::AsyncPanZoomController::ScrollSnap", &mApzc,
+          &AsyncPanZoomController::ScrollSnap,
+          ScrollSnapFlags::IntendedDirection |
+              ScrollSnapFlags::IntendedEndPosition));
+      return false;
+    }
+    return true;
+  }
+
+  virtual bool WantsRepaints() override { return false; }
+
+  // Tell the overscroll animation about the pan momentum event. For each axis,
+  // the overscroll animation may start, stop, or continue managing that axis in
+  // response to the pan momentum event
+  void HandlePanMomentum(const ParentLayerPoint& aDisplacement) {
+    float xOverscroll = mApzc.mX.GetOverscroll();
+    if ((xOverscroll > 0 && aDisplacement.x > 0) ||
+        (xOverscroll < 0 && aDisplacement.x < 0)) {
+      if (!mApzc.mX.IsOverscrollAnimationRunning()) {
+        // Start a new overscroll animation on this axis, if there is no
+        // overscroll animation running and if the pan momentum displacement
+        // the pan momentum displacement is the same direction of the current
+        // overscroll.
+        mApzc.mX.StartOverscrollAnimation(mApzc.mX.GetVelocity());
+        mOverscrollSideBits |=
+            xOverscroll > 0 ? SideBits::eRight : SideBits::eLeft;
+      }
+    } else if ((xOverscroll > 0 && aDisplacement.x < 0) ||
+               (xOverscroll < 0 && aDisplacement.x > 0)) {
+      // Otherwise, stop the animation in the direction so that it won't clobber
+      // subsequent pan momentum scrolling.
+      mApzc.mX.EndOverscrollAnimation();
+    }
+
+    // Same as above but for Y axis.
+    float yOverscroll = mApzc.mY.GetOverscroll();
+    if ((yOverscroll > 0 && aDisplacement.y > 0) ||
+        (yOverscroll < 0 && aDisplacement.y < 0)) {
+      if (!mApzc.mY.IsOverscrollAnimationRunning()) {
+        mApzc.mY.StartOverscrollAnimation(mApzc.mY.GetVelocity());
+        mOverscrollSideBits |=
+            yOverscroll > 0 ? SideBits::eBottom : SideBits::eTop;
+      }
+    } else if ((yOverscroll > 0 && aDisplacement.y < 0) ||
+               (yOverscroll < 0 && aDisplacement.y > 0)) {
+      mApzc.mY.EndOverscrollAnimation();
+    }
+  }
+
+  ScrollDirections GetDirections() const {
+    ScrollDirections directions;
+    if (mApzc.mX.IsOverscrollAnimationRunning()) {
+      directions += ScrollDirection::eHorizontal;
+    }
+    if (mApzc.mY.IsOverscrollAnimationRunning()) {
+      directions += ScrollDirection::eVertical;
+    }
+    return directions;
+  };
+
+  OverscrollAnimation* AsOverscrollAnimation() override { return this; }
+
+  bool IsManagingXAxis() const {
+    return mApzc.mX.IsOverscrollAnimationRunning();
+  }
+  bool IsManagingYAxis() const {
+    return mApzc.mY.IsOverscrollAnimationRunning();
+  }
+
+ private:
+  AsyncPanZoomController& mApzc;
+  SideBits mOverscrollSideBits;
+};
+
+// Base class for different overscroll effects;
+class OverscrollEffectBase {
+ public:
+  virtual ~OverscrollEffectBase() = default;
+
+  // Try to increase the amount of overscroll by |aOverscroll|. Limited to
+  // directions contained in |aOverscrollableDirections|. Components of
+  // |aOverscroll| in directions that are successfully consumed are dropped.
+  virtual void ConsumeOverscroll(
+      ParentLayerPoint& aOverscroll,
+      ScrollDirections aOverscrollableDirections) = 0;
+
+  // Relieve overscroll. Depending on the implementation, the relief may
+  // be immediate, or gradual (e.g. after an animation) but this starts
+  // the process. |aVelocity| is the current velocity of the APZC, and
+  // |aOverscrollSideBits| contains the side(s) at which the APZC is
+  // overscrolled.
+  virtual void RelieveOverscroll(const ParentLayerPoint& aVelocity,
+                                 SideBits aOverscrollSideBits) = 0;
+
+  virtual bool IsOverscrolled() const = 0;
+
+  // Similarly to RelieveOverscroll(), but has immediate effect
+  // (no animation).
+  virtual void ClearOverscroll() = 0;
+};
+
+// A generic overscroll effect, implemented by AsyncPanZoomController itself.
+class GenericOverscrollEffect : public OverscrollEffectBase {
+ public:
+  explicit GenericOverscrollEffect(AsyncPanZoomController& aApzc)
+      : mApzc(aApzc) {}
+
+  void ConsumeOverscroll(ParentLayerPoint& aOverscroll,
+                         ScrollDirections aOverscrollableDirections) override {
+    if (aOverscrollableDirections.contains(ScrollDirection::eHorizontal)) {
+      mApzc.mX.OverscrollBy(aOverscroll.x);
+      aOverscroll.x = 0;
+    }
+
+    if (aOverscrollableDirections.contains(ScrollDirection::eVertical)) {
+      mApzc.mY.OverscrollBy(aOverscroll.y);
+      aOverscroll.y = 0;
+    }
+
+    if (!aOverscrollableDirections.isEmpty()) {
+      mApzc.ScheduleComposite();
+    }
+  }
+
+  void RelieveOverscroll(const ParentLayerPoint& aVelocity,
+                         SideBits aOverscrollSideBits) override {
+    mApzc.StartOverscrollAnimation(aVelocity, aOverscrollSideBits);
+  }
+
+  bool IsOverscrolled() const override {
+    return mApzc.IsPhysicallyOverscrolled();
+  }
+
+  void ClearOverscroll() override { mApzc.ClearPhysicalOverscroll(); }
+
+ private:
+  AsyncPanZoomController& mApzc;
+};
+
+// A widget-specific overscroll effect, implemented by the widget via
+// GeckoContentController.
+class WidgetOverscrollEffect : public OverscrollEffectBase {
+ public:
+  explicit WidgetOverscrollEffect(AsyncPanZoomController& aApzc)
+      : mApzc(aApzc), mIsOverscrolled(false) {}
+
+  void ConsumeOverscroll(ParentLayerPoint& aOverscroll,
+                         ScrollDirections aOverscrollableDirections) override {
+    RefPtr<GeckoContentController> controller =
+        mApzc.GetGeckoContentController();
+    if (controller && !aOverscrollableDirections.isEmpty()) {
+      mIsOverscrolled = true;
+      controller->UpdateOverscrollOffset(mApzc.GetGuid(), aOverscroll.x,
+                                         aOverscroll.y, mApzc.IsRootContent());
+      aOverscroll = ParentLayerPoint();
+    }
+  }
+
+  void RelieveOverscroll(const ParentLayerPoint& aVelocity,
+                         SideBits aOverscrollSideBits) override {
+    RefPtr<GeckoContentController> controller =
+        mApzc.GetGeckoContentController();
+    // From APZC's point of view, consider it to no longer be overscrolled
+    // as soon as RelieveOverscroll() is called. The widget may use a
+    // delay or animation until the relieving of the overscroll is complete,
+    // but we don't have any insight into that.
+    mIsOverscrolled = false;
+    if (controller) {
+      controller->UpdateOverscrollVelocity(mApzc.GetGuid(), aVelocity.x,
+                                           aVelocity.y, mApzc.IsRootContent());
+    }
+  }
+
+  bool IsOverscrolled() const override { return mIsOverscrolled; }
+
+  void ClearOverscroll() override {
+    RelieveOverscroll(ParentLayerPoint(), SideBits() /* ignored */);
+  }
+
+ private:
+  AsyncPanZoomController& mApzc;
+  bool mIsOverscrolled;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_Overscroll_h
diff --git a/gfx/layers/apz/src/OverscrollHandoffState.cpp b/gfx/layers/apz/src/OverscrollHandoffState.cpp
new file mode 100644
index 0000000000..564d65c0e4
--- /dev/null
+++ b/gfx/layers/apz/src/OverscrollHandoffState.cpp
@@ -0,0 +1,245 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "OverscrollHandoffState.h"
+
+#include <algorithm>  // for std::stable_sort
+#include "mozilla/Assertions.h"
+#include "mozilla/FloatingPoint.h"
+#include "AsyncPanZoomController.h"
+
+namespace mozilla {
+namespace layers {
+
+OverscrollHandoffChain::~OverscrollHandoffChain() = default;
+
+void OverscrollHandoffChain::Add(AsyncPanZoomController* aApzc) {
+  mChain.push_back(aApzc);
+}
+
+struct CompareByScrollPriority {
+  bool operator()(const RefPtr<AsyncPanZoomController>& a,
+                  const RefPtr<AsyncPanZoomController>& b) const {
+    return a->HasScrollgrab() && !b->HasScrollgrab();
+  }
+};
+
+void OverscrollHandoffChain::SortByScrollPriority() {
+  // The sorting being stable ensures that the relative order between
+  // non-scrollgrabbing APZCs remains child -> parent.
+  // (The relative order between scrollgrabbing APZCs will also remain
+  // child -> parent, though that's just an artefact of the implementation
+  // and users of 'scrollgrab' should not rely on this.)
+  std::stable_sort(mChain.begin(), mChain.end(), CompareByScrollPriority());
+}
+
+const RefPtr<AsyncPanZoomController>& OverscrollHandoffChain::GetApzcAtIndex(
+    uint32_t aIndex) const {
+  MOZ_ASSERT(aIndex < Length());
+  return mChain[aIndex];
+}
+
+uint32_t OverscrollHandoffChain::IndexOf(
+    const AsyncPanZoomController* aApzc) const {
+  uint32_t i;
+  for (i = 0; i < Length(); ++i) {
+    if (mChain[i] == aApzc) {
+      break;
+    }
+  }
+  return i;
+}
+
+void OverscrollHandoffChain::ForEachApzc(APZCMethod aMethod) const {
+  for (uint32_t i = 0; i < Length(); ++i) {
+    (mChain[i]->*aMethod)();
+  }
+}
+
+bool OverscrollHandoffChain::AnyApzc(APZCPredicate aPredicate) const {
+  MOZ_ASSERT(Length() > 0);
+  for (uint32_t i = 0; i < Length(); ++i) {
+    if ((mChain[i]->*aPredicate)()) {
+      return true;
+    }
+  }
+  return false;
+}
+
+void OverscrollHandoffChain::FlushRepaints() const {
+  ForEachApzc(&AsyncPanZoomController::FlushRepaintForOverscrollHandoff);
+}
+
+void OverscrollHandoffChain::CancelAnimations(
+    CancelAnimationFlags aFlags) const {
+  MOZ_ASSERT(Length() > 0);
+  for (uint32_t i = 0; i < Length(); ++i) {
+    mChain[i]->CancelAnimation(aFlags);
+  }
+}
+
+void OverscrollHandoffChain::ClearOverscroll() const {
+  ForEachApzc(&AsyncPanZoomController::ClearOverscroll);
+}
+
+void OverscrollHandoffChain::SnapBackOverscrolledApzc(
+    const AsyncPanZoomController* aStart) const {
+  uint32_t i = IndexOf(aStart);
+  for (; i < Length(); ++i) {
+    AsyncPanZoomController* apzc = mChain[i];
+    if (!apzc->IsDestroyed()) {
+      apzc->SnapBackIfOverscrolled();
+    }
+  }
+}
+
+void OverscrollHandoffChain::SnapBackOverscrolledApzcForMomentum(
+    const AsyncPanZoomController* aStart,
+    const ParentLayerPoint& aVelocity) const {
+  uint32_t i = IndexOf(aStart);
+  for (; i < Length(); ++i) {
+    AsyncPanZoomController* apzc = mChain[i];
+    if (!apzc->IsDestroyed()) {
+      apzc->SnapBackIfOverscrolledForMomentum(aVelocity);
+    }
+  }
+}
+
+bool OverscrollHandoffChain::CanBePanned(
+    const AsyncPanZoomController* aApzc) const {
+  // Find |aApzc| in the handoff chain.
+  uint32_t i = IndexOf(aApzc);
+
+  // See whether any APZC in the handoff chain starting from |aApzc|
+  // has room to be panned.
+  for (uint32_t j = i; j < Length(); ++j) {
+    if (mChain[j]->IsPannable()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+bool OverscrollHandoffChain::CanScrollInDirection(
+    const AsyncPanZoomController* aApzc, ScrollDirection aDirection) const {
+  // Find |aApzc| in the handoff chain.
+  uint32_t i = IndexOf(aApzc);
+
+  // See whether any APZC in the handoff chain starting from |aApzc|
+  // has room to scroll in the given direction.
