Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

1 files changed, 3742 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..9dd296dd97
--- /dev/null
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -0,0 +1,3742 @@
+/* -*- 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 "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/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/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 "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()),
+      mRootLayersId(aRootLayersId),
+      mSampler(nullptr),
+      mUpdater(nullptr),
+      mTreeLock("APZCTreeLock"),
+      mMapLock("APZCMapLock"),
+      mRetainedTouchIdentifier(-1),
+      mInScrollbarTouchDrag(false),
+      mCurrentMousePosition(ScreenPoint(),
+                            "APZCTreeManager::mCurrentMousePosition"),
+      mApzcTreeLog("apzctree"),
+      mTestDataLock("APZTestDataLock"),
+      mDPI(160.0),
+      mHitTester(std::move(aHitTester)),
+      mScrollGenerationLock("APZScrollGenerationLock") {
+  RefPtr<APZCTreeManager> self(this);
+  NS_DispatchToMainThread(NS_NewRunnableFunction(
+      "layers::APZCTreeManager::APZCTreeManager",
+      [self] { self->mFlushObserver = new CheckerboardFlushObserver(self); }));
+  AsyncPanZoomController::InitializeGlobalState();
+  mApzcTreeLog.ConditionOnPrefFunction(StaticPrefs::apz_printtree);
+
+  if (!mHitTester) {
+    mHitTester = MakeUnique<WRHitTester>();
+  }
+  mHitTester->Initialize(this);
+}
+
+APZCTreeManager::~APZCTreeManager() = default;
+
+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).
+  }
+
+  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);
+  }
+}
+
+AsyncPanZoomController* APZCTreeManager::NewAPZCInstance(
+    LayersId aLayersId, GeckoContentController* aController) {
+  return new AsyncPanZoomController(
+      aLayersId, this, mInputQueue, aController,
+      AsyncPanZoomController::USE_GESTURE_DETECTOR);
+}
+
+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;
+  mAsyncZoomContainerSubtree = 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;
+            }
+            mAsyncZoomContainerSubtree = 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(
+        !mAsyncZoomContainerSubtree ||
+            !subtreesWithRootContentOutsideAsyncZoomContainer.Contains(
+                *mAsyncZoomContainerSubtree),
+        "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;
+      });
+
+  bool activeAnimations = AdvanceAnimationsInternal(lock, aSampleTime);
+  if (activeAnimations && controller) {
+    controller->ScheduleRenderOnCompositorThread(
+        wr::RenderReasons::ANIMATED_PROPERTY);
+  }
+
+  nsTArray<wr::WrTransformProperty> transforms;
+
+  // Sample async transforms on scrollable layers.
+  for (const auto& mapping : mApzcMap) {
+    AsyncPanZoomController* apzc = mapping.second.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);
+      }
+    }
+
+    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});
+
+      transforms.AppendElement(wr::ToWrTransformProperty(
+          *zoomAnimationId, LayoutDeviceToParentLayerMatrix4x4::Scaling(
+                                zoom.scale, zoom.scale, 1.0f) *
+                                AsyncTransformComponentMatrix::Translation(
+                                    asyncVisualTransform.mTranslation)));
+
+      aTxn.UpdateIsTransformAsyncZooming(*zoomAnimationId,
+                                         apzc->IsAsyncZooming());
+    }
+
+    nsTArray<wr::SampledScrollOffset> sampledOffsets =
+        apzc->GetSampledScrollOffsets();
+    aTxn.UpdateScrollPosition(wr::AsPipelineId(apzc->GetGuid().mLayersId),
+                              apzc->GetGuid().mScrollId, sampledOffsets);
+
+#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, 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.GetVisibleRegion(),
+                        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;
+  }
+
+  AsyncPanZoomController* apzc = nullptr;
+  // 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, 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, 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::eForHitTesting));
+      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()) {
+            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 | MODIFIER_OS)) != 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();
+      }
+
+      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));
+      }
+
+      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));
+    }
+    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 = ComputeTransformForNode(aScrollThumbNode.Get(lock));
+    }
+    // Only consider the translation, since we do not support both
+    // zooming and scrollbar dragging on any platform.
+    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 (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::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 nontransient async transform    (hereafter referred to as transform matrix LN)
+        L's transient async transform       (hereafter referred to as transform matrix LT)
+        M's CSS transform                   (hereafter referred to as transform matrix MC)
+        M's nontransient async transform    (hereafter referred to as transform matrix MN)
+        M's transient async transform       (hereafter referred to as transform matrix MT)
+        ...
+        R's CSS transform                   (hereafter referred to as transform matrix RC)
+        R's nontransient async transform    (hereafter referred to as transform matrix RN)
+        R's transient async transform       (hereafter referred to as transform matrix RT)
+
+   Also, for any layer, the async transform is the combination of its transient and non-transient
+   parts. That is, for any layer L:
+                  LA === LN * LT
+        LA.Inverse() === LT.Inverse() * LN.Inverse()
+
+   If we want user input to modify L's transient async transform, we have to first convert
+   user input from screen space to the coordinate space of L's transient async transform. Doing
+   this involves applying the following transforms (in order from top to bottom):
+        RT.Inverse()
+        RN.Inverse()
+        RC.Inverse()
+        ...
+        MT.Inverse()
+        MN.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
+                                     ->GetCurrentAsyncTransformWithOverscroll(
+                                         AsyncPanZoomController::eForHitTesting)
+                                     .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.
