Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 8022 insertions, 0 deletions

diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
new file mode 100644
index 0000000000..b51f3eccb7
--- /dev/null
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -0,0 +1,8022 @@
+/* -*- 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/. */
+
+/* rendering object to wrap rendering objects that should be scrollable */
+
+#include "nsGfxScrollFrame.h"
+
+#include "ScrollPositionUpdate.h"
+#include "mozilla/layers/LayersTypes.h"
+#include "nsIXULRuntime.h"
+#include "base/compiler_specific.h"
+#include "DisplayItemClip.h"
+#include "nsCOMPtr.h"
+#include "nsIDocumentViewer.h"
+#include "nsPresContext.h"
+#include "nsView.h"
+#include "nsViewportInfo.h"
+#include "nsContainerFrame.h"
+#include "nsGkAtoms.h"
+#include "nsNameSpaceManager.h"
+#include "mozilla/intl/BidiEmbeddingLevel.h"
+#include "mozilla/dom/DocumentInlines.h"
+#include "mozilla/gfx/gfxVars.h"
+#include "nsFontMetrics.h"
+#include "mozilla/dom/NodeInfo.h"
+#include "nsScrollbarFrame.h"
+#include "nsINode.h"
+#include "nsIScrollbarMediator.h"
+#include "nsITextControlFrame.h"
+#include "nsILayoutHistoryState.h"
+#include "nsNodeInfoManager.h"
+#include "nsContentCreatorFunctions.h"
+#include "nsStyleTransformMatrix.h"
+#include "mozilla/PresState.h"
+#include "nsContentUtils.h"
+#include "nsDisplayList.h"
+#include "nsHTMLDocument.h"
+#include "nsLayoutUtils.h"
+#include "nsBidiPresUtils.h"
+#include "nsBidiUtils.h"
+#include "nsDocShell.h"
+#include "mozilla/ContentEvents.h"
+#include "mozilla/DisplayPortUtils.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/PresShell.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/ScrollbarPreferences.h"
+#include "mozilla/ScrollingMetrics.h"
+#include "mozilla/StaticPrefs_bidi.h"
+#include "mozilla/StaticPrefs_browser.h"
+#include "mozilla/StaticPrefs_toolkit.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/SVGOuterSVGFrame.h"
+#include "mozilla/ViewportUtils.h"
+#include "mozilla/LookAndFeel.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/Event.h"
+#include "mozilla/dom/HTMLMarqueeElement.h"
+#include "mozilla/dom/ScrollTimeline.h"
+#include "mozilla/dom/BrowserChild.h"
+#include <stdint.h>
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Telemetry.h"
+#include "nsSubDocumentFrame.h"
+#include "mozilla/Attributes.h"
+#include "ScrollbarActivity.h"
+#include "nsRefreshDriver.h"
+#include "nsStyleConsts.h"
+#include "nsIScrollPositionListener.h"
+#include "StickyScrollContainer.h"
+#include "nsIFrameInlines.h"
+#include "gfxPlatform.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_general.h"
+#include "mozilla/StaticPrefs_layers.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_mousewheel.h"
+#include "mozilla/ToString.h"
+#include "ScrollAnimationPhysics.h"
+#include "ScrollAnimationBezierPhysics.h"
+#include "ScrollAnimationMSDPhysics.h"
+#include "ScrollSnap.h"
+#include "UnitTransforms.h"
+#include "nsSliderFrame.h"
+#include "ViewportFrame.h"
+#include "mozilla/gfx/gfxVars.h"
+#include "mozilla/layers/APZCCallbackHelper.h"
+#include "mozilla/layers/APZPublicUtils.h"
+#include "mozilla/layers/AxisPhysicsModel.h"
+#include "mozilla/layers/AxisPhysicsMSDModel.h"
+#include "mozilla/layers/ScrollingInteractionContext.h"
+#include "mozilla/layers/ScrollLinkedEffectDetector.h"
+#include "mozilla/Unused.h"
+#include "MobileViewportManager.h"
+#include "VisualViewport.h"
+#include "WindowRenderer.h"
+#include <algorithm>
+#include <cstdlib>  // for std::abs(int/long)
+#include <cmath>    // for std::abs(float/double)
+#include <tuple>    // for std::tie
+
+static mozilla::LazyLogModule sApzPaintSkipLog("apz.paintskip");
+#define PAINT_SKIP_LOG(...) \
+  MOZ_LOG(sApzPaintSkipLog, LogLevel::Debug, (__VA_ARGS__))
+static mozilla::LazyLogModule sScrollRestoreLog("scrollrestore");
+#define SCROLLRESTORE_LOG(...) \
+  MOZ_LOG(sScrollRestoreLog, LogLevel::Debug, (__VA_ARGS__))
+static mozilla::LazyLogModule sRootScrollbarsLog("rootscrollbars");
+#define ROOT_SCROLLBAR_LOG(...)                                  \
+  if (mIsRoot) {                                                 \
+    MOZ_LOG(sRootScrollbarsLog, LogLevel::Debug, (__VA_ARGS__)); \
+  }
+static mozilla::LazyLogModule sDisplayportLog("apz.displayport");
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using namespace mozilla::gfx;
+using namespace mozilla::layers;
+using namespace mozilla::layout;
+using nsStyleTransformMatrix::TransformReferenceBox;
+
+static ScrollDirections GetOverflowChange(const nsRect& aCurScrolledRect,
+                                          const nsRect& aPrevScrolledRect) {
+  ScrollDirections result;
+  if (aPrevScrolledRect.x != aCurScrolledRect.x ||
+      aPrevScrolledRect.width != aCurScrolledRect.width) {
+    result += ScrollDirection::eHorizontal;
+  }
+  if (aPrevScrolledRect.y != aCurScrolledRect.y ||
+      aPrevScrolledRect.height != aCurScrolledRect.height) {
+    result += ScrollDirection::eVertical;
+  }
+  return result;
+}
+
+/**
+ * This class handles the dispatching of scroll events to content.
+ *
+ * Scroll events are posted to the refresh driver via
+ * nsRefreshDriver::PostScrollEvent(), and they are fired during a refresh
+ * driver tick, after running requestAnimationFrame callbacks but before
+ * the style flush. This allows rAF callbacks to perform scrolling and have
+ * that scrolling be reflected on the same refresh driver tick, while at
+ * the same time allowing scroll event listeners to make style changes and
+ * have those style changes be reflected on the same refresh driver tick.
+ *
+ * ScrollEvents cannot be refresh observers, because none of the existing
+ * categories of refresh observers (FlushType::Style, FlushType::Layout,
+ * and FlushType::Display) are run at the desired time in a refresh driver
+ * tick. They behave similarly to refresh observers in that their presence
+ * causes the refresh driver to tick.
+ *
+ * ScrollEvents are one-shot runnables; the refresh driver drops them after
+ * running them.
+ */
+class nsHTMLScrollFrame::ScrollEvent : public Runnable {
+ public:
+  NS_DECL_NSIRUNNABLE
+  explicit ScrollEvent(nsHTMLScrollFrame* aHelper, bool aDelayed);
+  void Revoke() { mHelper = nullptr; }
+
+ private:
+  nsHTMLScrollFrame* mHelper;
+};
+
+class nsHTMLScrollFrame::ScrollEndEvent : public Runnable {
+ public:
+  NS_DECL_NSIRUNNABLE
+  explicit ScrollEndEvent(nsHTMLScrollFrame* aHelper);
+  void Revoke() { mHelper = nullptr; }
+
+ private:
+  nsHTMLScrollFrame* mHelper;
+};
+
+class nsHTMLScrollFrame::AsyncScrollPortEvent : public Runnable {
+ public:
+  NS_DECL_NSIRUNNABLE
+  explicit AsyncScrollPortEvent(nsHTMLScrollFrame* helper)
+      : Runnable("nsHTMLScrollFrame::AsyncScrollPortEvent"), mHelper(helper) {}
+  void Revoke() { mHelper = nullptr; }
+
+ private:
+  nsHTMLScrollFrame* mHelper;
+};
+
+class nsHTMLScrollFrame::ScrolledAreaEvent : public Runnable {
+ public:
+  NS_DECL_NSIRUNNABLE
+  explicit ScrolledAreaEvent(nsHTMLScrollFrame* helper)
+      : Runnable("nsHTMLScrollFrame::ScrolledAreaEvent"), mHelper(helper) {}
+  void Revoke() { mHelper = nullptr; }
+
+ private:
+  nsHTMLScrollFrame* mHelper;
+};
+
+//----------------------------------------------------------------------
+
+//----------nsHTMLScrollFrame-------------------------------------------
+
+class ScrollFrameActivityTracker final
+    : public nsExpirationTracker<nsHTMLScrollFrame, 4> {
+ public:
+  // Wait for 3-4s between scrolls before we remove our layers.