+  for (uint32_t j = i; j < Length(); ++j) {
+    if (mChain[j]->CanScroll(aDirection)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+bool OverscrollHandoffChain::HasOverscrolledApzc() const {
+  return AnyApzc(&AsyncPanZoomController::IsOverscrolled);
+}
+
+bool OverscrollHandoffChain::HasFastFlungApzc() const {
+  return AnyApzc(&AsyncPanZoomController::IsFlingingFast);
+}
+
+bool OverscrollHandoffChain::HasAutoscrollApzc() const {
+  return AnyApzc(&AsyncPanZoomController::IsAutoscroll);
+}
+
+RefPtr<AsyncPanZoomController> OverscrollHandoffChain::FindFirstScrollable(
+    const InputData& aInput, ScrollDirections* aOutAllowedScrollDirections,
+    IncludeOverscroll aIncludeOverscroll) const {
+  // Start by allowing scrolling in both directions. As we do handoff
+  // overscroll-behavior may restrict one or both of the directions.
+  *aOutAllowedScrollDirections += ScrollDirection::eVertical;
+  *aOutAllowedScrollDirections += ScrollDirection::eHorizontal;
+
+  for (size_t i = 0; i < Length(); i++) {
+    if (mChain[i]->CanScroll(aInput)) {
+      return mChain[i];
+    }
+
+    // If there is any directions we allow overscroll effects on the root
+    // content APZC (i.e. the overscroll-behavior of the root one is not
+    // `none`), we consider the APZC can be scrollable in terms of pan gestures
+    // because it causes overscrolling even if it's not able to scroll to the
+    // direction.
+    if (StaticPrefs::apz_overscroll_enabled() && bool(aIncludeOverscroll) &&
+        // FIXME: Bug 1707491: Drop this pan gesture input check.
+        aInput.mInputType == PANGESTURE_INPUT && mChain[i]->IsRootContent()) {
+      // Check whether the root content APZC is also overscrollable governed by
+      // overscroll-behavior in the same directions where we allow scrolling
+      // handoff and where we are going to scroll, if it matches we do handoff
+      // to the root content APZC.
+      // In other words, if the root content is not scrollable, we don't
+      // handoff.
+      ScrollDirections allowedOverscrollDirections =
+          mChain[i]->GetOverscrollableDirections();
+      ParentLayerPoint delta = mChain[i]->GetDeltaForEvent(aInput);
+      if (mChain[i]->IsZero(delta.x)) {
+        allowedOverscrollDirections -= ScrollDirection::eHorizontal;
+      }
+      if (mChain[i]->IsZero(delta.y)) {
+        allowedOverscrollDirections -= ScrollDirection::eVertical;
+      }
+
+      allowedOverscrollDirections &= *aOutAllowedScrollDirections;
+      if (!allowedOverscrollDirections.isEmpty()) {
+        *aOutAllowedScrollDirections = allowedOverscrollDirections;
+        return mChain[i];
+      }
+    }
+
+    *aOutAllowedScrollDirections &= mChain[i]->GetAllowedHandoffDirections();
+    if (aOutAllowedScrollDirections->isEmpty()) {
+      return nullptr;
+    }
+  }
+  return nullptr;
+}
+
+std::tuple<bool, const AsyncPanZoomController*>
+OverscrollHandoffChain::ScrollingDownWillMoveDynamicToolbar(
+    const AsyncPanZoomController* aApzc) const {
+  MOZ_ASSERT(aApzc && !aApzc->IsRootContent(),
+             "Should be used for non-root APZC");
+
+  for (uint32_t i = IndexOf(aApzc); i < Length(); i++) {
+    if (mChain[i]->IsRootContent()) {
+      bool scrollable = mChain[i]->CanVerticalScrollWithDynamicToolbar();
+      return {scrollable, scrollable ? mChain[i].get() : nullptr};
+    }
+
+    if (mChain[i]->CanScrollDownwards()) {
+      return {false, nullptr};
+    }
+  }
+
+  return {false, nullptr};
+}
+
+bool OverscrollHandoffChain::ScrollingUpWillTriggerPullToRefresh(
+    const AsyncPanZoomController* aApzc) const {
+  MOZ_ASSERT(aApzc && !aApzc->IsRootContent(),
+             "Should be used for non-root APZC");
+
+  for (uint32_t i = IndexOf(aApzc); i < Length(); i++) {
+    if (mChain[i]->IsRootContent()) {
+      return mChain[i]->CanOverscrollUpwards();
+    }
+
+    if (!mChain[i]->CanOverscrollUpwards()) {
+      return false;
+    }
+  }
+  return false;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/OverscrollHandoffState.h b/gfx/layers/apz/src/OverscrollHandoffState.h
new file mode 100644
index 0000000000..28c89edd74
--- /dev/null
+++ b/gfx/layers/apz/src/OverscrollHandoffState.h
@@ -0,0 +1,206 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_OverscrollHandoffChain_h
+#define mozilla_layers_OverscrollHandoffChain_h
+
+#include <vector>
+#include "mozilla/RefPtr.h"   // for RefPtr
+#include "nsISupportsImpl.h"  // for NS_INLINE_DECL_THREADSAFE_REFCOUNTING
+#include "APZUtils.h"         // for CancelAnimationFlags
+#include "mozilla/layers/LayersTypes.h"  // for Layer::ScrollDirection
+#include "Units.h"                       // for ScreenPoint
+
+namespace mozilla {
+
+class InputData;
+
+namespace layers {
+
+class AsyncPanZoomController;
+
+/**
+ * This class represents the chain of APZCs along which overscroll is handed
+ * off. It is created by APZCTreeManager by starting from an initial APZC which
+ * is the target for input events, and following the scroll parent ID links
+ * (often but not always corresponding to parent pointers in the APZC tree),
+ * then adjusting for scrollgrab.
+ */
+class OverscrollHandoffChain {
+ protected:
+  // Reference-counted classes cannot have public destructors.
+  ~OverscrollHandoffChain();
+
+ public:
+  // Threadsafe so that the controller and sampler threads can both maintain
+  // nsRefPtrs to the same handoff chain.
+  // Mutable so that we can pass around the class by
+  // RefPtr<const OverscrollHandoffChain> and thus enforce that, once built,
+  // the chain is not modified.
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(OverscrollHandoffChain)
+
+  /*
+   * Methods for building the handoff chain.
+   * These should be used only by
+   * AsyncPanZoomController::BuildOverscrollHandoffChain().
+   */
+  void Add(AsyncPanZoomController* aApzc);
+  void SortByScrollPriority();
+
+  /*
+   * Methods for accessing the handoff chain.
+   */
+  uint32_t Length() const { return mChain.size(); }
+  const RefPtr<AsyncPanZoomController>& GetApzcAtIndex(uint32_t aIndex) const;
+  // Returns Length() if |aApzc| is not on this chain.
+  uint32_t IndexOf(const AsyncPanZoomController* aApzc) const;
+
+  /*
+   * Convenience methods for performing operations on APZCs in the chain.
+   */
+
+  // Flush repaints all the way up the chain.
+  void FlushRepaints() const;
+
+  // Cancel animations all the way up the chain.
+  void CancelAnimations(CancelAnimationFlags aFlags = Default) const;
+
+  // Clear overscroll all the way up the chain.
+  void ClearOverscroll() const;
+
+  // Snap back the APZC that is overscrolled on the subset of the chain from
+  // |aStart| onwards, if any.
+  void SnapBackOverscrolledApzc(const AsyncPanZoomController* aStart) const;
+
+  // Similar to above SnapbackOverscrolledApzc but for pan gestures with
+  // momentum events, this function doesn't end up calling each APZC's
+  // ScrollSnap.
+  // |aVelocity| is the initial velocity of |aStart|.
+  void SnapBackOverscrolledApzcForMomentum(
+      const AsyncPanZoomController* aStart,
+      const ParentLayerPoint& aVelocity) const;
+
+  // Determine whether the given APZC, or any APZC further in the chain,
+  // has room to be panned.
+  bool CanBePanned(const AsyncPanZoomController* aApzc) const;
+
+  // Determine whether the given APZC, or any APZC further in the chain,
+  // can scroll in the given direction.
+  bool CanScrollInDirection(const AsyncPanZoomController* aApzc,
+                            ScrollDirection aDirection) const;
+
+  // Determine whether any APZC along this handoff chain is overscrolled.
+  bool HasOverscrolledApzc() const;
+
+  // Determine whether any APZC along this handoff chain has been flung fast.
+  bool HasFastFlungApzc() const;
+
+  // Determine whether any APZC along this handoff chain is autoscroll.
+  bool HasAutoscrollApzc() const;
+
+  // Find the first APZC in this handoff chain that can be scrolled by |aInput|.
+  // Since overscroll-behavior can restrict handoff in some directions,
+  // |aOutAllowedScrollDirections| is populated with the scroll directions
+  // in which scrolling of the returned APZC is allowed.
+  // |aIncludeOverscroll| is an optional flag whether to consider overscrollable
+  // as scrollable or not.
+  enum class IncludeOverscroll : bool { No, Yes };
+  RefPtr<AsyncPanZoomController> FindFirstScrollable(
+      const InputData& aInput, ScrollDirections* aOutAllowedScrollDirections,
+      IncludeOverscroll aIncludeOverscroll = IncludeOverscroll::Yes) const;
+
+  // Return a pair of true and the root content APZC if all non-root APZCs in
+  // this handoff chain starting from |aApzc| are not able to scroll downwards
+  // (i.e. there is no room to scroll downwards in each APZC respectively) and
+  // there is any contents covered by the dynamic toolbar, otherwise return a
+  // pair of false and nullptr.
+  std::tuple<bool, const AsyncPanZoomController*>
+  ScrollingDownWillMoveDynamicToolbar(
+      const AsyncPanZoomController* aApzc) const;
+
+  bool ScrollingUpWillTriggerPullToRefresh(
+      const AsyncPanZoomController* aApzc) const;
+
+ private:
+  std::vector<RefPtr<AsyncPanZoomController>> mChain;
+
+  typedef void (AsyncPanZoomController::*APZCMethod)();
+  typedef bool (AsyncPanZoomController::*APZCPredicate)() const;
+  void ForEachApzc(APZCMethod aMethod) const;
+  bool AnyApzc(APZCPredicate aPredicate) const;
+};
+
+/**
+ * This class groups the state maintained during overscroll handoff.
+ */
+struct OverscrollHandoffState {
+  OverscrollHandoffState(const OverscrollHandoffChain& aChain,
+                         const ScreenPoint& aPanDistance,
+                         ScrollSource aScrollSource)
+      : mChain(aChain),
+        mChainIndex(0),
+        mPanDistance(aPanDistance),
+        mScrollSource(aScrollSource) {}
+
+  // The chain of APZCs along which we hand off scroll.
+  // This is const to indicate that the chain does not change over the
+  // course of handoff.
+  const OverscrollHandoffChain& mChain;
+
+  // The index of the APZC in the chain that we are currently giving scroll to.
+  // This is non-const to indicate that this changes over the course of handoff.
+  uint32_t mChainIndex;
+
+  // The total distance since touch-start of the pan that triggered the
+  // handoff. This is const to indicate that it does not change over the
+  // course of handoff.
+  // The x/y components of this are non-negative.
+  const ScreenPoint mPanDistance;
+
+  ScrollSource mScrollSource;
+
+  // The total amount of actual movement that this scroll caused, including
+  // scrolling and changes to overscroll. This starts at zero and is accumulated
+  // over the course of the handoff.
+  ScreenPoint mTotalMovement;
+};
+
+/*
+ * This class groups the state maintained during fling handoff.
+ */
+struct FlingHandoffState {
+  // The velocity of the fling being handed off.
+  ParentLayerPoint mVelocity;
+
+  // The chain of APZCs along which we hand off the fling.
+  // Unlike in OverscrollHandoffState, this is stored by RefPtr because
+  // otherwise it may not stay alive for the entire handoff.
+  RefPtr<const OverscrollHandoffChain> mChain;
+
+  // The time duration between the touch start and the touch move that started
+  // the pan gesture which triggered this fling. In other words, the time it
+  // took for the finger to move enough to cross the touch slop threshold.
+  // Nothing if this fling was not immediately caused by a touch pan.
+  Maybe<TimeDuration> mTouchStartRestingTime;
+
+  // The slowest panning velocity encountered during the pan that triggered this
+  // fling.
+  ParentLayerCoord mMinPanVelocity;
+
+  // Whether handoff has happened by this point, or we're still process
+  // the original fling.
+  bool mIsHandoff;
+
+  // The single APZC that was scrolled by the pan that started this fling.
+  // The fling is only allowed to scroll this APZC, too.
+  // Used only if immediate scroll handoff is disallowed.