+ */
+ParentLayerToScreenMatrix4x4 APZCTreeManager::GetApzcToGeckoTransform(
+    const AsyncPanZoomController* aApzc,
+    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. 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.
+
+  // asyncUntransform is LA.Inverse()
+  Matrix4x4 asyncUntransform =
+      aApzc
+          ->GetCurrentAsyncTransformWithOverscroll(
+              AsyncPanZoomController::eForHitTesting, aComponents)
+          .Inverse()
+          .ToUnknownMatrix();
+
+  // aTransformToGeckoOut is initialized to LA.Inverse() * LD * MC * NC * OC *
+  // PC
+  result = asyncUntransform *
+           aApzc->GetTransformToLastDispatchedPaint(aComponents) *
+           aApzc->GetAncestorTransform();
+
+  for (AsyncPanZoomController* parent = aApzc->GetParent(); parent;
+       parent = parent->GetParent()) {
+    // aTransformToGeckoOut 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) *
+             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<ParentLayerToScreenMatrix4x4>(result);
+}
+
+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);
+}
+
+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) const {
+  MutexAutoLock lock(mMapLock);
+  return SidesStuckToRootContent(StickyPositionInfo(aNode), lock);
+}
+
+SideBits APZCTreeManager::SidesStuckToRootContent(
+    const StickyPositionInfo& aStickyInfo,
+    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(
+              AsyncPanZoomController::eForHitTesting,
+              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::ComputeTransformForNode(
+    const HitTestingTreeNode* aNode) const {
+  mTreeLock.AssertCurrentThreadIn();
+  // The async transforms applied here for hit-testing purposes, are intended
+  // to match the ones AsyncCompositionManager (or equivalent WebRender code)
+  // applies for rendering purposes.
+  // Note that with containerless scrolling, the layer structure looks like
+  // this:
+  //
+  //   root container layer
+  //     async zoom container layer
+  //       scrollable content layers (with scroll metadata)
+  //       fixed content layers (no scroll metadta, annotated isFixedPosition)
+  //     scrollbar layers
+  //
+  // The intended async transforms in this case are:
+  //  * On the async zoom container layer, the "visual" portion of the root
+  //    content APZC's async transform (which includes the zoom, and async
+  //    scrolling of the visual viewport relative to the layout viewport).
+  //  * On the scrollable layers bearing the root content APZC's scroll
+  //    metadata, the "layout" portion of the root content APZC's async
+  //    transform (which includes async scrolling of the layout viewport
+  //    relative to the scrollable rect origin).
+  if (AsyncPanZoomController* apzc = aNode->GetApzc()) {
+    // If the node represents scrollable content, apply the async transform
+    // from its APZC.
+    bool visualTransformIsInheritedFromAncestor =
+        /* we're the APZC whose visual transform might be on the async
+           zoom container */
+        apzc->IsRootContent() &&
+        /* there is an async zoom container on this subtree */
+        mAsyncZoomContainerSubtree == Some(aNode->GetLayersId()) &&
+        /* it's not us */
+        !aNode->GetAsyncZoomContainerId();
+    AsyncTransformComponents components =
+        visualTransformIsInheritedFromAncestor
+            ? AsyncTransformComponents{AsyncTransformComponent::eLayout}
+            : LayoutAndVisual;
+    return aNode->GetTransform() *
+           CompleteAsyncTransform(apzc->GetCurrentAsyncTransformWithOverscroll(
+               AsyncPanZoomController::eForHitTesting, components));
+  } else if (aNode->GetAsyncZoomContainerId()) {
+    if (AsyncPanZoomController* rootContent =
+            FindRootContentApzcForLayersId(aNode->GetLayersId())) {
+      return aNode->GetTransform() *
+             CompleteAsyncTransform(
+                 rootContent->GetCurrentAsyncTransformWithOverscroll(
+                     AsyncPanZoomController::eForHitTesting,
+                     {AsyncTransformComponent::eVisual}));
+    }
+  } else if (aNode->IsScrollThumbNode()) {
+    // If the node represents a scrollbar thumb, 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));
+          });
+    }
+  } else if (IsFixedToRootContent(aNode)) {
+    ParentLayerPoint translation;
+    {
+      MutexAutoLock mapLock(mMapLock);
+      translation = ViewAs<ParentLayerPixel>(
+          apz::ComputeFixedMarginsOffset(
+              GetCompositorFixedLayerMargins(mapLock),
+              aNode->GetFixedPosSides(), mGeckoFixedLayerMargins),
+          PixelCastJustification::ScreenIsParentLayerForRoot);
+    }
+    return aNode->GetTransform() *
+           CompleteAsyncTransform(
+               AsyncTransformComponentMatrix::Translation(translation));
+  }
+  SideBits sides = SidesStuckToRootContent(aNode);
+  if (sides != SideBits::eNone) {
+    ParentLayerPoint translation;
+    {
+      MutexAutoLock mapLock(mMapLock);
+      translation = ViewAs<ParentLayerPixel>(
+          apz::ComputeFixedMarginsOffset(
+              GetCompositorFixedLayerMargins(mapLock), 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()),
+          PixelCastJustification::ScreenIsParentLayerForRoot);
+    }
+    return aNode->GetTransform() *
+           CompleteAsyncTransform(
+               AsyncTransformComponentMatrix::Translation(translation));
+  }
+  // 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 {
+              messages.AppendElement(MatrixMessage(
+                  Some(parent->GetTransformToGecko()),
+                  parent->GetRemoteDocumentScreenRect(), 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 = aTop;
+  mCompositorFixedLayerMargins.bottom = 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