+  // That's 4 generations of 1s each.
+  enum { TIMEOUT_MS = 1000 };
+  explicit ScrollFrameActivityTracker(nsIEventTarget* aEventTarget)
+      : nsExpirationTracker<nsHTMLScrollFrame, 4>(
+            TIMEOUT_MS, "ScrollFrameActivityTracker", aEventTarget) {}
+  ~ScrollFrameActivityTracker() { AgeAllGenerations(); }
+
+  virtual void NotifyExpired(nsHTMLScrollFrame* aObject) override {
+    RemoveObject(aObject);
+    aObject->MarkNotRecentlyScrolled();
+  }
+};
+static StaticAutoPtr<ScrollFrameActivityTracker> gScrollFrameActivityTracker;
+
+nsHTMLScrollFrame* NS_NewHTMLScrollFrame(PresShell* aPresShell,
+                                         ComputedStyle* aStyle, bool aIsRoot) {
+  return new (aPresShell)
+      nsHTMLScrollFrame(aStyle, aPresShell->GetPresContext(), aIsRoot);
+}
+
+NS_IMPL_FRAMEARENA_HELPERS(nsHTMLScrollFrame)
+
+nsHTMLScrollFrame::nsHTMLScrollFrame(ComputedStyle* aStyle,
+                                     nsPresContext* aPresContext,
+                                     nsIFrame::ClassID aID, bool aIsRoot)
+    : nsContainerFrame(aStyle, aPresContext, aID),
+      mHScrollbarBox(nullptr),
+      mVScrollbarBox(nullptr),
+      mScrolledFrame(nullptr),
+      mScrollCornerBox(nullptr),
+      mResizerBox(nullptr),
+      mReferenceFrameDuringPainting(nullptr),
+      mAsyncScroll(nullptr),
+      mAsyncSmoothMSDScroll(nullptr),
+      mLastScrollOrigin(ScrollOrigin::None),
+      mDestination(0, 0),
+      mRestorePos(-1, -1),
+      mLastPos(-1, -1),
+      mApzScrollPos(0, 0),
+      mLastUpdateFramesPos(-1, -1),
+      mScrollParentID(mozilla::layers::ScrollableLayerGuid::NULL_SCROLL_ID),
+      mAnchor(this),
+      mCurrentAPZScrollAnimationType(APZScrollAnimationType::No),
+      mIsFirstScrollableFrameSequenceNumber(Nothing()),
+      mInScrollingGesture(InScrollingGesture::No),
+      mAllowScrollOriginDowngrade(false),
+      mHadDisplayPortAtLastFrameUpdate(false),
+      mHasVerticalScrollbar(false),
+      mHasHorizontalScrollbar(false),
+      mOnlyNeedVScrollbarToScrollVVInsideLV(false),
+      mOnlyNeedHScrollbarToScrollVVInsideLV(false),
+      mFrameIsUpdatingScrollbar(false),
+      mDidHistoryRestore(false),
+      mIsRoot(aIsRoot),
+      mSuppressScrollbarUpdate(false),
+      mSkippedScrollbarLayout(false),
+      mHadNonInitialReflow(false),
+      mFirstReflow(true),
+      mHorizontalOverflow(false),
+      mVerticalOverflow(false),
+      mPostedReflowCallback(false),
+      mMayHaveDirtyFixedChildren(false),
+      mUpdateScrollbarAttributes(false),
+      mHasBeenScrolledRecently(false),
+      mWillBuildScrollableLayer(false),
+      mIsParentToActiveScrollFrames(false),
+      mHasBeenScrolled(false),
+      mIgnoreMomentumScroll(false),
+      mTransformingByAPZ(false),
+      mScrollableByAPZ(false),
+      mZoomableByAPZ(false),
+      mHasOutOfFlowContentInsideFilter(false),
+      mSuppressScrollbarRepaints(false),
+      mIsUsingMinimumScaleSize(false),
+      mMinimumScaleSizeChanged(false),
+      mProcessingScrollEvent(false),
+      mApzAnimationRequested(false),
+      mApzAnimationTriggeredByScriptRequested(false),
+      mReclampVVOffsetInReflowFinished(false),
+      mMayScheduleScrollAnimations(false),
+#ifdef MOZ_WIDGET_ANDROID
+      mHasVerticalOverflowForDynamicToolbar(false),
+#endif
+      mVelocityQueue(PresContext()) {
+  AppendScrollUpdate(ScrollPositionUpdate::NewScrollframe(nsPoint()));
+
+  if (UsesOverlayScrollbars()) {
+    mScrollbarActivity = new ScrollbarActivity(this);
+  }
+
+  if (mIsRoot) {
+    mZoomableByAPZ = PresShell()->GetZoomableByAPZ();
+  }
+}
+
+nsHTMLScrollFrame::~nsHTMLScrollFrame() = default;
+
+void nsHTMLScrollFrame::ScrollbarActivityStarted() const {
+  if (mScrollbarActivity) {
+    mScrollbarActivity->ActivityStarted();
+  }
+}
+
+void nsHTMLScrollFrame::ScrollbarActivityStopped() const {
+  if (mScrollbarActivity) {
+    mScrollbarActivity->ActivityStopped();
+  }
+}
+
+void nsHTMLScrollFrame::Destroy(DestroyContext& aContext) {
+  DestroyAbsoluteFrames(aContext);
+  if (mIsRoot) {
+    PresShell()->ResetVisualViewportOffset();
+  }
+
+  mAnchor.Destroy();
+
+  if (mScrollbarActivity) {
+    mScrollbarActivity->Destroy();
+    mScrollbarActivity = nullptr;
+  }
+
+  // Unbind the content created in CreateAnonymousContent later...
+  aContext.AddAnonymousContent(mHScrollbarContent.forget());
+  aContext.AddAnonymousContent(mVScrollbarContent.forget());
+  aContext.AddAnonymousContent(mScrollCornerContent.forget());
+  aContext.AddAnonymousContent(mResizerContent.forget());
+
+  if (mPostedReflowCallback) {
+    PresShell()->CancelReflowCallback(this);
+    mPostedReflowCallback = false;
+  }
+
+  if (mDisplayPortExpiryTimer) {
+    mDisplayPortExpiryTimer->Cancel();
+    mDisplayPortExpiryTimer = nullptr;
+  }
+  if (mActivityExpirationState.IsTracked()) {
+    gScrollFrameActivityTracker->RemoveObject(this);
+  }
+  if (gScrollFrameActivityTracker && gScrollFrameActivityTracker->IsEmpty()) {
+    gScrollFrameActivityTracker = nullptr;
+  }
+
+  if (mScrollActivityTimer) {
+    mScrollActivityTimer->Cancel();
+    mScrollActivityTimer = nullptr;
+  }
+  RemoveObservers();
+  if (mScrollEvent) {
+    mScrollEvent->Revoke();
+  }
+  if (mScrollEndEvent) {
+    mScrollEndEvent->Revoke();
+  }
+  nsContainerFrame::Destroy(aContext);
+}
+
+void nsHTMLScrollFrame::SetInitialChildList(ChildListID aListID,
+                                            nsFrameList&& aChildList) {
+  nsContainerFrame::SetInitialChildList(aListID, std::move(aChildList));
+  ReloadChildFrames();
+}
+
+void nsHTMLScrollFrame::AppendFrames(ChildListID aListID,
+                                     nsFrameList&& aFrameList) {
+  NS_ASSERTION(aListID == FrameChildListID::Principal,
+               "Only main list supported");
+  mFrames.AppendFrames(nullptr, std::move(aFrameList));
+  ReloadChildFrames();
+}
+
+void nsHTMLScrollFrame::InsertFrames(ChildListID aListID, nsIFrame* aPrevFrame,
+                                     const nsLineList::iterator* aPrevFrameLine,
+                                     nsFrameList&& aFrameList) {
+  NS_ASSERTION(aListID == FrameChildListID::Principal,
+               "Only main list supported");
+  NS_ASSERTION(!aPrevFrame || aPrevFrame->GetParent() == this,
+               "inserting after sibling frame with different parent");
+  mFrames.InsertFrames(nullptr, aPrevFrame, std::move(aFrameList));
+  ReloadChildFrames();
+}
+
+void nsHTMLScrollFrame::RemoveFrame(DestroyContext& aContext,
+                                    ChildListID aListID, nsIFrame* aOldFrame) {
+  NS_ASSERTION(aListID == FrameChildListID::Principal,
+               "Only main list supported");
+  mFrames.DestroyFrame(aContext, aOldFrame);
+  ReloadChildFrames();
+}
+
+/**
+ HTML scrolling implementation
+
+ All other things being equal, we prefer layouts with fewer scrollbars showing.