+  RefPtr<const AsyncPanZoomController> mScrolledApzc;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* mozilla_layers_OverscrollHandoffChain_h */
diff --git a/gfx/layers/apz/src/OvershootDetector.cpp b/gfx/layers/apz/src/OvershootDetector.cpp
new file mode 100644
index 0000000000..c713ce8d1a
--- /dev/null
+++ b/gfx/layers/apz/src/OvershootDetector.cpp
@@ -0,0 +1,69 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "OvershootDetector.h"
+
+#include "InputData.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+
+namespace mozilla {
+namespace layers {
+
+const TimeDuration kOvershootInterval = TimeDuration::FromMilliseconds(500);
+
+static Maybe<Side> GetScrollDirection(const ScrollWheelInput& aInput) {
+  // If the wheel input is scrolling on both axes, we just take the y-axis.
+  // This is mostly out of laziness, because this code is for experiment
+  // purposes and just needs to handle the cases that will dominate in the data.
+  if (aInput.mDeltaY > 0) {
+    return Some(Side::eSideBottom);
+  }
+  if (aInput.mDeltaY < 0) {
+    return Some(Side::eSideTop);
+  }
+  if (aInput.mDeltaX > 0) {
+    return Some(Side::eSideRight);
+  }
+  if (aInput.mDeltaX < 0) {
+    return Some(Side::eSideLeft);
+  }
+  return Nothing();
+}
+
+void OvershootDetector::Update(const ScrollWheelInput& aInput) {
+  TimeStamp inputTime = aInput.mTimeStamp;
+  Maybe<Side> inputDirection = GetScrollDirection(aInput);
+  if (mLastTimeStamp && (inputTime - mLastTimeStamp) < kOvershootInterval &&
+      mLastDirection && inputDirection) {
+    // The new input we got happened within the kOvershootInterval of the last
+    // input. Let's see if the direction is reversed; if so, we accumulate to
+    // telemetry.
+    bool reversed = false;
+    switch (*mLastDirection) {
+      case Side::eSideBottom:
+        reversed = (*inputDirection == Side::eSideTop);
+        break;
+      case Side::eSideTop:
+        reversed = (*inputDirection == Side::eSideBottom);
+        break;
+      case Side::eSideRight:
+        reversed = (*inputDirection == Side::eSideLeft);
+        break;
+      case Side::eSideLeft:
+        reversed = (*inputDirection == Side::eSideRight);
+        break;
+    }
+    if (reversed) {
+      Telemetry::ScalarAdd(Telemetry::ScalarID::APZ_SCROLLWHEEL_OVERSHOOT, 1);
+    }
+  }
+
+  mLastTimeStamp = inputTime;
+  mLastDirection = inputDirection;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/OvershootDetector.h b/gfx/layers/apz/src/OvershootDetector.h
new file mode 100644
index 0000000000..55beee5df2
--- /dev/null
+++ b/gfx/layers/apz/src/OvershootDetector.h
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_OvershootDetector_h_
+#define mozilla_layers_OvershootDetector_h_
+
+#include "mozilla/gfx/Types.h"  // for Side
+#include "mozilla/Maybe.h"
+#include "mozilla/TimeStamp.h"
+
+namespace mozilla {
+
+class ScrollWheelInput;
+
+namespace layers {
+
+/*
+ * Handles detection and telemetry reporting of overshoot events, which are
+ * situations where the user changes scrolling direction shortly after a
+ * previous scroll in the other direction, indicating that the user likely
+ * scrolled too far and had to backtrack.
+ *
+ * Currently this data collection is focused on forms of input that trigger
+ * animated rather than instant scrolling. That includes ScrollWheelInput
+ * (currently implemented) and KeyboardInput (planned in bug 1838568).
+ */
+class OvershootDetector {
+ public:
+  OvershootDetector() = default;
+
+  // Updates the internal state machine with a new incoming scrollwheel input.
+  void Update(const ScrollWheelInput& aInput);
+
+ private:
+  TimeStamp mLastTimeStamp;
+  Maybe<mozilla::Side> mLastDirection;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_OvershootDetector_h_
diff --git a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp
new file mode 100644
index 0000000000..28730ae5aa
--- /dev/null
+++ b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp
@@ -0,0 +1,74 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "PotentialCheckerboardDurationTracker.h"
+
+#include "mozilla/Telemetry.h"           // for Telemetry
+#include "mozilla/glean/GleanMetrics.h"  // for Glean telemetry
+
+namespace mozilla {
+namespace layers {
+
+PotentialCheckerboardDurationTracker::PotentialCheckerboardDurationTracker()
+    : mInCheckerboard(false), mInTransform(false) {}
+
+void PotentialCheckerboardDurationTracker::CheckerboardSeen() {
+  // This might get called while mInCheckerboard is already true
+  if (!Tracking()) {
+    mCurrentPeriodStart = TimeStamp::Now();
+  }
+  mInCheckerboard = true;
+}
+
+void PotentialCheckerboardDurationTracker::CheckerboardDone(
+    bool aRecordTelemetry) {
+  MOZ_ASSERT(Tracking());
+  mInCheckerboard = false;
+  if (!Tracking()) {
+    if (aRecordTelemetry) {
+      mozilla::glean::gfx_checkerboard::potential_duration
+          .AccumulateRawDuration(mCurrentPeriodStart - TimeStamp::Now());
+    }
+  }
+}
+
+void PotentialCheckerboardDurationTracker::InTransform(bool aInTransform,
+                                                       bool aRecordTelemetry) {
+  if (aInTransform == mInTransform) {
+    // no-op
+    return;
+  }
+
+  if (!Tracking()) {
+    // Because !Tracking(), mInTransform must be false, and so aInTransform
+    // must be true (or we would have early-exited this function already).
+    // Therefore, we are starting a potential checkerboard period.
+    mInTransform = aInTransform;
+    mCurrentPeriodStart = TimeStamp::Now();
+    return;
+  }
+
+  mInTransform = aInTransform;
+
+  if (!Tracking()) {
+    // Tracking() must have been true at the start of this function, or we
+    // would have taken the other !Tracking branch above. If it's false now,
+    // it means we just stopped tracking, so we are ending a potential
+    // checkerboard period.
+    if (aRecordTelemetry) {
+      mozilla::Telemetry::AccumulateTimeDelta(
+          mozilla::Telemetry::CHECKERBOARD_POTENTIAL_DURATION,
+          mCurrentPeriodStart);
+    }
+  }
+}
+
+bool PotentialCheckerboardDurationTracker::Tracking() const {
+  return mInTransform || mInCheckerboard;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h
new file mode 100644
index 0000000000..58786f32af
--- /dev/null
+++ b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h
@@ -0,0 +1,61 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_PotentialCheckerboardDurationTracker_h
+#define mozilla_layers_PotentialCheckerboardDurationTracker_h
+
+#include "mozilla/TimeStamp.h"
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * This class allows the owner to track the duration of time considered
+ * "potentially checkerboarding". This is the union of two possibly-intersecting
+ * sets of time periods. The first set is that in which checkerboarding was
+ * actually happening, since by definition it could potentially be happening.
+ * The second set is that in which the APZC is actively transforming content
+ * in the compositor, since it could potentially transform it so as to display
+ * checkerboarding to the user.
+ * The caller of this class calls the appropriate methods to indicate the start
+ * and stop of these two sets, and this class manages accumulating the union
+ * of the various durations.
+ */
+class PotentialCheckerboardDurationTracker {
+ public:
+  PotentialCheckerboardDurationTracker();
+
+  /**
+   * This should be called if checkerboarding is encountered. It can be called
+   * multiple times during a checkerboard event.
+   */
+  void CheckerboardSeen();
+  /**
+   * This should be called when checkerboarding is done. It must have been
+   * preceded by one or more calls to CheckerboardSeen().
+   */
+  void CheckerboardDone(bool aRecordTelemetry);
+
+  /**
+   * This should be called at composition time, to indicate if the APZC is in
+   * a transforming state or not.
+   */
+  void InTransform(bool aInTransform, bool aRecordTelemetry);
+
+ private:
+  bool Tracking() const;
+
+ private:
+  bool mInCheckerboard;
+  bool mInTransform;
+
+  TimeStamp mCurrentPeriodStart;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_PotentialCheckerboardDurationTracker_h
diff --git a/gfx/layers/apz/src/QueuedInput.cpp b/gfx/layers/apz/src/QueuedInput.cpp
new file mode 100644
index 0000000000..87ffe7250e
--- /dev/null
+++ b/gfx/layers/apz/src/QueuedInput.cpp
@@ -0,0 +1,44 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "QueuedInput.h"
+
+#include "AsyncPanZoomController.h"
+#include "InputBlockState.h"
+#include "InputData.h"
+#include "OverscrollHandoffState.h"
+
+namespace mozilla {
+namespace layers {
+
+QueuedInput::QueuedInput(const MultiTouchInput& aInput, TouchBlockState& aBlock)
+    : mInput(MakeUnique<MultiTouchInput>(aInput)), mBlock(&aBlock) {}
+
+QueuedInput::QueuedInput(const ScrollWheelInput& aInput,
+                         WheelBlockState& aBlock)
+    : mInput(MakeUnique<ScrollWheelInput>(aInput)), mBlock(&aBlock) {}
+
+QueuedInput::QueuedInput(const MouseInput& aInput, DragBlockState& aBlock)
+    : mInput(MakeUnique<MouseInput>(aInput)), mBlock(&aBlock) {}
+
+QueuedInput::QueuedInput(const PanGestureInput& aInput,
+                         PanGestureBlockState& aBlock)
+    : mInput(MakeUnique<PanGestureInput>(aInput)), mBlock(&aBlock) {}
+
+QueuedInput::QueuedInput(const PinchGestureInput& aInput,
+                         PinchGestureBlockState& aBlock)
+    : mInput(MakeUnique<PinchGestureInput>(aInput)), mBlock(&aBlock) {}
+
+QueuedInput::QueuedInput(const KeyboardInput& aInput,
+                         KeyboardBlockState& aBlock)
+    : mInput(MakeUnique<KeyboardInput>(aInput)), mBlock(&aBlock) {}
+
+InputData* QueuedInput::Input() { return mInput.get(); }
+
+InputBlockState* QueuedInput::Block() { return mBlock.get(); }
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/QueuedInput.h b/gfx/layers/apz/src/QueuedInput.h
new file mode 100644
index 0000000000..fcc2f2090a
--- /dev/null
+++ b/gfx/layers/apz/src/QueuedInput.h
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_QueuedInput_h
+#define mozilla_layers_QueuedInput_h
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla {
+
+class InputData;
+class MultiTouchInput;
+class ScrollWheelInput;
+class MouseInput;
+class PanGestureInput;
+class PinchGestureInput;
+class KeyboardInput;
+
+namespace layers {
+
+class InputBlockState;
+class TouchBlockState;
+class WheelBlockState;
+class DragBlockState;
+class PanGestureBlockState;
+class PinchGestureBlockState;
+class KeyboardBlockState;
+
+/**
+ * This lightweight class holds a pointer to an input event that has not yet
+ * been completely processed, along with the input block that the input event
+ * is associated with.
+ */
+class QueuedInput {
+ public:
+  QueuedInput(const MultiTouchInput& aInput, TouchBlockState& aBlock);
+  QueuedInput(const ScrollWheelInput& aInput, WheelBlockState& aBlock);
+  QueuedInput(const MouseInput& aInput, DragBlockState& aBlock);
+  QueuedInput(const PanGestureInput& aInput, PanGestureBlockState& aBlock);
+  QueuedInput(const PinchGestureInput& aInput, PinchGestureBlockState& aBlock);
+  QueuedInput(const KeyboardInput& aInput, KeyboardBlockState& aBlock);
+
+  InputData* Input();
+  InputBlockState* Block();
+
+ private:
+  // A copy of the input event that is provided to the constructor. This must
+  // be non-null, and is owned by this QueuedInput instance (hence the
+  // UniquePtr).
+  UniquePtr<InputData> mInput;
+  // A pointer to the block that the input event is associated with. This must
+  // be non-null.
+  RefPtr<InputBlockState> mBlock;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_QueuedInput_h
diff --git a/gfx/layers/apz/src/RecentEventsBuffer.h b/gfx/layers/apz/src/RecentEventsBuffer.h
new file mode 100644
index 0000000000..d1ae5797af
--- /dev/null
+++ b/gfx/layers/apz/src/RecentEventsBuffer.h
@@ -0,0 +1,83 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_RecentEventsBuffer_h
+#define mozilla_layers_RecentEventsBuffer_h
+
+#include <deque>
+
+#include "mozilla/TimeStamp.h"
+
+namespace mozilla {
+namespace layers {
+/**
+ * RecentEventsBuffer: maintains an age constrained buffer of events
+ *
+ * Intended for use with elements of type InputData, but the only requirement
+ * is a member "mTimeStamp" of type TimeStamp
+ */
+template <typename Event>
+class RecentEventsBuffer {
+ public:
+  explicit RecentEventsBuffer(TimeDuration maxAge);
+
+  void push(Event event);
+  void clear();
+
+  typedef typename std::deque<Event>::size_type size_type;
+  size_type size() { return mBuffer.size(); }
+
+  // Delegate to container for iterators
+  typedef typename std::deque<Event>::iterator iterator;
+  typedef typename std::deque<Event>::const_iterator const_iterator;
+  iterator begin() { return mBuffer.begin(); }
+  iterator end() { return mBuffer.end(); }
+  const_iterator cbegin() const { return mBuffer.cbegin(); }
+  const_iterator cend() const { return mBuffer.cend(); }
+
+  // Also delegate for front/back
+  typedef typename std::deque<Event>::reference reference;
+  typedef typename std::deque<Event>::const_reference const_reference;
+  reference front() { return mBuffer.front(); }
+  reference back() { return mBuffer.back(); }
+  const_reference front() const { return mBuffer.front(); }
+  const_reference back() const { return mBuffer.back(); }
+
+ private:
+  TimeDuration mMaxAge;
+  std::deque<Event> mBuffer;
+};
+
+template <typename Event>
+RecentEventsBuffer<Event>::RecentEventsBuffer(TimeDuration maxAge)
+    : mMaxAge(maxAge), mBuffer() {}
+
+template <typename Event>
+void RecentEventsBuffer<Event>::push(Event event) {
+  // Events must be pushed in chronological order
+  MOZ_ASSERT(mBuffer.empty() || mBuffer.back().mTimeStamp <= event.mTimeStamp);
+
+  mBuffer.push_back(event);
+
+  // Flush all events older than the given lifetime
+  TimeStamp bound = event.mTimeStamp - mMaxAge;
+  while (!mBuffer.empty()) {
+    if (mBuffer.front().mTimeStamp >= bound) {
+      break;
+    }
+    mBuffer.pop_front();
+  }
+}
+
+template <typename Event>
+void RecentEventsBuffer<Event>::clear() {
+  mBuffer.clear();
+}
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_RecentEventsBuffer_h
diff --git a/gfx/layers/apz/src/SampledAPZCState.cpp b/gfx/layers/apz/src/SampledAPZCState.cpp
new file mode 100644
index 0000000000..712a46a3b1
--- /dev/null
+++ b/gfx/layers/apz/src/SampledAPZCState.cpp
@@ -0,0 +1,111 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SampledAPZCState.h"
+#include "APZUtils.h"
+
+namespace mozilla {
+namespace layers {
+
+SampledAPZCState::SampledAPZCState() {}
+
+SampledAPZCState::SampledAPZCState(const FrameMetrics& aMetrics)
+    : mLayoutViewport(aMetrics.GetLayoutViewport()),
+      mVisualScrollOffset(aMetrics.GetVisualScrollOffset()),
+      mZoom(aMetrics.GetZoom()) {
+  RemoveFractionalAsyncDelta();
+}
+
+SampledAPZCState::SampledAPZCState(const FrameMetrics& aMetrics,
+                                   Maybe<CompositionPayload>&& aPayload,
+                                   APZScrollGeneration aGeneration)
+    : mLayoutViewport(aMetrics.GetLayoutViewport()),
+      mVisualScrollOffset(aMetrics.GetVisualScrollOffset()),
+      mZoom(aMetrics.GetZoom()),
+      mScrollPayload(std::move(aPayload)),
+      mGeneration(aGeneration) {
+  RemoveFractionalAsyncDelta();
+}
+
+bool SampledAPZCState::operator==(const SampledAPZCState& aOther) const {
+  // The payload doesn't factor into equality, that just comes along for
+  // the ride.
+  return mLayoutViewport.IsEqualEdges(aOther.mLayoutViewport) &&
+         mVisualScrollOffset == aOther.mVisualScrollOffset &&
+         mZoom == aOther.mZoom;
+}
+
+bool SampledAPZCState::operator!=(const SampledAPZCState& aOther) const {
+  return !(*this == aOther);
+}
+
+Maybe<CompositionPayload> SampledAPZCState::TakeScrollPayload() {
+  return std::move(mScrollPayload);
+}
+
+void SampledAPZCState::UpdateScrollProperties(const FrameMetrics& aMetrics) {
+  mLayoutViewport = aMetrics.GetLayoutViewport();
+  mVisualScrollOffset = aMetrics.GetVisualScrollOffset();
+}
+
+void SampledAPZCState::UpdateScrollPropertiesWithRelativeDelta(
+    const FrameMetrics& aMetrics, const CSSPoint& aRelativeDelta) {
+  mVisualScrollOffset += aRelativeDelta;
+  KeepLayoutViewportEnclosingVisualViewport(aMetrics);
+}
+
+void SampledAPZCState::UpdateZoomProperties(const FrameMetrics& aMetrics) {
+  mZoom = aMetrics.GetZoom();
+}
+
+void SampledAPZCState::ClampVisualScrollOffset(const FrameMetrics& aMetrics) {
+  // Make sure that we use the local mZoom to do these calculations, because the
+  // one on aMetrics might be newer.
+  CSSRect scrollRange = FrameMetrics::CalculateScrollRange(
+      aMetrics.GetScrollableRect(), aMetrics.GetCompositionBounds(), mZoom);
+  mVisualScrollOffset = scrollRange.ClampPoint(mVisualScrollOffset);
+
+  KeepLayoutViewportEnclosingVisualViewport(aMetrics);
+}
+
+void SampledAPZCState::ZoomBy(float aScale) { mZoom.scale *= aScale; }
+
+void SampledAPZCState::RemoveFractionalAsyncDelta() {
+  // This function is a performance hack. With non-WebRender, having small
+  // fractional deltas between the layout offset and scroll offset on
+  // container layers can trigger the creation of a temporary surface during
+  // composition, because it produces a non-integer translation that doesn't
+  // play well with layer clips. So we detect the case where the delta is
+  // uselessly small (0.01 parentlayer pixels or less) and tweak the sampled
+  // scroll offset to eliminate it. By doing this here at sample time rather
+  // than elsewhere in the pipeline we are least likely to break assumptions
+  // and invariants elsewhere in the code, since sampling effectively takes
+  // a snapshot of APZ state (decoupling it from APZ assumptions) and provides
+  // it as an input to the compositor (so all compositor state should be
+  // internally consistent based on this input).
+  if (mLayoutViewport.TopLeft() == mVisualScrollOffset) {
+    return;
+  }
+  const ParentLayerCoord EPSILON = 0.01;
+  ParentLayerPoint paintedOffset = mLayoutViewport.TopLeft() * mZoom;
+  ParentLayerPoint asyncOffset = mVisualScrollOffset * mZoom;
+  if (FuzzyEqualsAdditive(paintedOffset.x, asyncOffset.x, EPSILON) &&
+      FuzzyEqualsAdditive(paintedOffset.y, asyncOffset.y, EPSILON)) {
+    mVisualScrollOffset = mLayoutViewport.TopLeft();
+  }
+}
+
+void SampledAPZCState::KeepLayoutViewportEnclosingVisualViewport(
+    const FrameMetrics& aMetrics) {
+  FrameMetrics::KeepLayoutViewportEnclosingVisualViewport(
+      CSSRect(mVisualScrollOffset,
+              FrameMetrics::CalculateCompositedSizeInCssPixels(
+                  aMetrics.GetCompositionBounds(), mZoom)),
+      aMetrics.GetScrollableRect(), mLayoutViewport);
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/SampledAPZCState.h b/gfx/layers/apz/src/SampledAPZCState.h
new file mode 100644
index 0000000000..a521eeaf87
--- /dev/null
+++ b/gfx/layers/apz/src/SampledAPZCState.h
@@ -0,0 +1,72 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_SampledAPZCState_h
+#define mozilla_layers_SampledAPZCState_h
+
+#include "FrameMetrics.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ScrollGeneration.h"
+
+namespace mozilla {
+namespace layers {
+
+class SampledAPZCState {
+ public:
+  SampledAPZCState();
+  explicit SampledAPZCState(const FrameMetrics& aMetrics);
+  SampledAPZCState(const FrameMetrics& aMetrics,
+                   Maybe<CompositionPayload>&& aPayload,
+                   APZScrollGeneration aGeneration);
+
+  bool operator==(const SampledAPZCState& aOther) const;
+  bool operator!=(const SampledAPZCState& aOther) const;
+
+  CSSRect GetLayoutViewport() const { return mLayoutViewport; }
+  CSSPoint GetVisualScrollOffset() const { return mVisualScrollOffset; }
+  CSSToParentLayerScale GetZoom() const { return mZoom; }
+  Maybe<CompositionPayload> TakeScrollPayload();
+  const APZScrollGeneration& Generation() const { return mGeneration; }
+
+  void UpdateScrollProperties(const FrameMetrics& aMetrics);
+  void UpdateScrollPropertiesWithRelativeDelta(const FrameMetrics& aMetrics,
+                                               const CSSPoint& aRelativeDelta);
+
+  void UpdateZoomProperties(const FrameMetrics& aMetrics);
+
+  /**
+   * Re-clamp mVisualScrollOffset to the scroll range specified by the provided
+   * metrics. This only needs to be called if the scroll offset changes
+   * outside of AsyncPanZoomController::SampleCompositedAsyncTransform().
+   * It also recalculates mLayoutViewport so that it continues to enclose
+   * the visual viewport. This only needs to be called if the
+   * layout viewport changes outside of SampleCompositedAsyncTransform().
+   */
+  void ClampVisualScrollOffset(const FrameMetrics& aMetrics);
+
+  void ZoomBy(float aScale);
+
+ private:
+  // These variables cache the layout viewport, scroll offset, and zoom stored
+  // in |Metrics()| at the time this class was constructed.
+  CSSRect mLayoutViewport;
+  CSSPoint mVisualScrollOffset;
+  CSSToParentLayerScale mZoom;
+  // An optional payload that rides along with the sampled state.
+  Maybe<CompositionPayload> mScrollPayload;
+  APZScrollGeneration mGeneration;
+
+  void RemoveFractionalAsyncDelta();
+  // A handy wrapper to call
+  // FrameMetrics::KeepLayoutViewportEnclosingVisualViewport with this
+  // SampledAPZCState and the given |aMetrics|.
+  void KeepLayoutViewportEnclosingVisualViewport(const FrameMetrics& aMetrics);
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_SampledAPZCState_h
diff --git a/gfx/layers/apz/src/ScrollThumbUtils.cpp b/gfx/layers/apz/src/ScrollThumbUtils.cpp
new file mode 100644
index 0000000000..814fa59759
--- /dev/null
+++ b/gfx/layers/apz/src/ScrollThumbUtils.cpp
@@ -0,0 +1,341 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScrollThumbUtils.h"
+#include "AsyncPanZoomController.h"
+#include "FrameMetrics.h"
+#include "UnitTransforms.h"
+#include "Units.h"
+#include "gfxPlatform.h"
+#include "mozilla/gfx/Matrix.h"
+#include "mozilla/StaticPrefs_toolkit.h"
+
+namespace mozilla {
+namespace layers {
+namespace apz {
+
+struct AsyncScrollThumbTransformer {
+  // Inputs
+  const LayerToParentLayerMatrix4x4& mCurrentTransform;
+  const gfx::Matrix4x4& mScrollableContentTransform;
+  AsyncPanZoomController* mApzc;
+  const FrameMetrics& mMetrics;
+  const ScrollbarData& mScrollbarData;
+  bool mScrollbarIsDescendant;
+
+  // Intermediate results
+  AsyncTransformComponentMatrix mAsyncTransform;
+  AsyncTransformComponentMatrix mScrollbarTransform;
+
+  LayerToParentLayerMatrix4x4 ComputeTransform();
+
+ private:
+  // Helper functions for ComputeTransform().
+
+  // If the thumb's orientation is along |aAxis|, add transformations
+  // of the thumb into |mScrollbarTransform|.
+  void ApplyTransformForAxis(const Axis& aAxis);
+
+  enum class ScrollThumbExtent { Start, End };
+
+  // Scale the thumb by |aScale| along |aAxis|, while keeping constant the
+  // position of the top denoted by |aExtent|.
+  void ScaleThumbBy(const Axis& aAxis, float aScale, ScrollThumbExtent aExtent);
+
+  // Translate the thumb along |aAxis| by |aTranslation| in "scrollbar space"
+  // (CSS pixels along the scrollbar track, similar to e.g.
+  // |mScrollbarData.mThumbStart|).
+  void TranslateThumb(const Axis& aAxis, OuterCSSCoord aTranslation);
+};
+
+void AsyncScrollThumbTransformer::TranslateThumb(const Axis& aAxis,
+                                                 OuterCSSCoord aTranslation) {
+  aAxis.PostTranslate(
+      mScrollbarTransform,
+      ViewAs<CSSPixel>(aTranslation,
+                       PixelCastJustification::CSSPixelsOfSurroundingContent) *
+          mMetrics.GetDevPixelsPerCSSPixel() *
+          LayoutDeviceToParentLayerScale(1.0));
+}
+
+void AsyncScrollThumbTransformer::ScaleThumbBy(const Axis& aAxis, float aScale,
+                                               ScrollThumbExtent aExtent) {
+  // To keep the position of the top of the thumb constant, the thumb needs to
+  // translated to compensate for the scale applied. The origin with respect to
+  // which the scale is applied is the origin of the layer tree, rather than
+  // the origin of the scroll thumb. This means that the space between the
+  // origin and the top of thumb (including the part of the scrollbar track
+  // above the thumb, the part of the scrollbar above the track (i.e. a
+  // scrollbar button if present), plus whatever content is above the scroll
+  // frame) is scaled too, effectively translating the thumb. We undo that
+  // translation here. (One can think of the adjustment being done to the
+  // translation here as a change of basis. We have a method to help with that,
+  // Matrix4x4::ChangeBasis(), but it wouldn't necessarily make the code cleaner
+  // in this case).