+*/
+
+namespace mozilla {
+
+enum class ShowScrollbar : uint8_t {
+  Auto,
+  Always,
+  // Never is a misnomer. We can still get a scrollbar if we need to scroll the
+  // visual viewport inside the layout viewport. Thus this enum is best thought
+  // of as value used by layout, which does not know about the visual viewport.
+  // The visual viewport does not affect any layout sizes, so this is sound.
+  Never,
+};
+
+static ShowScrollbar ShouldShowScrollbar(StyleOverflow aOverflow) {
+  switch (aOverflow) {
+    case StyleOverflow::Scroll:
+      return ShowScrollbar::Always;
+    case StyleOverflow::Hidden:
+      return ShowScrollbar::Never;
+    default:
+    case StyleOverflow::Auto:
+      return ShowScrollbar::Auto;
+  }
+}
+
+struct MOZ_STACK_CLASS ScrollReflowInput {
+  // === Filled in by the constructor. Members in this section shouldn't change
+  // their values after the constructor. ===
+  const ReflowInput& mReflowInput;
+  ShowScrollbar mHScrollbar;
+  // If the horizontal scrollbar is allowed (even if mHScrollbar ==
+  // ShowScrollbar::Never) provided that it is for scrolling the visual viewport
+  // inside the layout viewport only.
+  bool mHScrollbarAllowedForScrollingVVInsideLV = true;
+  ShowScrollbar mVScrollbar;
+  // If the vertical scrollbar is allowed (even if mVScrollbar ==
+  // ShowScrollbar::Never) provided that it is for scrolling the visual viewport
+  // inside the layout viewport only.
+  bool mVScrollbarAllowedForScrollingVVInsideLV = true;
+  nsMargin mComputedBorder;
+
+  // === Filled in by ReflowScrolledFrame ===
+  OverflowAreas mContentsOverflowAreas;
+  // The scrollbar gutter sizes used in the most recent reflow of
+  // mScrolledFrame. The writing-mode is the same as the scroll
+  // container.
+  LogicalMargin mScrollbarGutterFromLastReflow;
+  // True if the most recent reflow of mScrolledFrame is with the
+  // horizontal scrollbar.
+  bool mReflowedContentsWithHScrollbar = false;
+  // True if the most recent reflow of mScrolledFrame is with the
+  // vertical scrollbar.
+  bool mReflowedContentsWithVScrollbar = false;
+
+  // === Filled in when TryLayout succeeds ===
+  // The size of the inside-border area
+  nsSize mInsideBorderSize;
+  // Whether we decided to show the horizontal scrollbar in the most recent
+  // TryLayout.
+  bool mShowHScrollbar = false;
+  // Whether we decided to show the vertical scrollbar in the most recent
+  // TryLayout.
+  bool mShowVScrollbar = false;
+  // If mShow(H|V)Scrollbar is true then
+  // mOnlyNeed(V|H)ScrollbarToScrollVVInsideLV indicates if the only reason we
+  // need that scrollbar is to scroll the visual viewport inside the layout
+  // viewport. These scrollbars are special in that even if they are layout
+  // scrollbars they do not take up any layout space.
+  bool mOnlyNeedHScrollbarToScrollVVInsideLV = false;
+  bool mOnlyNeedVScrollbarToScrollVVInsideLV = false;
+
+  ScrollReflowInput(nsHTMLScrollFrame* aFrame, const ReflowInput& aReflowInput);
+
+  nscoord VScrollbarMinHeight() const { return mVScrollbarPrefSize.height; }
+  nscoord VScrollbarPrefWidth() const { return mVScrollbarPrefSize.width; }
+  nscoord HScrollbarMinWidth() const { return mHScrollbarPrefSize.width; }
+  nscoord HScrollbarPrefHeight() const { return mHScrollbarPrefSize.height; }
+
+  // Returns the sizes occupied by the scrollbar gutters. If aShowVScroll or
+  // aShowHScroll is true, the sizes occupied by the scrollbars are also
+  // included.
+  nsMargin ScrollbarGutter(bool aShowVScrollbar, bool aShowHScrollbar,
+                           bool aScrollbarOnRight) const {
+    if (mOverlayScrollbars) {
+      return mScrollbarGutter;
+    }
+    nsMargin gutter = mScrollbarGutter;
+    if (aShowVScrollbar && gutter.right == 0 && gutter.left == 0) {
+      const nscoord w = VScrollbarPrefWidth();
+      if (aScrollbarOnRight) {
+        gutter.right = w;
+      } else {
+        gutter.left = w;
+      }
+    }
+    if (aShowHScrollbar && gutter.bottom == 0) {
+      // The horizontal scrollbar is always at the bottom side.
+      gutter.bottom = HScrollbarPrefHeight();
+    }
+    return gutter;
+  }
+
+  bool OverlayScrollbars() const { return mOverlayScrollbars; }
+
+ private:
+  // Filled in by the constructor. Put variables here to keep them unchanged
+  // after initializing them in the constructor.
+  nsSize mVScrollbarPrefSize;
+  nsSize mHScrollbarPrefSize;
+  bool mOverlayScrollbars = false;
+  // The scrollbar gutter sizes resolved from the scrollbar-gutter and
+  // scrollbar-width property.
+  nsMargin mScrollbarGutter;
+};
+
+ScrollReflowInput::ScrollReflowInput(nsHTMLScrollFrame* aFrame,
+                                     const ReflowInput& aReflowInput)
+    : mReflowInput(aReflowInput),
+      mComputedBorder(aReflowInput.ComputedPhysicalBorderPadding() -
+                      aReflowInput.ComputedPhysicalPadding()),
+      mScrollbarGutterFromLastReflow(aFrame->GetWritingMode()) {
+  ScrollStyles styles = aFrame->GetScrollStyles();
+  mHScrollbar = ShouldShowScrollbar(styles.mHorizontal);
+  mVScrollbar = ShouldShowScrollbar(styles.mVertical);
+  mOverlayScrollbars = aFrame->UsesOverlayScrollbars();
+
+  if (nsScrollbarFrame* scrollbar = aFrame->GetScrollbarBox(false)) {
+    scrollbar->SetScrollbarMediatorContent(mReflowInput.mFrame->GetContent());
+    mHScrollbarPrefSize = scrollbar->ScrollbarMinSize();
+    // A zero minimum size is a bug with non-overlay scrollbars. That means
+    // we'll always try to place the scrollbar, even if it will ultimately not
+    // fit, see bug 1809630. XUL collapsing is the exception because the
+    // front-end uses it.
+    MOZ_ASSERT(mHScrollbarPrefSize.width && mHScrollbarPrefSize.height,
+               "Shouldn't have a zero horizontal scrollbar-size");
+  } else {
+    mHScrollbar = ShowScrollbar::Never;
+    mHScrollbarAllowedForScrollingVVInsideLV = false;
+  }
+  if (nsScrollbarFrame* scrollbar = aFrame->GetScrollbarBox(true)) {
+    scrollbar->SetScrollbarMediatorContent(mReflowInput.mFrame->GetContent());
+    mVScrollbarPrefSize = scrollbar->ScrollbarMinSize();
+    // See above.
+    MOZ_ASSERT(mVScrollbarPrefSize.width && mVScrollbarPrefSize.height,
+               "Shouldn't have a zero vertical scrollbar-size");
+  } else {
+    mVScrollbar = ShowScrollbar::Never;
+    mVScrollbarAllowedForScrollingVVInsideLV = false;
+  }
+
+  const auto* scrollbarStyle =
+      nsLayoutUtils::StyleForScrollbar(mReflowInput.mFrame);
+  // Hide the scrollbar when the scrollbar-width is set to none.
+  //
+  // Note: In some cases this is unnecessary, because scrollbar-width:none
+  // makes us suppress scrollbars in CreateAnonymousContent. But if this frame
+  // initially had a non-'none' scrollbar-width and dynamically changed to
+  // 'none', then we'll need to handle it here.
+  const auto scrollbarWidth = scrollbarStyle->StyleUIReset()->ScrollbarWidth();
+  if (scrollbarWidth == StyleScrollbarWidth::None) {
+    mHScrollbar = ShowScrollbar::Never;
+    mHScrollbarAllowedForScrollingVVInsideLV = false;
+    mVScrollbar = ShowScrollbar::Never;
+    mVScrollbarAllowedForScrollingVVInsideLV = false;
+  }
+
+  mScrollbarGutter = aFrame->ComputeStableScrollbarGutter(
+      scrollbarWidth, scrollbarStyle->StyleDisplay()->mScrollbarGutter);
+}
+
+}  // namespace mozilla
+
+static nsSize ComputeInsideBorderSize(const ScrollReflowInput& aState,
+                                      const nsSize& aDesiredInsideBorderSize) {
+  // aDesiredInsideBorderSize is the frame size; i.e., it includes
+  // borders and padding (but the scrolled child doesn't have
+  // borders). The scrolled child has the same padding as us.
+  const WritingMode wm = aState.mReflowInput.GetWritingMode();
+  const LogicalSize desiredInsideBorderSize(wm, aDesiredInsideBorderSize);
+  LogicalSize contentSize = aState.mReflowInput.ComputedSize();
+  const LogicalMargin padding = aState.mReflowInput.ComputedLogicalPadding(wm);
+
+  if (contentSize.ISize(wm) == NS_UNCONSTRAINEDSIZE) {
+    contentSize.ISize(wm) =
+        desiredInsideBorderSize.ISize(wm) - padding.IStartEnd(wm);
+  }
+  if (contentSize.BSize(wm) == NS_UNCONSTRAINEDSIZE) {
+    contentSize.BSize(wm) =
+        desiredInsideBorderSize.BSize(wm) - padding.BStartEnd(wm);
+  }
+
+  contentSize.ISize(wm) =
+      aState.mReflowInput.ApplyMinMaxISize(contentSize.ISize(wm));
+  contentSize.BSize(wm) =
+      aState.mReflowInput.ApplyMinMaxBSize(contentSize.BSize(wm));
+
+  return (contentSize + padding.Size(wm)).GetPhysicalSize(wm);
+}
+
+/**
+ * Assuming that we know the metrics for our wrapped frame and
+ * whether the horizontal and/or vertical scrollbars are present,
+ * compute the resulting layout and return true if the layout is
+ * consistent. If the layout is consistent then we fill in the
+ * computed fields of the ScrollReflowInput.
+ *
+ * The layout is consistent when both scrollbars are showing if and only
+ * if they should be showing. A horizontal scrollbar should be showing if all
+ * following conditions are met:
+ * 1) the style is not HIDDEN
+ * 2) our inside-border height is at least the scrollbar height (i.e., the
+ * scrollbar fits vertically)
+ * 3) the style is SCROLL, or the kid's overflow-area XMost is
+ * greater than the scrollport width
+ *
+ * @param aForce if true, then we just assume the layout is consistent.
+ */
+bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput& aState,
+                                  ReflowOutput* aKidMetrics,
+                                  bool aAssumeHScroll, bool aAssumeVScroll,
+                                  bool aForce) {
+  if ((aState.mVScrollbar == ShowScrollbar::Never && aAssumeVScroll) ||
+      (aState.mHScrollbar == ShowScrollbar::Never && aAssumeHScroll)) {
+    NS_ASSERTION(!aForce, "Shouldn't be forcing a hidden scrollbar to show!");
+    return false;
+  }
+
+  const auto wm = GetWritingMode();
+  const nsMargin scrollbarGutter = aState.ScrollbarGutter(
+      aAssumeVScroll, aAssumeHScroll, IsScrollbarOnRight());
+  const LogicalMargin logicalScrollbarGutter(wm, scrollbarGutter);
+
+  const bool inlineEndsGutterChanged =
+      aState.mScrollbarGutterFromLastReflow.IStartEnd(wm) !=
+      logicalScrollbarGutter.IStartEnd(wm);
+  const bool blockEndsGutterChanged =
+      aState.mScrollbarGutterFromLastReflow.BStartEnd(wm) !=
+      logicalScrollbarGutter.BStartEnd(wm);
+  const bool shouldReflowScrolledFrame =
+      inlineEndsGutterChanged ||
+      (blockEndsGutterChanged && ScrolledContentDependsOnBSize(aState));
+
+  if (shouldReflowScrolledFrame) {
+    if (blockEndsGutterChanged) {
+      nsLayoutUtils::MarkIntrinsicISizesDirtyIfDependentOnBSize(mScrolledFrame);
+    }
+    aKidMetrics->mOverflowAreas.Clear();
+    ROOT_SCROLLBAR_LOG(
+        "TryLayout reflowing scrolled frame with scrollbars h=%d, v=%d\n",
+        aAssumeHScroll, aAssumeVScroll);
+    ReflowScrolledFrame(aState, aAssumeHScroll, aAssumeVScroll, aKidMetrics);
+  }
+
+  const nsSize scrollbarGutterSize(scrollbarGutter.LeftRight(),
+                                   scrollbarGutter.TopBottom());
+
+  // First, compute our inside-border size and scrollport size
+  nsSize kidSize = GetContainSizeAxes().ContainSize(
+      aKidMetrics->PhysicalSize(), *aState.mReflowInput.mFrame);
+  const nsSize desiredInsideBorderSize = kidSize + scrollbarGutterSize;
+  aState.mInsideBorderSize =
+      ComputeInsideBorderSize(aState, desiredInsideBorderSize);
+
+  nsSize layoutSize =
+      mIsUsingMinimumScaleSize ? mMinimumScaleSize : aState.mInsideBorderSize;
+
+  const nsSize scrollPortSize =
+      Max(nsSize(0, 0), layoutSize - scrollbarGutterSize);
+  if (mIsUsingMinimumScaleSize) {
+    mICBSize =
+        Max(nsSize(0, 0), aState.mInsideBorderSize - scrollbarGutterSize);
+  }
+
+  nsSize visualViewportSize = scrollPortSize;
+  ROOT_SCROLLBAR_LOG("TryLayout with VV %s\n",
+                     ToString(visualViewportSize).c_str());
+  mozilla::PresShell* presShell = PresShell();
+  // Note: we check for a non-null MobileViepwortManager here, but ideally we
+  // should be able to drop that clause as well. It's just that in some cases
+  // with extension popups the composition size comes back as stale, because
+  // the content viewer is only resized after the popup contents are reflowed.
+  // That case also happens to have no APZ and no MVM, so we use that as a
+  // way to detect the scenario. Bug 1648669 tracks removing this clause.
+  if (mIsRoot && presShell->GetMobileViewportManager()) {
+    visualViewportSize = nsLayoutUtils::CalculateCompositionSizeForFrame(
+        this, false, &layoutSize);
+    visualViewportSize =
+        Max(nsSize(0, 0), visualViewportSize - scrollbarGutterSize);
+
+    float resolution = presShell->GetResolution();
+    visualViewportSize.width /= resolution;
+    visualViewportSize.height /= resolution;
+    ROOT_SCROLLBAR_LOG("TryLayout now with VV %s\n",
+                       ToString(visualViewportSize).c_str());
+  }
+
+  nsRect overflowRect = aState.mContentsOverflowAreas.ScrollableOverflow();
+  // If the content height expanded by the minimum-scale will be taller than
+  // the scrollable overflow area, we need to expand the area here to tell
+  // properly whether we need to render the overlay vertical scrollbar.
+  // NOTE: This expanded size should NOT be used for non-overley scrollbars
+  // cases since putting the vertical non-overlay scrollbar will make the
+  // content width narrow a little bit, which in turn the minimum scale value
+  // becomes a bit bigger than before, then the vertical scrollbar is no longer
+  // needed, which means the content width becomes the original width, then the
+  // minimum-scale is changed to the original one, and so forth.
+  if (UsesOverlayScrollbars() && mIsUsingMinimumScaleSize &&
+      mMinimumScaleSize.height > overflowRect.YMost()) {
+    overflowRect.height += mMinimumScaleSize.height - overflowRect.YMost();
+  }
+  nsRect scrolledRect =
+      GetUnsnappedScrolledRectInternal(overflowRect, scrollPortSize);
+  ROOT_SCROLLBAR_LOG(
+      "TryLayout scrolledRect:%s overflowRect:%s scrollportSize:%s\n",
+      ToString(scrolledRect).c_str(), ToString(overflowRect).c_str(),
+      ToString(scrollPortSize).c_str());
+  nscoord oneDevPixel = PresContext()->DevPixelsToAppUnits(1);
+
+  bool showHScrollbar = aAssumeHScroll;
+  bool showVScrollbar = aAssumeVScroll;
+  if (!aForce) {
+    nsSize sizeToCompare = visualViewportSize;
+    if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+      sizeToCompare = scrollPortSize;
+    }
+
+    // No need to compute showHScrollbar if we got ShowScrollbar::Never.