+  const OuterCSSCoord scrollTrackOrigin =
+      aAxis.GetPointOffset(
+          mMetrics.CalculateCompositionBoundsInOuterCssPixels().TopLeft()) +
+      mScrollbarData.mScrollTrackStart;
+  OuterCSSCoord thumbExtent = scrollTrackOrigin + mScrollbarData.mThumbStart;
+  if (aExtent == ScrollThumbExtent::End) {
+    thumbExtent += mScrollbarData.mThumbLength;
+  }
+  const OuterCSSCoord thumbExtentScaled = thumbExtent * aScale;
+  const OuterCSSCoord thumbExtentDelta = thumbExtentScaled - thumbExtent;
+
+  aAxis.PostScale(mScrollbarTransform, aScale);
+  TranslateThumb(aAxis, -thumbExtentDelta);
+}
+
+void AsyncScrollThumbTransformer::ApplyTransformForAxis(const Axis& aAxis) {
+  ParentLayerCoord asyncScroll = aAxis.GetTransformTranslation(mAsyncTransform);
+  const float asyncZoom = aAxis.GetTransformScale(mAsyncTransform);
+  const ParentLayerCoord overscroll =
+      aAxis.GetPointOffset(mApzc->GetOverscrollAmount());
+
+  bool haveAsyncZoom = !FuzzyEqualsAdditive(asyncZoom, 1.f);
+  if (!haveAsyncZoom && mApzc->IsZero(asyncScroll) &&
+      mApzc->IsZero(overscroll)) {
+    return;
+  }
+
+  OuterCSSCoord translation;
+  float scale = 1.0;
+
+  bool recalcMode = StaticPrefs::apz_scrollthumb_recalc();
+  if (recalcMode) {
+    // In this branch (taken when apz.scrollthumb.recalc=true), |translation|
+    // and |scale| are computed using the approach implemented in bug 1554795
+    // of fully recalculating the desired position and size using the logic
+    // that attempts to closely match the main-thread calculation.
+
+    const CSSRect visualViewportRect = mApzc->GetCurrentAsyncVisualViewport(
+        AsyncPanZoomController::eForCompositing);
+    const CSSCoord visualViewportLength =
+        aAxis.GetRectLength(visualViewportRect);
+
+    const CSSCoord maxMinPosDifference =
+        CSSCoord(
+            aAxis.GetRectLength(mMetrics.GetScrollableRect()).Truncated()) -
+        visualViewportLength;
+
+    OuterCSSCoord effectiveThumbLength = mScrollbarData.mThumbLength;
+
+    if (haveAsyncZoom) {
+      // The calculations here closely follow the main thread calculations at
+      // https://searchfox.org/mozilla-central/rev/0bf957f909ae1f3d19b43fd4edfc277342554836/layout/generic/nsGfxScrollFrame.cpp#6902-6927
+      // and
+      // https://searchfox.org/mozilla-central/rev/0bf957f909ae1f3d19b43fd4edfc277342554836/layout/xul/nsSliderFrame.cpp#587-614
+      // Any modifications there should be reflected here as well.
+      const CSSCoord pageIncrementMin =
+          static_cast<int>(visualViewportLength * 0.8);
+      CSSCoord pageIncrement;
+
+      CSSToLayoutDeviceScale deviceScale = mMetrics.GetDevPixelsPerCSSPixel();
+      if (*mScrollbarData.mDirection == ScrollDirection::eVertical) {
+        const CSSCoord lineScrollAmount =
+            (mApzc->GetScrollMetadata().GetLineScrollAmount() / deviceScale)
+                .height;
+        const double kScrollMultiplier =
+            StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+        CSSCoord increment = lineScrollAmount * kScrollMultiplier;
+
+        pageIncrement =
+            std::max(visualViewportLength - increment, pageIncrementMin);
+      } else {
+        pageIncrement = pageIncrementMin;
+      }
+
+      float ratio = pageIncrement / (maxMinPosDifference + pageIncrement);
+
+      OuterCSSCoord desiredThumbLength{
+          std::max(mScrollbarData.mThumbMinLength,
+                   mScrollbarData.mScrollTrackLength * ratio)};
+
+      // Round the thumb length to an integer number of LayoutDevice pixels, to
+      // match the main-thread behaviour.
+      auto outerDeviceScale = ViewAs<OuterCSSToLayoutDeviceScale>(
+          deviceScale, PixelCastJustification::CSSPixelsOfSurroundingContent);
+      desiredThumbLength =
+          LayoutDeviceCoord((desiredThumbLength * outerDeviceScale).Rounded()) /
+          outerDeviceScale;
+
+      effectiveThumbLength = desiredThumbLength;
+
+      scale = desiredThumbLength / mScrollbarData.mThumbLength;
+    }
+
+    // Subtracting the offset of the scrollable rect is needed for right-to-left
+    // pages.
+    const CSSCoord curPos = aAxis.GetRectOffset(visualViewportRect) -
+                            aAxis.GetRectOffset(mMetrics.GetScrollableRect());
+
+    const CSSToOuterCSSScale thumbPosRatio(
+        (maxMinPosDifference != 0)
+            ? float((mScrollbarData.mScrollTrackLength - effectiveThumbLength) /
+                    maxMinPosDifference)
+            : 1.f);
+
+    const OuterCSSCoord desiredThumbPos = curPos * thumbPosRatio;
+
+    translation = desiredThumbPos - mScrollbarData.mThumbStart;
+  } else {
+    // In this branch (taken when apz.scrollthumb.recalc=false), |translation|
+    // and |scale| are computed using the pre-bug1554795 approach of turning
+    // the async scroll and zoom deltas into transforms to apply to the
+    // main-thread thumb position and size.
+
+    // The scroll thumb needs to be scaled in the direction of scrolling by the
+    // inverse of the async zoom. This is because zooming in decreases the
+    // fraction of the whole srollable rect that is in view.
+    scale = 1.f / asyncZoom;
+
+    // Note: |metrics.GetZoom()| doesn't yet include the async zoom.
+    CSSToParentLayerScale effectiveZoom =
+        CSSToParentLayerScale(mMetrics.GetZoom().scale * asyncZoom);
+
+    if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+      // As computed by GetCurrentAsyncTransform, asyncScrollY is
+      //   asyncScrollY = -(GetEffectiveScrollOffset -
+      //   mLastContentPaintMetrics.GetLayoutScrollOffset()) *
+      //   effectiveZoom
+      // where GetEffectiveScrollOffset includes the visual viewport offset that
+      // the main thread knows about plus any async scrolling to the visual
+      // viewport offset that the main thread does not (yet) know about. We want
+      // asyncScrollY to be
+      //   asyncScrollY = -(GetEffectiveScrollOffset -
+      //   mLastContentPaintMetrics.GetVisualScrollOffset()) * effectiveZoom
+      // because the main thread positions the scrollbars at the visual viewport
+      // offset that it knows about. (aMetrics is mLastContentPaintMetrics)
+
+      asyncScroll -= aAxis.GetPointOffset((mMetrics.GetLayoutScrollOffset() -
+                                           mMetrics.GetVisualScrollOffset()) *
+                                          effectiveZoom);
+    }
+
+    // Here we convert the scrollbar thumb ratio into a true unitless ratio by
+    // dividing out the conversion factor from the scrollframe's parent's space
+    // to the scrollframe's space.
+    float unitlessThumbRatio = mScrollbarData.mThumbRatio /
+                               (mMetrics.GetPresShellResolution() * asyncZoom);
+
+    // The scroll thumb needs to be translated in opposite direction of the
+    // async scroll. This is because scrolling down, which translates the layer
+    // content up, should result in moving the scroll thumb down.
+    ParentLayerCoord translationPL = -asyncScroll * unitlessThumbRatio;
+
+    // The translation we computed is in the scroll frame's ParentLayer space.
+    // This includes the full cumulative resolution, even if we are a subframe.
+    // However, the resulting transform is used in a context where the scrollbar
+    // is already subject to the resolutions of enclosing scroll frames. To
+    // avoid double application of these enclosing resolutions, divide them out,
+    // leaving only the local resolution if any.
+    translationPL /= (mMetrics.GetCumulativeResolution().scale /
+                      mMetrics.GetPresShellResolution());
+
+    // Convert translation to CSS pixels as this is what TranslateThumb expects.
+    translation = ViewAs<OuterCSSPixel>(
+        translationPL / (mMetrics.GetDevPixelsPerCSSPixel() *
+                         LayoutDeviceToParentLayerScale(1.0)),
+        PixelCastJustification::CSSPixelsOfSurroundingContent);
+  }
+
+  // When scaling the thumb to account for the async zoom, keep the position
+  // of the start of the thumb (which corresponds to the scroll offset)
+  // constant.
+  if (haveAsyncZoom) {
+    ScaleThumbBy(aAxis, scale, ScrollThumbExtent::Start);
+  }
+
+  // If the page is overscrolled, additionally squish the thumb in accordance
+  // with the overscroll amount.
+  if (overscroll != 0) {
+    float overscrollScale =
+        1.0f - (std::abs(overscroll.value) /
+                aAxis.GetRectLength(mMetrics.GetCompositionBounds()));
+    MOZ_ASSERT(overscrollScale > 0.0f && overscrollScale <= 1.0f);
+    // If we're overscrolled at the top, keep the top of the thumb in place
+    // as we squish it. If we're overscrolled at the bottom, keep the bottom of
+    // the thumb in place.
+    ScaleThumbBy(
+        aAxis, overscrollScale,
+        overscroll < 0 ? ScrollThumbExtent::Start : ScrollThumbExtent::End);
+  }
+
+  TranslateThumb(aAxis, translation);
+}
+
+LayerToParentLayerMatrix4x4 AsyncScrollThumbTransformer::ComputeTransform() {
+  // We only apply the transform if the scroll-target layer has non-container
+  // children (i.e. when it has some possibly-visible content). This is to
+  // avoid moving scroll-bars in the situation that only a scroll information
+  // layer has been built for a scroll frame, as this would result in a
+  // disparity between scrollbars and visible content.
+  if (mMetrics.IsScrollInfoLayer()) {
+    return LayerToParentLayerMatrix4x4{};
+  }
+
+  MOZ_RELEASE_ASSERT(mApzc);
+
+  mAsyncTransform =
+      mApzc->GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing);
+
+  // |mAsyncTransform| represents the amount by which we have scrolled and
+  // zoomed since the last paint. Because the scrollbar was sized and positioned
+  // based on the painted content, we need to adjust it based on asyncTransform
+  // so that it reflects what the user is actually seeing now.
+  if (*mScrollbarData.mDirection == ScrollDirection::eVertical) {
+    ApplyTransformForAxis(mApzc->mY);
+  }
+  if (*mScrollbarData.mDirection == ScrollDirection::eHorizontal) {
+    ApplyTransformForAxis(mApzc->mX);
+  }
+
+  LayerToParentLayerMatrix4x4 transform =
+      mCurrentTransform * mScrollbarTransform;
+
+  AsyncTransformComponentMatrix compensation;
+  // If the scrollbar layer is a child of the content it is a scrollbar for,
+  // then we need to adjust for any async transform (including an overscroll
+  // transform) on the content. This needs to be cancelled out because layout
+  // positions and sizes the scrollbar on the assumption that there is no async
+  // transform, and without this adjustment the scrollbar will end up in the
+  // wrong place.
+  //
+  // Note that since the async transform is applied on top of the content's
+  // regular transform, we need to make sure to unapply the async transform in
+  // the same coordinate space. This requires applying the content transform
+  // and then unapplying it after unapplying the async transform.