+    if (aState.mHScrollbar != ShowScrollbar::Never) {
+      showHScrollbar =
+          aState.mHScrollbar == ShowScrollbar::Always ||
+          scrolledRect.XMost() >= sizeToCompare.width + oneDevPixel ||
+          scrolledRect.x <= -oneDevPixel;
+      // TODO(emilio): This should probably check this scrollbar's minimum size
+      // in both axes, for consistency?
+      if (aState.mHScrollbar == ShowScrollbar::Auto &&
+          scrollPortSize.width < aState.HScrollbarMinWidth()) {
+        showHScrollbar = false;
+      }
+      ROOT_SCROLLBAR_LOG("TryLayout wants H Scrollbar: %d =? %d\n",
+                         showHScrollbar, aAssumeHScroll);
+    }
+
+    // No need to compute showVScrollbar if we got ShowScrollbar::Never.
+    if (aState.mVScrollbar != ShowScrollbar::Never) {
+      showVScrollbar =
+          aState.mVScrollbar == ShowScrollbar::Always ||
+          scrolledRect.YMost() >= sizeToCompare.height + oneDevPixel ||
+          scrolledRect.y <= -oneDevPixel;
+      // TODO(emilio): This should probably check this scrollbar's minimum size
+      // in both axes, for consistency?
+      if (aState.mVScrollbar == ShowScrollbar::Auto &&
+          scrollPortSize.height < aState.VScrollbarMinHeight()) {
+        showVScrollbar = false;
+      }
+      ROOT_SCROLLBAR_LOG("TryLayout wants V Scrollbar: %d =? %d\n",
+                         showVScrollbar, aAssumeVScroll);
+    }
+
+    if (showHScrollbar != aAssumeHScroll || showVScrollbar != aAssumeVScroll) {
+      const nsMargin wantedScrollbarGutter = aState.ScrollbarGutter(
+          showVScrollbar, showHScrollbar, IsScrollbarOnRight());
+      // We report an inconsistent layout only when the desired visibility of
+      // the scrollbars can change the size of the scrollbar gutters.
+      if (scrollbarGutter != wantedScrollbarGutter) {
+        return false;
+      }
+    }
+  }
+
+  // If we reach here, the layout is consistent. Record the desired visibility
+  // of the scrollbars.
+  aState.mShowHScrollbar = showHScrollbar;
+  aState.mShowVScrollbar = showVScrollbar;
+  const nsPoint scrollPortOrigin(
+      aState.mComputedBorder.left + scrollbarGutter.left,
+      aState.mComputedBorder.top + scrollbarGutter.top);
+  SetScrollPort(nsRect(scrollPortOrigin, scrollPortSize));
+
+  if (mIsRoot && gfxPlatform::UseDesktopZoomingScrollbars()) {
+    bool vvChanged = true;
+    const bool overlay = aState.OverlayScrollbars();
+    // This loop can run at most twice since we can only add a scrollbar once.
+    // At this point we've already decided that this layout is consistent so we
+    // will return true. Scrollbars added here never take up layout space even
+    // if they are layout scrollbars so any changes made here will not make us
+    // return false.
+    while (vvChanged) {
+      vvChanged = false;
+      if (!aState.mShowHScrollbar &&
+          aState.mHScrollbarAllowedForScrollingVVInsideLV) {
+        if (ScrollPort().width >= visualViewportSize.width + oneDevPixel &&
+            (overlay ||
+             visualViewportSize.width >= aState.HScrollbarMinWidth())) {
+          vvChanged = true;
+          if (!overlay) {
+            visualViewportSize.height -= aState.HScrollbarPrefHeight();
+          }
+          aState.mShowHScrollbar = true;
+          aState.mOnlyNeedHScrollbarToScrollVVInsideLV = true;
+          ROOT_SCROLLBAR_LOG("TryLayout added H scrollbar for VV, VV now %s\n",
+                             ToString(visualViewportSize).c_str());
+        }
+      }
+
+      if (!aState.mShowVScrollbar &&
+          aState.mVScrollbarAllowedForScrollingVVInsideLV) {
+        if (ScrollPort().height >= visualViewportSize.height + oneDevPixel &&
+            (overlay ||
+             visualViewportSize.height >= aState.VScrollbarMinHeight())) {
+          vvChanged = true;
+          if (!overlay) {
+            visualViewportSize.width -= aState.VScrollbarPrefWidth();
+          }
+          aState.mShowVScrollbar = true;
+          aState.mOnlyNeedVScrollbarToScrollVVInsideLV = true;
+          ROOT_SCROLLBAR_LOG("TryLayout added V scrollbar for VV, VV now %s\n",
+                             ToString(visualViewportSize).c_str());
+        }
+      }
+    }
+  }
+
+  return true;
+}
+
+bool nsHTMLScrollFrame::ScrolledContentDependsOnBSize(
+    const ScrollReflowInput& aState) const {
+  return mScrolledFrame->HasAnyStateBits(
+             NS_FRAME_CONTAINS_RELATIVE_BSIZE |
+             NS_FRAME_DESCENDANT_INTRINSIC_ISIZE_DEPENDS_ON_BSIZE) ||
+         aState.mReflowInput.ComputedBSize() != NS_UNCONSTRAINEDSIZE ||
+         aState.mReflowInput.ComputedMinBSize() > 0 ||
+         aState.mReflowInput.ComputedMaxBSize() != NS_UNCONSTRAINEDSIZE;
+}
+
+void nsHTMLScrollFrame::ReflowScrolledFrame(ScrollReflowInput& aState,
+                                            bool aAssumeHScroll,
+                                            bool aAssumeVScroll,
+                                            ReflowOutput* aMetrics) {
+  const WritingMode wm = GetWritingMode();
+
+  // these could be NS_UNCONSTRAINEDSIZE ... std::min arithmetic should
+  // be OK
+  LogicalMargin padding = aState.mReflowInput.ComputedLogicalPadding(wm);
+  nscoord availISize =
+      aState.mReflowInput.ComputedISize() + padding.IStartEnd(wm);
+
+  nscoord computedBSize = aState.mReflowInput.ComputedBSize();
+  nscoord computedMinBSize = aState.mReflowInput.ComputedMinBSize();
+  nscoord computedMaxBSize = aState.mReflowInput.ComputedMaxBSize();
+  if (!ShouldPropagateComputedBSizeToScrolledContent()) {
+    computedBSize = NS_UNCONSTRAINEDSIZE;
+    computedMinBSize = 0;
+    computedMaxBSize = NS_UNCONSTRAINEDSIZE;
+  }
+
+  const LogicalMargin scrollbarGutter(
+      wm, aState.ScrollbarGutter(aAssumeVScroll, aAssumeHScroll,
+                                 IsScrollbarOnRight()));
+  if (const nscoord inlineEndsGutter = scrollbarGutter.IStartEnd(wm);
+      inlineEndsGutter > 0) {
+    availISize = std::max(0, availISize - inlineEndsGutter);
+  }
+  if (const nscoord blockEndsGutter = scrollbarGutter.BStartEnd(wm);
+      blockEndsGutter > 0) {
+    if (computedBSize != NS_UNCONSTRAINEDSIZE) {
+      computedBSize = std::max(0, computedBSize - blockEndsGutter);
+    }
+    computedMinBSize = std::max(0, computedMinBSize - blockEndsGutter);
+    if (computedMaxBSize != NS_UNCONSTRAINEDSIZE) {
+      computedMaxBSize = std::max(0, computedMaxBSize - blockEndsGutter);
+    }
+  }
+
+  nsPresContext* presContext = PresContext();
+
+  // Pass InitFlags::CallerWillInit so we can pass in the correct padding.
+  ReflowInput kidReflowInput(presContext, aState.mReflowInput, mScrolledFrame,
+                             LogicalSize(wm, availISize, NS_UNCONSTRAINEDSIZE),
+                             Nothing(), ReflowInput::InitFlag::CallerWillInit);
+  const WritingMode kidWM = kidReflowInput.GetWritingMode();
+  kidReflowInput.Init(presContext, Nothing(), Nothing(),
+                      Some(padding.ConvertTo(kidWM, wm)));
+  kidReflowInput.mFlags.mAssumingHScrollbar = aAssumeHScroll;
+  kidReflowInput.mFlags.mAssumingVScrollbar = aAssumeVScroll;
+  kidReflowInput.mFlags.mTreatBSizeAsIndefinite =
+      aState.mReflowInput.mFlags.mTreatBSizeAsIndefinite;
+  kidReflowInput.SetComputedBSize(computedBSize);
+  kidReflowInput.SetComputedMinBSize(computedMinBSize);
+  kidReflowInput.SetComputedMaxBSize(computedMaxBSize);
+  if (aState.mReflowInput.IsBResizeForWM(kidWM)) {
+    kidReflowInput.SetBResize(true);
+  }
+  if (aState.mReflowInput.IsBResizeForPercentagesForWM(kidWM)) {
+    kidReflowInput.mFlags.mIsBResizeForPercentages = true;
+  }
+
+  // Temporarily set mHasHorizontalScrollbar/mHasVerticalScrollbar to
+  // reflect our assumptions while we reflow the child.
+  bool didHaveHorizontalScrollbar = mHasHorizontalScrollbar;
+  bool didHaveVerticalScrollbar = mHasVerticalScrollbar;
+  mHasHorizontalScrollbar = aAssumeHScroll;
+  mHasVerticalScrollbar = aAssumeVScroll;
+
+  nsReflowStatus status;
+  // No need to pass a true container-size to ReflowChild or
+  // FinishReflowChild, because it's only used there when positioning
+  // the frame (i.e. if ReflowChildFlags::NoMoveFrame isn't set)
+  const nsSize dummyContainerSize;
+  ReflowChild(mScrolledFrame, presContext, *aMetrics, kidReflowInput, wm,
+              LogicalPoint(wm), dummyContainerSize,
+              ReflowChildFlags::NoMoveFrame, status);
+
+  mHasHorizontalScrollbar = didHaveHorizontalScrollbar;
+  mHasVerticalScrollbar = didHaveVerticalScrollbar;
+
+  // Don't resize or position the view (if any) because we're going to resize
+  // it to the correct size anyway in PlaceScrollArea. Allowing it to
+  // resize here would size it to the natural height of the frame,
+  // which will usually be different from the scrollport height;
+  // invalidating the difference will cause unnecessary repainting.
+  FinishReflowChild(
+      mScrolledFrame, presContext, *aMetrics, &kidReflowInput, wm,
+      LogicalPoint(wm), dummyContainerSize,
+      ReflowChildFlags::NoMoveFrame | ReflowChildFlags::NoSizeView);
+
+  if (mScrolledFrame->HasAnyStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE)) {
+    // Propagate NS_FRAME_CONTAINS_RELATIVE_BSIZE from our inner scrolled frame
+    // to ourselves so that our containing block is aware of it.
+    //
+    // Note: If the scrolled frame has any child whose block-size depends on the
+    // containing block's block-size, the NS_FRAME_CONTAINS_RELATIVE_BSIZE bit
+    // is set on the scrolled frame when initializing the child's ReflowInput in
+    // ReflowInput::InitResizeFlags(). Therefore, we propagate the bit here
+    // after we reflowed the scrolled frame.
+    AddStateBits(NS_FRAME_CONTAINS_RELATIVE_BSIZE);
+  }
+
+  // XXX Some frames (e.g. nsFrameFrame, nsTextFrame) don't
+  // bother setting their mOverflowArea. This is wrong because every frame
+  // should always set mOverflowArea. In fact nsFrameFrame doesn't
+  // support the 'outline' property because of this. Rather than fix the
+  // world right now, just fix up the overflow area if necessary. Note that we
+  // don't check HasOverflowRect() because it could be set even though the
+  // overflow area doesn't include the frame bounds.
+  aMetrics->UnionOverflowAreasWithDesiredBounds();
+
+  auto* disp = StyleDisplay();
+  if (MOZ_UNLIKELY(disp->mOverflowClipBoxInline ==
+                   StyleOverflowClipBox::ContentBox)) {
+    // The scrolled frame is scrollable in the inline axis with
+    // `overflow-clip-box:content-box`. To prevent its content from being
+    // clipped at the scroll container's padding edges, we inflate its
+    // children's scrollable overflow area with its inline padding, and union
+    // its scrollable overflow area with its children's inflated scrollable
+    // overflow area.
+    OverflowAreas childOverflow;
+    mScrolledFrame->UnionChildOverflow(childOverflow);
+    nsRect childScrollableOverflow = childOverflow.ScrollableOverflow();
+
+    const LogicalMargin inlinePadding =
+        padding.ApplySkipSides(LogicalSides(wm, eLogicalSideBitsBBoth));
+    childScrollableOverflow.Inflate(inlinePadding.GetPhysicalMargin(wm));
+
+    nsRect& so = aMetrics->ScrollableOverflow();
+    so = so.UnionEdges(childScrollableOverflow);
+  }
+
+  aState.mContentsOverflowAreas = aMetrics->mOverflowAreas;
+  aState.mScrollbarGutterFromLastReflow = scrollbarGutter;
+  aState.mReflowedContentsWithHScrollbar = aAssumeHScroll;
+  aState.mReflowedContentsWithVScrollbar = aAssumeVScroll;
+}
+
+bool nsHTMLScrollFrame::GuessHScrollbarNeeded(const ScrollReflowInput& aState) {
+  if (aState.mHScrollbar != ShowScrollbar::Auto) {
+    // no guessing required
+    return aState.mHScrollbar == ShowScrollbar::Always;
+  }
+  // We only care about scrollbars that might take up space when trying to guess
+  // if we need a scrollbar, so we ignore scrollbars only created to scroll the
+  // visual viewport inside the layout viewport because they take up no layout
+  // space.
+  return mHasHorizontalScrollbar && !mOnlyNeedHScrollbarToScrollVVInsideLV;
+}
+
+bool nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowInput& aState) {
+  if (aState.mVScrollbar != ShowScrollbar::Auto) {
+    // no guessing required
+    return aState.mVScrollbar == ShowScrollbar::Always;
+  }
+
+  // If we've had at least one non-initial reflow, then just assume
+  // the state of the vertical scrollbar will be what we determined
+  // last time.
+  if (mHadNonInitialReflow) {
+    // We only care about scrollbars that might take up space when trying to
+    // guess if we need a scrollbar, so we ignore scrollbars only created to
+    // scroll the visual viewport inside the layout viewport because they take
+    // up no layout space.
+    return mHasVerticalScrollbar && !mOnlyNeedVScrollbarToScrollVVInsideLV;
+  }
+
+  // If this is the initial reflow, guess false because usually
+  // we have very little content by then.
+  if (InInitialReflow()) return false;
+
+  if (mIsRoot) {
+    nsIFrame* f = mScrolledFrame->PrincipalChildList().FirstChild();
+    if (f && f->IsSVGOuterSVGFrame() &&
+        static_cast<SVGOuterSVGFrame*>(f)->VerticalScrollbarNotNeeded()) {
+      // Common SVG case - avoid a bad guess.
+      return false;
+    }
+    // Assume that there will be a scrollbar; it seems to me
+    // that 'most pages' do have a scrollbar, and anyway, it's cheaper
+    // to do an extra reflow for the pages that *don't* need a
+    // scrollbar (because on average they will have less content).
+    return true;
+  }
+
+  // For non-viewports, just guess that we don't need a scrollbar.
+  // XXX I wonder if statistically this is the right idea; I'm
+  // basically guessing that there are a lot of overflow:auto DIVs
+  // that get their intrinsic size and don't overflow
+  return false;
+}
+
+bool nsHTMLScrollFrame::InInitialReflow() const {
+  // We're in an initial reflow if NS_FRAME_FIRST_REFLOW is set, unless we're a
+  // root scrollframe.  In that case we want to skip this clause altogether.
+  // The guess here is that there are lots of overflow:auto divs out there that
+  // end up auto-sizing so they don't overflow, and that the root basically
+  // always needs a scrollbar if it did last time we loaded this page (good
+  // assumption, because our initial reflow is no longer synchronous).
+  return !mIsRoot && HasAnyStateBits(NS_FRAME_FIRST_REFLOW);
+}
+
+void nsHTMLScrollFrame::ReflowContents(ScrollReflowInput& aState,
+                                       const ReflowOutput& aDesiredSize) {
+  const WritingMode desiredWm = aDesiredSize.GetWritingMode();
+  ReflowOutput kidDesiredSize(desiredWm);
+  ReflowScrolledFrame(aState, GuessHScrollbarNeeded(aState),
+                      GuessVScrollbarNeeded(aState), &kidDesiredSize);
+
+  // There's an important special case ... if the child appears to fit
+  // in the inside-border rect (but overflows the scrollport), we
+  // should try laying it out without a vertical scrollbar. It will
+  // usually fit because making the available-width wider will not
+  // normally make the child taller. (The only situation I can think
+  // of is when you have a line containing %-width inline replaced
+  // elements whose percentages sum to more than 100%, so increasing
+  // the available width makes the line break where it was fitting
+  // before.) If we don't treat this case specially, then we will
+  // decide that showing scrollbars is OK because the content
+  // overflows when we're showing scrollbars and we won't try to
+  // remove the vertical scrollbar.