+  if (mScrollbarIsDescendant) {
+    AsyncTransformComponentMatrix overscroll =
+        mApzc->GetOverscrollTransform(AsyncPanZoomController::eForCompositing);
+    gfx::Matrix4x4 asyncUntransform =
+        (mAsyncTransform * overscroll).Inverse().ToUnknownMatrix();
+    const gfx::Matrix4x4& contentTransform = mScrollableContentTransform;
+    gfx::Matrix4x4 contentUntransform = contentTransform.Inverse();
+
+    compensation *= ViewAs<AsyncTransformComponentMatrix>(
+        contentTransform * asyncUntransform * contentUntransform);
+  }
+  transform = transform * compensation;
+
+  return transform;
+}
+
+LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumb(
+    const LayerToParentLayerMatrix4x4& aCurrentTransform,
+    const gfx::Matrix4x4& aScrollableContentTransform,
+    AsyncPanZoomController* aApzc, const FrameMetrics& aMetrics,
+    const ScrollbarData& aScrollbarData, bool aScrollbarIsDescendant) {
+  return AsyncScrollThumbTransformer{
+      aCurrentTransform, aScrollableContentTransform, aApzc, aMetrics,
+      aScrollbarData,    aScrollbarIsDescendant}
+      .ComputeTransform();
+}
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/ScrollThumbUtils.h b/gfx/layers/apz/src/ScrollThumbUtils.h
new file mode 100644
index 0000000000..49a45de3a7
--- /dev/null
+++ b/gfx/layers/apz/src/ScrollThumbUtils.h
@@ -0,0 +1,51 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_ScrollThumbUtils_h
+#define mozilla_layers_ScrollThumbUtils_h
+
+#include "LayersTypes.h"
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+struct FrameMetrics;
+struct ScrollbarData;
+
+namespace apz {
+/**
+ * Compute the updated shadow transform for a scroll thumb layer that
+ * reflects async scrolling of the associated scroll frame.
+ *
+ * @param aCurrentTransform The current shadow transform on the scroll thumb
+ *    layer, as returned by Layer::GetLocalTransform() or similar.
+ * @param aScrollableContentTransform The current content transform on the
+ *    scrollable content, as returned by Layer::GetTransform().
+ * @param aApzc The APZC that scrolls the scroll frame.
+ * @param aMetrics The metrics associated with the scroll frame, reflecting
+ *    the last paint of the associated content. Note: this metrics should
+ *    NOT reflect async scrolling or zooming, i.e. they should be the layer
+ *    tree's copy of the metrics, or APZC's last-content-paint metrics.
+ * @param aScrollbarData The scrollbar data for the the scroll thumb layer.
+ * @param aScrollbarIsDescendant True iff. the scroll thumb layer is a
+ *    descendant of the layer bearing the scroll frame's metrics.
+ * @return The new shadow transform for the scroll thumb layer, including
+ *    any pre- or post-scales.
+ */
+LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumb(
+    const LayerToParentLayerMatrix4x4& aCurrentTransform,
+    const gfx::Matrix4x4& aScrollableContentTransform,
+    AsyncPanZoomController* aApzc, const FrameMetrics& aMetrics,
+    const ScrollbarData& aScrollbarData, bool aScrollbarIsDescendant);
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_ScrollThumbUtils_h
diff --git a/gfx/layers/apz/src/SimpleVelocityTracker.cpp b/gfx/layers/apz/src/SimpleVelocityTracker.cpp
new file mode 100644
index 0000000000..87cae10d51
--- /dev/null
+++ b/gfx/layers/apz/src/SimpleVelocityTracker.cpp
@@ -0,0 +1,135 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SimpleVelocityTracker.h"
+
+#include "mozilla/ServoStyleConsts.h"  // for StyleComputedTimingFunction
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPtr.h"  // for StaticAutoPtr
+
+static mozilla::LazyLogModule sApzSvtLog("apz.simplevelocitytracker");
+#define SVT_LOG(...) MOZ_LOG(sApzSvtLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+// When we compute the velocity we do so by taking two input events and
+// dividing the distance delta over the time delta. In some cases the time
+// delta can be really small, which can make the velocity computation very
+// volatile. To avoid this we impose a minimum time delta below which we do
+// not recompute the velocity.
+const TimeDuration MIN_VELOCITY_SAMPLE_TIME = TimeDuration::FromMilliseconds(5);
+
+extern StaticAutoPtr<StyleComputedTimingFunction> gVelocityCurveFunction;
+
+SimpleVelocityTracker::SimpleVelocityTracker(Axis* aAxis)
+    : mAxis(aAxis), mVelocitySamplePos(0) {}
+
+void SimpleVelocityTracker::StartTracking(ParentLayerCoord aPos,
+                                          TimeStamp aTimestamp) {
+  Clear();
+  mVelocitySampleTime = aTimestamp;
+  mVelocitySamplePos = aPos;
+}
+
+Maybe<float> SimpleVelocityTracker::AddPosition(ParentLayerCoord aPos,
+                                                TimeStamp aTimestamp) {
+  if (aTimestamp <= mVelocitySampleTime + MIN_VELOCITY_SAMPLE_TIME) {
+    // See also the comment on MIN_VELOCITY_SAMPLE_TIME.
+    // We don't update either mVelocitySampleTime or mVelocitySamplePos so that
+    // eventually when we do get an event with the required time delta we use
+    // the corresponding distance delta as well.
+    SVT_LOG("%p|%s skipping velocity computation for small time delta %f ms\n",
+            mAxis->OpaqueApzcPointer(), mAxis->Name(),
+            (aTimestamp - mVelocitySampleTime).ToMilliseconds());
+    return Nothing();
+  }
+
+  float newVelocity =
+      (float)(mVelocitySamplePos - aPos) /
+      (float)(aTimestamp - mVelocitySampleTime).ToMilliseconds();
+
+  newVelocity = ApplyFlingCurveToVelocity(newVelocity);
+
+  SVT_LOG("%p|%s updating velocity to %f with touch\n",
+          mAxis->OpaqueApzcPointer(), mAxis->Name(), newVelocity);
+  mVelocitySampleTime = aTimestamp;
+  mVelocitySamplePos = aPos;
+
+  AddVelocityToQueue(aTimestamp, newVelocity);
+
+  return Some(newVelocity);
+}
+
+Maybe<float> SimpleVelocityTracker::ComputeVelocity(TimeStamp aTimestamp) {
+  float velocity = 0;
+  int count = 0;
+  for (const auto& e : mVelocityQueue) {
+    TimeDuration timeDelta = (aTimestamp - e.first);
+    if (timeDelta < TimeDuration::FromMilliseconds(
+                        StaticPrefs::apz_velocity_relevance_time_ms())) {
+      count++;
+      velocity += e.second;
+    }
+  }
+  mVelocityQueue.Clear();
+  if (count > 1) {
+    velocity /= count;
+  }
+  return Some(velocity);
+}
+
+void SimpleVelocityTracker::Clear() { mVelocityQueue.Clear(); }
+
+void SimpleVelocityTracker::AddVelocityToQueue(TimeStamp aTimestamp,
+                                               float aVelocity) {
+  mVelocityQueue.AppendElement(std::make_pair(aTimestamp, aVelocity));
+  if (mVelocityQueue.Length() >
+      StaticPrefs::apz_max_velocity_queue_size_AtStartup()) {
+    mVelocityQueue.RemoveElementAt(0);
+  }
+}
+
+float SimpleVelocityTracker::ApplyFlingCurveToVelocity(float aVelocity) const {
+  float newVelocity = aVelocity;
+  if (StaticPrefs::apz_max_velocity_inches_per_ms() > 0.0f) {
+    bool velocityIsNegative = (newVelocity < 0);
+    newVelocity = fabs(newVelocity);
+
+    float maxVelocity =
+        mAxis->ToLocalVelocity(StaticPrefs::apz_max_velocity_inches_per_ms());
+    newVelocity = std::min(newVelocity, maxVelocity);
+
+    if (StaticPrefs::apz_fling_curve_threshold_inches_per_ms() > 0.0f &&
+        StaticPrefs::apz_fling_curve_threshold_inches_per_ms() <
+            StaticPrefs::apz_max_velocity_inches_per_ms()) {
+      float curveThreshold = mAxis->ToLocalVelocity(
+          StaticPrefs::apz_fling_curve_threshold_inches_per_ms());
+      if (newVelocity > curveThreshold) {
+        // here, 0 < curveThreshold < newVelocity <= maxVelocity, so we apply
+        // the curve
+        float scale = maxVelocity - curveThreshold;
+        float funcInput = (newVelocity - curveThreshold) / scale;
+        float funcOutput =
+            gVelocityCurveFunction->At(funcInput, /* aBeforeFlag = */ false);
+        float curvedVelocity = (funcOutput * scale) + curveThreshold;
+        SVT_LOG("%p|%s curving up velocity from %f to %f\n",
+                mAxis->OpaqueApzcPointer(), mAxis->Name(), newVelocity,
+                curvedVelocity);
+        newVelocity = curvedVelocity;
+      }
+    }
+
+    if (velocityIsNegative) {
+      newVelocity = -newVelocity;
+    }
+  }
+
+  return newVelocity;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/SimpleVelocityTracker.h b/gfx/layers/apz/src/SimpleVelocityTracker.h
new file mode 100644
index 0000000000..1778dee065
--- /dev/null
+++ b/gfx/layers/apz/src/SimpleVelocityTracker.h
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_VelocityTracker_h
+#define mozilla_layers_VelocityTracker_h
+
+#include <utility>
+#include <cstdint>
+
+#include "Axis.h"
+#include "mozilla/Attributes.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace layers {
+
+class SimpleVelocityTracker : public VelocityTracker {
+ public:
+  explicit SimpleVelocityTracker(Axis* aAxis);
+  void StartTracking(ParentLayerCoord aPos, TimeStamp aTimestamp) override;
+  Maybe<float> AddPosition(ParentLayerCoord aPos,
+                           TimeStamp aTimestamp) override;
+  Maybe<float> ComputeVelocity(TimeStamp aTimestamp) override;
+  void Clear() override;
+
+ private:
+  void AddVelocityToQueue(TimeStamp aTimestamp, float aVelocity);
+  float ApplyFlingCurveToVelocity(float aVelocity) const;
+
+  // The Axis that uses this velocity tracker.
+  // This is a raw pointer because the Axis owns the velocity tracker
+  // by UniquePtr, so the velocity tracker cannot outlive the Axis.
+  Axis* MOZ_NON_OWNING_REF mAxis;
+
+  // A queue of (timestamp, velocity) pairs; these are the historical
+  // velocities at the given timestamps. Velocities are in screen pixels per ms.
+  // This member can only be accessed on the controller/UI thread.
+  nsTArray<std::pair<TimeStamp, float>> mVelocityQueue;
+
+  // mVelocitySampleTime and mVelocitySamplePos are the time and position
+  // used in the last velocity sampling. They get updated when a new sample is
+  // taken (which may not happen on every input event, if the time delta is too
+  // small).
+  TimeStamp mVelocitySampleTime;
+  ParentLayerCoord mVelocitySamplePos;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/SmoothMsdScrollAnimation.cpp b/gfx/layers/apz/src/SmoothMsdScrollAnimation.cpp
new file mode 100644
index 0000000000..8342dc157f
--- /dev/null
+++ b/gfx/layers/apz/src/SmoothMsdScrollAnimation.cpp
@@ -0,0 +1,139 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "SmoothMsdScrollAnimation.h"
+#include "AsyncPanZoomController.h"
+
+namespace mozilla {
+namespace layers {
+
+SmoothMsdScrollAnimation::SmoothMsdScrollAnimation(
+    AsyncPanZoomController& aApzc, const CSSPoint& aInitialPosition,
+    const CSSPoint& aInitialVelocity, const CSSPoint& aDestination,
+    double aSpringConstant, double aDampingRatio,
+    ScrollSnapTargetIds&& aSnapTargetIds,
+    ScrollTriggeredByScript aTriggeredByScript)
+    : mApzc(aApzc),
+      mXAxisModel(aInitialPosition.x, aDestination.x, aInitialVelocity.x,
+                  aSpringConstant, aDampingRatio),
+      mYAxisModel(aInitialPosition.y, aDestination.y, aInitialVelocity.y,
+                  aSpringConstant, aDampingRatio),
+      mSnapTargetIds(std::move(aSnapTargetIds)),
+      mTriggeredByScript(aTriggeredByScript) {}
+
+bool SmoothMsdScrollAnimation::DoSample(FrameMetrics& aFrameMetrics,
+                                        const TimeDuration& aDelta) {
+  CSSToParentLayerScale zoom(aFrameMetrics.GetZoom());
+  if (zoom == CSSToParentLayerScale(0)) {
+    return false;
+  }
+  CSSPoint oneParentLayerPixel =
+      ParentLayerPoint(1, 1) / aFrameMetrics.GetZoom();
+  if (mXAxisModel.IsFinished(oneParentLayerPixel.x) &&
+      mYAxisModel.IsFinished(oneParentLayerPixel.y)) {
+    // Set the scroll offset to the exact destination. If we allow the scroll
+    // offset to end up being a bit off from the destination, we can get
+    // artefacts like "scroll to the next snap point in this direction"
+    // scrolling to the snap point we're already supposed to be at.
+    mApzc.ClampAndSetVisualScrollOffset(
+        CSSPoint(mXAxisModel.GetDestination(), mYAxisModel.GetDestination()));
+    return false;
+  }
+
+  mXAxisModel.Simulate(aDelta);
+  mYAxisModel.Simulate(aDelta);
+
+  CSSPoint position =
+      CSSPoint(mXAxisModel.GetPosition(), mYAxisModel.GetPosition());
+  CSSPoint css_velocity =
+      CSSPoint(mXAxisModel.GetVelocity(), mYAxisModel.GetVelocity());
+
+  // Convert from pixels/second to pixels/ms
+  ParentLayerPoint velocity =
+      ParentLayerPoint(css_velocity.x, css_velocity.y) / 1000.0f;
+
+  // Keep the velocity updated for the Axis class so that any animations
+  // chained off of the smooth scroll will inherit it.
+  if (mXAxisModel.IsFinished(oneParentLayerPixel.x)) {
+    mApzc.mX.SetVelocity(0);
+  } else {
+    mApzc.mX.SetVelocity(velocity.x);
+  }
+  if (mYAxisModel.IsFinished(oneParentLayerPixel.y)) {
+    mApzc.mY.SetVelocity(0);
+  } else {
+    mApzc.mY.SetVelocity(velocity.y);
+  }
+  // If we overscroll, hand off to a fling animation that will complete the
+  // spring back.
+  ParentLayerPoint displacement =
+      (position - aFrameMetrics.GetVisualScrollOffset()) * zoom;
+
+  ParentLayerPoint overscroll;
+  ParentLayerPoint adjustedOffset;
+  mApzc.mX.AdjustDisplacement(displacement.x, adjustedOffset.x, overscroll.x);
+  mApzc.mY.AdjustDisplacement(displacement.y, adjustedOffset.y, overscroll.y);
+  mApzc.ScrollBy(adjustedOffset / zoom);
+  // The smooth scroll may have caused us to reach the end of our scroll
+  // range. This can happen if either the
+  // layout.css.scroll-behavior.damping-ratio preference is set to less than 1
+  // (underdamped) or if a smooth scroll inherits velocity from a fling
+  // gesture.
+  if (!IsZero(overscroll / zoom)) {
+    // Hand off a fling with the remaining momentum to the next APZC in the
+    // overscroll handoff chain.
+
+    // We may have reached the end of the scroll range along one axis but
+    // not the other. In such a case we only want to hand off the relevant
+    // component of the fling.
+    if (mApzc.IsZero(overscroll.x)) {
+      velocity.x = 0;
+    } else if (mApzc.IsZero(overscroll.y)) {
+      velocity.y = 0;
+    }
+
+    // To hand off the fling, we attempt to find a target APZC and start a new
+    // fling with the same velocity on that APZC. For simplicity, the actual
+    // overscroll of the current sample is discarded rather than being handed
+    // off. The compositor should sample animations sufficiently frequently
+    // that this is not noticeable. The target APZC is chosen by seeing if
+    // there is an APZC further in the handoff chain which is pannable; if
+    // there isn't, we take the new fling ourselves, entering an overscrolled
+    // state.
+    // Note: APZC is holding mRecursiveMutex, so directly calling
+    // HandleSmoothScrollOverscroll() (which acquires the tree lock) would
+    // violate the lock ordering. Instead we schedule
+    // HandleSmoothScrollOverscroll() to be called after mRecursiveMutex is
+    // released.
+    mDeferredTasks.AppendElement(NewRunnableMethod<ParentLayerPoint, SideBits>(
+        "layers::AsyncPanZoomController::HandleSmoothScrollOverscroll", &mApzc,
+        &AsyncPanZoomController::HandleSmoothScrollOverscroll, velocity,
+        apz::GetOverscrollSideBits(overscroll)));
+    return false;
+  }
+
+  return true;
+}
+
+void SmoothMsdScrollAnimation::SetDestination(
+    const CSSPoint& aNewDestination, ScrollSnapTargetIds&& aSnapTargetIds,
+    ScrollTriggeredByScript aTriggeredByScript) {
+  mXAxisModel.SetDestination(aNewDestination.x);
+  mYAxisModel.SetDestination(aNewDestination.y);
+  mSnapTargetIds = std::move(aSnapTargetIds);
+  mTriggeredByScript = aTriggeredByScript;
+}
+
+CSSPoint SmoothMsdScrollAnimation::GetDestination() const {
+  return CSSPoint(mXAxisModel.GetDestination(), mYAxisModel.GetDestination());
+}
+
+SmoothMsdScrollAnimation*
+SmoothMsdScrollAnimation::AsSmoothMsdScrollAnimation() {
+  return this;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/SmoothMsdScrollAnimation.h b/gfx/layers/apz/src/SmoothMsdScrollAnimation.h
new file mode 100644
index 0000000000..1f2247c473
--- /dev/null
+++ b/gfx/layers/apz/src/SmoothMsdScrollAnimation.h
@@ -0,0 +1,61 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_SmoothMsdScrollAnimation_h_
+#define mozilla_layers_SmoothMsdScrollAnimation_h_
+
+#include "AsyncPanZoomAnimation.h"
+#include "mozilla/layers/AxisPhysicsMSDModel.h"
+#include "mozilla/ScrollPositionUpdate.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+class SmoothMsdScrollAnimation final : public AsyncPanZoomAnimation {
+ public:
+  SmoothMsdScrollAnimation(AsyncPanZoomController& aApzc,
+                           const CSSPoint& aInitialPosition,
+                           const CSSPoint& aInitialVelocity,
+                           const CSSPoint& aDestination, double aSpringConstant,
+                           double aDampingRatio,
+                           ScrollSnapTargetIds&& aSnapTargetIds,
+                           ScrollTriggeredByScript aTriggeredByScript);
+
+  /**
+   * Advances a smooth scroll simulation based on the time passed in |aDelta|.
+   * This should be called whenever sampling the content transform for this
+   * frame. Returns true if the smooth scroll should be advanced by one frame,
+   * or false if the smooth scroll has ended.
+   */
+  bool DoSample(FrameMetrics& aFrameMetrics,
+                const TimeDuration& aDelta) override;
+
+  void SetDestination(const CSSPoint& aNewDestination,
+                      ScrollSnapTargetIds&& aSnapTargetIds,
+                      ScrollTriggeredByScript aTriggeredByScript);
+  CSSPoint GetDestination() const;
+  SmoothMsdScrollAnimation* AsSmoothMsdScrollAnimation() override;
+
+  bool WasTriggeredByScript() const override {
+    return mTriggeredByScript == ScrollTriggeredByScript::Yes;
+  }
+
+  ScrollSnapTargetIds TakeSnapTargetIds() { return std::move(mSnapTargetIds); }
+
+ private:
+  AsyncPanZoomController& mApzc;
+  AxisPhysicsMSDModel mXAxisModel;
+  AxisPhysicsMSDModel mYAxisModel;
+  ScrollSnapTargetIds mSnapTargetIds;
+  ScrollTriggeredByScript mTriggeredByScript;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif
diff --git a/gfx/layers/apz/src/SmoothScrollAnimation.cpp b/gfx/layers/apz/src/SmoothScrollAnimation.cpp
new file mode 100644
index 0000000000..7efec2bd54
--- /dev/null
+++ b/gfx/layers/apz/src/SmoothScrollAnimation.cpp
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SmoothScrollAnimation.h"
+#include "ScrollAnimationBezierPhysics.h"
+#include "ScrollPositionUpdate.h"
+#include "apz/src/GenericScrollAnimation.h"
+#include "mozilla/layers/APZPublicUtils.h"
+
+namespace mozilla {
+namespace layers {
+
+SmoothScrollAnimation::SmoothScrollAnimation(AsyncPanZoomController& aApzc,
+                                             const nsPoint& aInitialPosition,
+                                             ScrollOrigin aOrigin)
+    : GenericScrollAnimation(aApzc, aInitialPosition, aOrigin),
+      mOrigin(aOrigin),
+      mTriggeredByScript(ScrollTriggeredByScript::No) {}
+
+SmoothScrollAnimation* SmoothScrollAnimation::AsSmoothScrollAnimation() {
+  return this;
+}
+
+void SmoothScrollAnimation::UpdateDestinationAndSnapTargets(
+    TimeStamp aTime, const nsPoint& aDestination,
+    const nsSize& aCurrentVelocity, ScrollSnapTargetIds&& aSnapTargetIds,
+    ScrollTriggeredByScript aTriggeredByScript) {
+  GenericScrollAnimation::UpdateDestination(aTime, aDestination,
+                                            aCurrentVelocity);
+  mSnapTargetIds = std::move(aSnapTargetIds);
+  mTriggeredByScript = aTriggeredByScript;
+}
+
+ScrollOrigin SmoothScrollAnimation::GetScrollOrigin() const { return mOrigin; }
+
+ScrollOrigin SmoothScrollAnimation::GetScrollOriginForAction(
+    KeyboardScrollAction::KeyboardScrollActionType aAction) {
+  switch (aAction) {
+    case KeyboardScrollAction::eScrollCharacter:
+    case KeyboardScrollAction::eScrollLine: {
+      return ScrollOrigin::Lines;
+    }
+    case KeyboardScrollAction::eScrollPage:
+      return ScrollOrigin::Pages;
+    case KeyboardScrollAction::eScrollComplete:
+      return ScrollOrigin::Other;
+    default:
+      MOZ_ASSERT(false, "Unknown keyboard scroll action type");
+      return ScrollOrigin::Other;
+  }
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/SmoothScrollAnimation.h b/gfx/layers/apz/src/SmoothScrollAnimation.h
new file mode 100644
index 0000000000..8096875a51
--- /dev/null
+++ b/gfx/layers/apz/src/SmoothScrollAnimation.h
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_SmoothScrollAnimation_h_
+#define mozilla_layers_SmoothScrollAnimation_h_
+
+#include "GenericScrollAnimation.h"
+#include "ScrollPositionUpdate.h"
+#include "mozilla/ScrollOrigin.h"
+#include "mozilla/layers/KeyboardScrollAction.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+class SmoothScrollAnimation : public GenericScrollAnimation {
+ public:
+  SmoothScrollAnimation(AsyncPanZoomController& aApzc,
+                        const nsPoint& aInitialPosition, ScrollOrigin aOrigin);
+
+  void UpdateDestinationAndSnapTargets(
+      TimeStamp aTime, const nsPoint& aDestination,
+      const nsSize& aCurrentVelocity, ScrollSnapTargetIds&& aSnapTargetIds,
+      ScrollTriggeredByScript aTriggeredByScript);
+
+  SmoothScrollAnimation* AsSmoothScrollAnimation() override;
+  bool WasTriggeredByScript() const override {
+    return mTriggeredByScript == ScrollTriggeredByScript::Yes;
+  }
+  ScrollSnapTargetIds TakeSnapTargetIds() { return std::move(mSnapTargetIds); }
+  ScrollOrigin GetScrollOrigin() const;
+  static ScrollOrigin GetScrollOriginForAction(
+      KeyboardScrollAction::KeyboardScrollActionType aAction);
+
+ private:
+  ScrollOrigin mOrigin;
+  ScrollSnapTargetIds mSnapTargetIds;
+  ScrollTriggeredByScript mTriggeredByScript;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_SmoothScrollAnimation_h_
diff --git a/gfx/layers/apz/src/WRHitTester.cpp b/gfx/layers/apz/src/WRHitTester.cpp
new file mode 100644
index 0000000000..873400976f
--- /dev/null
+++ b/gfx/layers/apz/src/WRHitTester.cpp
@@ -0,0 +1,247 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "WRHitTester.h"
+#include "AsyncPanZoomController.h"
+#include "APZCTreeManager.h"
+#include "TreeTraversal.h"  // for BreadthFirstSearch
+#include "mozilla/gfx/CompositorHitTestInfo.h"
+#include "mozilla/webrender/WebRenderAPI.h"
+#include "nsDebug.h"        // for NS_ASSERTION
+#include "nsIXULRuntime.h"  // for FissionAutostart
+#include "mozilla/gfx/Matrix.h"
+
+#define APZCTM_LOG(...) \
+  MOZ_LOG(APZCTreeManager::sLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+using mozilla::gfx::CompositorHitTestFlags;
+using mozilla::gfx::CompositorHitTestInvisibleToHit;
+
+static bool CheckCloseToIdentity(const gfx::Matrix4x4& aMatrix) {
+  // We allow a factor of 1/2048 in the multiply part of the matrix, so that if
+  // we multiply by a point on a screen of size 2048 we would be off by at most
+  // 1 pixel approximately.
+  const float multiplyEps = 1 / 2048.f;
+  // We allow 1 pixel in the translate part of the matrix.
+  const float translateEps = 1.f;
+
+  if (!FuzzyEqualsAdditive(aMatrix._11, 1.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._12, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._13, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._14, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._21, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._22, 1.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._23, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._24, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._31, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._32, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._33, 1.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._34, 0.f, multiplyEps) ||
+      !FuzzyEqualsAdditive(aMatrix._41, 0.f, translateEps) ||
+      !FuzzyEqualsAdditive(aMatrix._42, 0.f, translateEps) ||
+      !FuzzyEqualsAdditive(aMatrix._43, 0.f, translateEps) ||
+      !FuzzyEqualsAdditive(aMatrix._44, 1.f, multiplyEps)) {
+    return false;
+  }
+  return true;
+}
+
+// Checks that within the constraints of floating point math we can invert it
+// reasonably enough that multiplying by the computed inverse is close to the
+// identity.