+
+  // Detecting when we enter this special case is important for when
+  // people design layouts that exactly fit the container "most of the
+  // time".
+
+  // XXX Is this check really sufficient to catch all the incremental cases
+  // where the ideal case doesn't have a scrollbar?
+  if ((aState.mReflowedContentsWithHScrollbar ||
+       aState.mReflowedContentsWithVScrollbar) &&
+      aState.mVScrollbar != ShowScrollbar::Always &&
+      aState.mHScrollbar != ShowScrollbar::Always) {
+    nsSize kidSize = GetContainSizeAxes().ContainSize(
+        kidDesiredSize.PhysicalSize(), *aState.mReflowInput.mFrame);
+    nsSize insideBorderSize = ComputeInsideBorderSize(aState, kidSize);
+    nsRect scrolledRect = GetUnsnappedScrolledRectInternal(
+        kidDesiredSize.ScrollableOverflow(), insideBorderSize);
+    if (nsRect(nsPoint(0, 0), insideBorderSize).Contains(scrolledRect)) {
+      // Let's pretend we had no scrollbars coming in here
+      kidDesiredSize.mOverflowAreas.Clear();
+      ReflowScrolledFrame(aState, false, false, &kidDesiredSize);
+    }
+  }
+
+  if (IsRootScrollFrameOfDocument()) {
+    UpdateMinimumScaleSize(aState.mContentsOverflowAreas.ScrollableOverflow(),
+                           kidDesiredSize.PhysicalSize());
+  }
+
+  // Try vertical scrollbar settings that leave the vertical scrollbar
+  // unchanged. Do this first because changing the vertical scrollbar setting is
+  // expensive, forcing a reflow always.
+
+  // Try leaving the horizontal scrollbar unchanged first. This will be more
+  // efficient.
+  ROOT_SCROLLBAR_LOG("Trying layout1 with %d, %d\n",
+                     aState.mReflowedContentsWithHScrollbar,
+                     aState.mReflowedContentsWithVScrollbar);
+  if (TryLayout(aState, &kidDesiredSize, aState.mReflowedContentsWithHScrollbar,
+                aState.mReflowedContentsWithVScrollbar, false)) {
+    return;
+  }
+  ROOT_SCROLLBAR_LOG("Trying layout2 with %d, %d\n",
+                     !aState.mReflowedContentsWithHScrollbar,
+                     aState.mReflowedContentsWithVScrollbar);
+  if (TryLayout(aState, &kidDesiredSize,
+                !aState.mReflowedContentsWithHScrollbar,
+                aState.mReflowedContentsWithVScrollbar, false)) {
+    return;
+  }
+
+  // OK, now try toggling the vertical scrollbar. The performance advantage
+  // of trying the status-quo horizontal scrollbar state
+  // does not exist here (we'll have to reflow due to the vertical scrollbar
+  // change), so always try no horizontal scrollbar first.
+  bool newVScrollbarState = !aState.mReflowedContentsWithVScrollbar;
+  ROOT_SCROLLBAR_LOG("Trying layout3 with %d, %d\n", false, newVScrollbarState);
+  if (TryLayout(aState, &kidDesiredSize, false, newVScrollbarState, false)) {
+    return;
+  }
+  ROOT_SCROLLBAR_LOG("Trying layout4 with %d, %d\n", true, newVScrollbarState);
+  if (TryLayout(aState, &kidDesiredSize, true, newVScrollbarState, false)) {
+    return;
+  }
+
+  // OK, we're out of ideas. Try again enabling whatever scrollbars we can
+  // enable and force the layout to stick even if it's inconsistent.
+  // This just happens sometimes.
+  ROOT_SCROLLBAR_LOG("Giving up, adding both scrollbars...\n");
+  TryLayout(aState, &kidDesiredSize, aState.mHScrollbar != ShowScrollbar::Never,
+            aState.mVScrollbar != ShowScrollbar::Never, true);
+}
+
+void nsHTMLScrollFrame::PlaceScrollArea(ScrollReflowInput& aState,
+                                        const nsPoint& aScrollPosition) {
+  // Set the x,y of the scrolled frame to the correct value
+  mScrolledFrame->SetPosition(ScrollPort().TopLeft() - aScrollPosition);
+
+  // Recompute our scrollable overflow, taking perspective children into
+  // account. Note that this only recomputes the overflow areas stored on the
+  // helper (which are used to compute scrollable length and scrollbar thumb
+  // sizes) but not the overflow areas stored on the frame. This seems to work
+  // for now, but it's possible that we may need to update both in the future.
+  AdjustForPerspective(aState.mContentsOverflowAreas.ScrollableOverflow());
+
+  // Preserve the width or height of empty rects
+  const nsSize portSize = ScrollPort().Size();
+  nsRect scrolledRect = GetUnsnappedScrolledRectInternal(
+      aState.mContentsOverflowAreas.ScrollableOverflow(), portSize);
+  nsRect scrolledArea =
+      scrolledRect.UnionEdges(nsRect(nsPoint(0, 0), portSize));
+
+  // Store the new overflow area. Note that this changes where an outline
+  // of the scrolled frame would be painted, but scrolled frames can't have
+  // outlines (the outline would go on this scrollframe instead).
+  // Using FinishAndStoreOverflow is needed so the overflow rect gets set
+  // correctly.  It also messes with the overflow rect in the 'clip' case, but
+  // scrolled frames can't have 'overflow' either.
+  // This needs to happen before SyncFrameViewAfterReflow so
+  // HasOverflowRect() will return the correct value.
+  OverflowAreas overflow(scrolledArea, scrolledArea);
+  mScrolledFrame->FinishAndStoreOverflow(overflow, mScrolledFrame->GetSize());
+
+  // Note that making the view *exactly* the size of the scrolled area
+  // is critical, since the view scrolling code uses the size of the
+  // scrolled view to clamp scroll requests.
+  // Normally the mScrolledFrame won't have a view but in some cases it
+  // might create its own.
+  nsContainerFrame::SyncFrameViewAfterReflow(
+      mScrolledFrame->PresContext(), mScrolledFrame, mScrolledFrame->GetView(),
+      scrolledArea, ReflowChildFlags::Default);
+}
+
+nscoord nsHTMLScrollFrame::IntrinsicScrollbarGutterSizeAtInlineEdges() {
+  const bool isVerticalWM = GetWritingMode().IsVertical();
+  if (PresContext()->UseOverlayScrollbars()) {
+    return 0;
+  }
+
+  const auto* styleForScrollbar = nsLayoutUtils::StyleForScrollbar(this);
+  const auto& styleScrollbarWidth =
+      styleForScrollbar->StyleUIReset()->ScrollbarWidth();
+  if (styleScrollbarWidth == StyleScrollbarWidth::None) {
+    // Scrollbar shouldn't appear at all with "scrollbar-width: none".
+    return 0;
+  }
+
+  const auto& styleScrollbarGutter =
+      styleForScrollbar->StyleDisplay()->mScrollbarGutter;
+  ScrollStyles ss = GetScrollStyles();
+  const StyleOverflow& inlineEndStyleOverflow =
+      isVerticalWM ? ss.mHorizontal : ss.mVertical;
+
+  // Return the scrollbar-gutter size only if we have "overflow:scroll" or
+  // non-auto "scrollbar-gutter", so early-return here if the conditions aren't
+  // satisfied.
+  if (inlineEndStyleOverflow != StyleOverflow::Scroll &&
+      styleScrollbarGutter == StyleScrollbarGutter::AUTO) {
+    return 0;
+  }
+
+  const nscoord scrollbarSize =
+      GetNonOverlayScrollbarSize(PresContext(), styleScrollbarWidth);
+  const auto bothEdges =
+      bool(styleScrollbarGutter & StyleScrollbarGutter::BOTH_EDGES);
+  return bothEdges ? scrollbarSize * 2 : scrollbarSize;
+}
+
+nsMargin nsHTMLScrollFrame::ComputeStableScrollbarGutter(
+    const StyleScrollbarWidth& aStyleScrollbarWidth,
+    const StyleScrollbarGutter& aStyleScrollbarGutter) const {
+  if (PresContext()->UseOverlayScrollbars()) {
+    // Overlay scrollbars do not consume space per spec.
+    return {};
+  }
+
+  if (aStyleScrollbarWidth == StyleScrollbarWidth::None) {
+    // Scrollbar shouldn't appear at all with "scrollbar-width: none".
+    return {};
+  }
+
+  if (aStyleScrollbarGutter == StyleScrollbarGutter::AUTO) {
+    // Scrollbars create space depending on the 'overflow' property and whether
+    // the content overflows. Callers need to check this scenario if they want
+    // to consider the space created by the actual scrollbars.