+static bool CheckInvertibleWithFinitePrecision(const gfx::Matrix4x4& aMatrix) {
+  auto inverse = aMatrix.MaybeInverse();
+  if (inverse.isNothing()) {
+    // Should we return false?
+    return true;
+  }
+  if (!CheckCloseToIdentity(aMatrix * *inverse)) {
+    return false;
+  }
+  if (!CheckCloseToIdentity(*inverse * aMatrix)) {
+    return false;
+  }
+  return true;
+}
+
+IAPZHitTester::HitTestResult WRHitTester::GetAPZCAtPoint(
+    const ScreenPoint& aHitTestPoint,
+    const RecursiveMutexAutoLock& aProofOfTreeLock) {
+  HitTestResult hit;
+  RefPtr<wr::WebRenderAPI> wr = mTreeManager->GetWebRenderAPI();
+  if (!wr) {
+    // If WebRender isn't running, fall back to the root APZC.
+    // This is mostly for the benefit of GTests which do not
+    // run a WebRender instance, but gracefully falling back
+    // here allows those tests which are not specifically
+    // testing the hit-test algorithm to still work.
+    hit.mTargetApzc = FindRootApzcForLayersId(GetRootLayersId());
+    hit.mHitResult = CompositorHitTestFlags::eVisibleToHitTest;
+    return hit;
+  }
+
+  APZCTM_LOG("Hit-testing point %s with WR\n", ToString(aHitTestPoint).c_str());
+  std::vector<wr::WrHitResult> results =
+      wr->HitTest(wr::ToWorldPoint(aHitTestPoint));
+
+  Maybe<wr::WrHitResult> chosenResult;
+  for (const wr::WrHitResult& result : results) {
+    ScrollableLayerGuid guid{result.mLayersId, 0, result.mScrollId};
+    APZCTM_LOG("Examining result with guid %s hit info 0x%x... ",
+               ToString(guid).c_str(), result.mHitInfo.serialize());
+    if (result.mHitInfo == CompositorHitTestInvisibleToHit) {
+      APZCTM_LOG("skipping due to invisibility.\n");
+      continue;
+    }
+    RefPtr<HitTestingTreeNode> node =
+        GetTargetNode(guid, &ScrollableLayerGuid::EqualsIgnoringPresShell);
+    if (!node) {
+      APZCTM_LOG("no corresponding node found, falling back to root.\n");
+
+#ifdef DEBUG
+      // We can enter here during normal codepaths for cases where the
+      // nsDisplayCompositorHitTestInfo item emitted a scrollId of
+      // NULL_SCROLL_ID to the webrender display list. The semantics of that
+      // is to fall back to the root APZC for the layers id, so that's what
+      // we do here.
+      // If we enter this codepath and scrollId is not NULL_SCROLL_ID, then
+      // that's more likely to be due to a race condition between rebuilding
+      // the APZ tree and updating the WR scene/hit-test information, resulting
+      // in WR giving us a hit result for a scene that is not active in APZ.
+      // Such a scenario would need debugging and fixing.
+      // In non-Fission mode, make this assertion non-fatal because there is
+      // a known issue related to inactive scroll frames that can cause this
+      // to fire (see bug 1634763), which is fixed in Fission mode and not
+      // worth fixing in non-Fission mode.
+      if (FissionAutostart()) {
+        MOZ_ASSERT(result.mScrollId == ScrollableLayerGuid::NULL_SCROLL_ID);
+      } else {
+        NS_ASSERTION(
+            result.mScrollId == ScrollableLayerGuid::NULL_SCROLL_ID,
+            "Inconsistency between WebRender display list and APZ scroll data");
+      }
+#endif
+      node = FindRootNodeForLayersId(result.mLayersId);
+      if (!node) {
+        // Should never happen, but handle gracefully in release builds just
+        // in case.
+        MOZ_ASSERT(false);
+        chosenResult = Some(result);
+        break;
+      }
+    }
+    MOZ_ASSERT(node->GetApzc());  // any node returned must have an APZC
+    EventRegionsOverride flags = node->GetEventRegionsOverride();
+    if (flags & EventRegionsOverride::ForceEmptyHitRegion) {
+      // This result is inside a subtree that is invisible to hit-testing.
+      APZCTM_LOG("skipping due to FEHR subtree.\n");
+      continue;
+    }
+
+    if (!CheckInvertibleWithFinitePrecision(
+            mTreeManager->GetScreenToApzcTransform(node->GetApzc())
+                .ToUnknownMatrix())) {
+      APZCTM_LOG("skipping due to check inverse accuracy\n");
+      continue;
+    }
+
+    APZCTM_LOG("selecting as chosen result.\n");
+    chosenResult = Some(result);
+    hit.mTargetApzc = node->GetApzc();
+    if (flags & EventRegionsOverride::ForceDispatchToContent) {
+      chosenResult->mHitInfo += CompositorHitTestFlags::eApzAwareListeners;
+    }
+    break;
+  }
+  if (!chosenResult) {
+    return hit;
+  }
+
+  MOZ_ASSERT(hit.mTargetApzc);
+  hit.mLayersId = chosenResult->mLayersId;
+  ScrollableLayerGuid::ViewID scrollId = chosenResult->mScrollId;
+  gfx::CompositorHitTestInfo hitInfo = chosenResult->mHitInfo;
+  Maybe<uint64_t> animationId = chosenResult->mAnimationId;
+  SideBits sideBits = chosenResult->mSideBits;
+
+  APZCTM_LOG("Successfully matched APZC %p (hit result 0x%x)\n",
+             hit.mTargetApzc.get(), hitInfo.serialize());
+
+  const bool isScrollbar =
+      hitInfo.contains(gfx::CompositorHitTestFlags::eScrollbar);
+  const bool isScrollbarThumb =
+      hitInfo.contains(gfx::CompositorHitTestFlags::eScrollbarThumb);
+  const ScrollDirection direction =
+      hitInfo.contains(gfx::CompositorHitTestFlags::eScrollbarVertical)
+          ? ScrollDirection::eVertical
+          : ScrollDirection::eHorizontal;
+  HitTestingTreeNode* scrollbarNode = nullptr;
+  if (isScrollbar || isScrollbarThumb) {
+    scrollbarNode = BreadthFirstSearch<ReverseIterator>(
+        GetRootNode(), [&](HitTestingTreeNode* aNode) {
+          return (aNode->GetLayersId() == hit.mLayersId) &&
+                 (aNode->IsScrollbarNode() == isScrollbar) &&
+                 (aNode->IsScrollThumbNode() == isScrollbarThumb) &&
+                 (aNode->GetScrollbarDirection() == direction) &&
+                 (aNode->GetScrollTargetId() == scrollId);
+        });
+  }
+
+  hit.mHitResult = hitInfo;
+
+  if (scrollbarNode) {
+    RefPtr<HitTestingTreeNode> scrollbarRef = scrollbarNode;
+    InitializeHitTestingTreeNodeAutoLock(hit.mScrollbarNode, aProofOfTreeLock,
+                                         scrollbarRef);
+  }
+
+  hit.mFixedPosSides = sideBits;
+  if (animationId.isSome()) {
+    RefPtr<HitTestingTreeNode> positionedNode = nullptr;
+
+    positionedNode = BreadthFirstSearch<ReverseIterator>(
+        GetRootNode(), [&](HitTestingTreeNode* aNode) {
+          return (aNode->GetFixedPositionAnimationId() == animationId ||
+                  aNode->GetStickyPositionAnimationId() == animationId);
+        });
+
+    if (positionedNode) {
+      MOZ_ASSERT(positionedNode->GetLayersId() == chosenResult->mLayersId,
+                 "Found node layers id does not match the hit result");
+      MOZ_ASSERT((positionedNode->GetFixedPositionAnimationId().isSome() ||
+                  positionedNode->GetStickyPositionAnimationId().isSome()),
+                 "A a matching fixed/sticky position node should be found");
+      InitializeHitTestingTreeNodeAutoLock(hit.mNode, aProofOfTreeLock,
+                                           positionedNode);
+    }
+
+#if defined(MOZ_WIDGET_ANDROID)
+    if (hit.mNode && hit.mNode->GetFixedPositionAnimationId().isSome()) {
+      // If the hit element is a fixed position element, the side bits from
+      // the hit-result item tag are used. For now just ensure that these
+      // match what is found in the hit-testing tree node.
+      MOZ_ASSERT(sideBits == hit.mNode->GetFixedPosSides(),
+                 "Fixed position side bits do not match");
+    } else if (hit.mTargetApzc && hit.mTargetApzc->IsRootContent()) {
+      // If the hit element is not a fixed position element, then the hit test
+      // result item's side bits should not be populated.
+      MOZ_ASSERT(sideBits == SideBits::eNone,
+                 "Hit test results have side bits only for pos:fixed");
+    }
+#endif
+  }
+
+  hit.mHitOverscrollGutter =
+      hit.mTargetApzc && hit.mTargetApzc->IsInOverscrollGutter(aHitTestPoint);
+
+  return hit;
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/WRHitTester.h b/gfx/layers/apz/src/WRHitTester.h
new file mode 100644
index 0000000000..abb9de1a66
--- /dev/null
+++ b/gfx/layers/apz/src/WRHitTester.h
@@ -0,0 +1,26 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_WRHitTester_h
+#define mozilla_layers_WRHitTester_h
+
+#include "IAPZHitTester.h"
+
+namespace mozilla {
+namespace layers {
+
+// IAPZHitTester implementation for WebRender.
+class WRHitTester : public IAPZHitTester {
+ public:
+  virtual HitTestResult GetAPZCAtPoint(
+      const ScreenPoint& aHitTestPoint,
+      const RecursiveMutexAutoLock& aProofOfTreeLock) override;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // define mozilla_layers_WRHitTester_h
diff --git a/gfx/layers/apz/src/WheelScrollAnimation.cpp b/gfx/layers/apz/src/WheelScrollAnimation.cpp
new file mode 100644
index 0000000000..ddc88615de
--- /dev/null
+++ b/gfx/layers/apz/src/WheelScrollAnimation.cpp
@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "WheelScrollAnimation.h"
+
+#include <tuple>
+#include "AsyncPanZoomController.h"
+#include "mozilla/StaticPrefs_general.h"
+#include "mozilla/layers/APZPublicUtils.h"
+#include "nsPoint.h"
+#include "ScrollAnimationBezierPhysics.h"
+
+namespace mozilla {
+namespace layers {
+
+static ScrollOrigin OriginForDeltaType(
+    ScrollWheelInput::ScrollDeltaType aDeltaType) {
+  switch (aDeltaType) {
+    case ScrollWheelInput::SCROLLDELTA_PAGE:
+      return ScrollOrigin::Pages;
+    case ScrollWheelInput::SCROLLDELTA_PIXEL:
+      return ScrollOrigin::Pixels;
+    case ScrollWheelInput::SCROLLDELTA_LINE:
+      return ScrollOrigin::MouseWheel;
+  }
+  // Shouldn't happen, pick a default.
+  return ScrollOrigin::MouseWheel;
+}
+
+WheelScrollAnimation::WheelScrollAnimation(
+    AsyncPanZoomController& aApzc, const nsPoint& aInitialPosition,
+    ScrollWheelInput::ScrollDeltaType aDeltaType)
+    : GenericScrollAnimation(aApzc, aInitialPosition,
+                             OriginForDeltaType(aDeltaType)) {
+  MOZ_ASSERT(StaticPrefs::general_smoothScroll(),
+             "We shouldn't be creating a WheelScrollAnimation if smooth "
+             "scrolling is disabled");
+  mDirectionForcedToOverscroll =
+      mApzc.mScrollMetadata.GetDisregardedDirection();
+}
+
+}  // namespace layers
+}  // namespace mozilla
diff --git a/gfx/layers/apz/src/WheelScrollAnimation.h b/gfx/layers/apz/src/WheelScrollAnimation.h
new file mode 100644
index 0000000000..7c039ef3fd
--- /dev/null
+++ b/gfx/layers/apz/src/WheelScrollAnimation.h
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_layers_WheelScrollAnimation_h_
+#define mozilla_layers_WheelScrollAnimation_h_
+
+#include "GenericScrollAnimation.h"
+#include "InputData.h"
+
+namespace mozilla {
+namespace layers {
+
+class AsyncPanZoomController;
+
+class WheelScrollAnimation : public GenericScrollAnimation {
+ public:
+  WheelScrollAnimation(AsyncPanZoomController& aApzc,
+                       const nsPoint& aInitialPosition,
+                       ScrollWheelInput::ScrollDeltaType aDeltaType);
+
+  WheelScrollAnimation* AsWheelScrollAnimation() override { return this; }
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_WheelScrollAnimation_h_