+    return {};
+  }
+
+  const bool bothEdges =
+      bool(aStyleScrollbarGutter & StyleScrollbarGutter::BOTH_EDGES);
+  const bool isVerticalWM = GetWritingMode().IsVertical();
+  const nscoord scrollbarSize =
+      GetNonOverlayScrollbarSize(PresContext(), aStyleScrollbarWidth);
+
+  nsMargin scrollbarGutter;
+  if (bothEdges) {
+    if (isVerticalWM) {
+      scrollbarGutter.top = scrollbarGutter.bottom = scrollbarSize;
+    } else {
+      scrollbarGutter.left = scrollbarGutter.right = scrollbarSize;
+    }
+  } else {
+    MOZ_ASSERT(bool(aStyleScrollbarGutter & StyleScrollbarGutter::STABLE),
+               "scrollbar-gutter value should be 'stable'!");
+    if (isVerticalWM) {
+      // The horizontal scrollbar-gutter is always at the bottom side.
+      scrollbarGutter.bottom = scrollbarSize;
+    } else if (IsScrollbarOnRight()) {
+      scrollbarGutter.right = scrollbarSize;
+    } else {
+      scrollbarGutter.left = scrollbarSize;
+    }
+  }
+  return scrollbarGutter;
+}
+
+// Legacy, this sucks!
+static bool IsMarqueeScrollbox(const nsIFrame& aScrollFrame) {
+  if (!aScrollFrame.GetContent()) {
+    return false;
+  }
+  if (MOZ_LIKELY(!aScrollFrame.GetContent()->HasBeenInUAWidget())) {
+    return false;
+  }
+  MOZ_ASSERT(aScrollFrame.GetParent() &&
+             aScrollFrame.GetParent()->GetContent());
+  return aScrollFrame.GetParent() &&
+         HTMLMarqueeElement::FromNodeOrNull(
+             aScrollFrame.GetParent()->GetContent());
+}
+
+/* virtual */
+nscoord nsHTMLScrollFrame::GetMinISize(gfxContext* aRenderingContext) {
+  nscoord result = [&] {
+    if (const Maybe<nscoord> containISize = ContainIntrinsicISize()) {
+      return *containISize;
+    }
+    if (MOZ_UNLIKELY(IsMarqueeScrollbox(*this))) {
+      return 0;
+    }
+    return mScrolledFrame->GetMinISize(aRenderingContext);
+  }();
+
+  DISPLAY_MIN_INLINE_SIZE(this, result);
+  return result + IntrinsicScrollbarGutterSizeAtInlineEdges();
+}
+
+/* virtual */
+nscoord nsHTMLScrollFrame::GetPrefISize(gfxContext* aRenderingContext) {
+  const Maybe<nscoord> containISize = ContainIntrinsicISize();
+  nscoord result = containISize
+                       ? *containISize
+                       : mScrolledFrame->GetPrefISize(aRenderingContext);
+  DISPLAY_PREF_INLINE_SIZE(this, result);
+  return NSCoordSaturatingAdd(result,
+                              IntrinsicScrollbarGutterSizeAtInlineEdges());
+}
+
+// When we have perspective set on the outer scroll frame, and transformed
+// children (possibly with preserve-3d) then the effective transform on the
+// child depends on the offset to the scroll frame, which changes as we scroll.
+// This perspective transform can cause the element to move relative to the
+// scrolled inner frame, which would cause the scrollable length changes during
+// scrolling if we didn't account for it. Since we don't want scrollHeight/Width
+// and the size of scrollbar thumbs to change during scrolling, we compute the
+// scrollable overflow by determining the scroll position at which the child
+// becomes completely visible within the scrollport rather than using the union
+// of the overflow areas at their current position.
+static void GetScrollableOverflowForPerspective(
+    nsIFrame* aScrolledFrame, nsIFrame* aCurrentFrame, const nsRect aScrollPort,
+    nsPoint aOffset, nsRect& aScrolledFrameOverflowArea) {
+  // Iterate over all children except pop-ups.
+  for (const auto& [list, listID] : aCurrentFrame->ChildLists()) {
+    if (listID == FrameChildListID::Popup) {
+      continue;
+    }
+
+    for (nsIFrame* child : list) {
+      nsPoint offset = aOffset;
+
+      // When we reach a direct child of the scroll, then we record the offset
+      // to convert from that frame's coordinate into the scroll frame's
+      // coordinates. Preserve-3d descendant frames use the same offset as their
+      // ancestors, since TransformRect already converts us into the coordinate
+      // space of the preserve-3d root.
+      if (aScrolledFrame == aCurrentFrame) {
+        offset = child->GetPosition();
+      }
+
+      if (child->Extend3DContext()) {
+        // If we're a preserve-3d frame, then recurse and include our
+        // descendants since overflow of preserve-3d frames is only included
+        // in the post-transform overflow area of the preserve-3d root frame.
+        GetScrollableOverflowForPerspective(aScrolledFrame, child, aScrollPort,
+                                            offset, aScrolledFrameOverflowArea);
+      }
+
+      // If we're transformed, then we want to consider the possibility that
+      // this frame might move relative to the scrolled frame when scrolling.
+      // For preserve-3d, leaf frames have correct overflow rects relative to
+      // themselves. preserve-3d 'nodes' (intermediate frames and the root) have
+      // only their untransformed children included in their overflow relative
+      // to self, which is what we want to include here.
+      if (child->IsTransformed()) {
+        // Compute the overflow rect for this leaf transform frame in the
+        // coordinate space of the scrolled frame.
+        nsPoint scrollPos = aScrolledFrame->GetPosition();
+        nsRect preScroll, postScroll;
+        {
+          // TODO: Can we reuse the reference box?
+          TransformReferenceBox refBox(child);
+          preScroll = nsDisplayTransform::TransformRect(
+              child->ScrollableOverflowRectRelativeToSelf(), child, refBox);
+        }
+
+        // Temporarily override the scroll position of the scrolled frame by
+        // 10 CSS pixels, and then recompute what the overflow rect would be.
+        // This scroll position may not be valid, but that shouldn't matter
+        // for our calculations.
+        {
+          aScrolledFrame->SetPosition(scrollPos + nsPoint(600, 600));
+          TransformReferenceBox refBox(child);
+          postScroll = nsDisplayTransform::TransformRect(
+              child->ScrollableOverflowRectRelativeToSelf(), child, refBox);
+          aScrolledFrame->SetPosition(scrollPos);
+        }
+
+        // Compute how many app units the overflow rects moves by when we adjust
+        // the scroll position by 1 app unit.
+        double rightDelta =
+            (postScroll.XMost() - preScroll.XMost() + 600.0) / 600.0;
+        double bottomDelta =
+            (postScroll.YMost() - preScroll.YMost() + 600.0) / 600.0;
+
+        // We can't ever have negative scrolling.
+        NS_ASSERTION(rightDelta > 0.0f && bottomDelta > 0.0f,
+                     "Scrolling can't be reversed!");
+
+        // Move preScroll into the coordinate space of the scrollport.
+        preScroll += offset + scrollPos;
+
+        // For each of the four edges of preScroll, figure out how far they
+        // extend beyond the scrollport. Ignore negative values since that means
+        // that side is already scrolled in to view and we don't need to add
+        // overflow to account for it.
+        nsMargin overhang(std::max(0, aScrollPort.Y() - preScroll.Y()),
+                          std::max(0, preScroll.XMost() - aScrollPort.XMost()),
+                          std::max(0, preScroll.YMost() - aScrollPort.YMost()),
+                          std::max(0, aScrollPort.X() - preScroll.X()));
+
+        // Scale according to rightDelta/bottomDelta to adjust for the different
+        // scroll rates.
+        overhang.top = NSCoordSaturatingMultiply(
+            overhang.top, static_cast<float>(1 / bottomDelta));
+        overhang.right = NSCoordSaturatingMultiply(
+            overhang.right, static_cast<float>(1 / rightDelta));
+        overhang.bottom = NSCoordSaturatingMultiply(
+            overhang.bottom, static_cast<float>(1 / bottomDelta));
+        overhang.left = NSCoordSaturatingMultiply(
+            overhang.left, static_cast<float>(1 / rightDelta));
+
+        // Take the minimum overflow rect that would allow the current scroll
+        // position, using the size of the scroll port and offset by the
+        // inverse of the scroll position.
+        nsRect overflow = aScrollPort - scrollPos;
+
+        // Expand it by our margins to get an overflow rect that would allow all
+        // edges of our transformed content to be scrolled into view.
+        overflow.Inflate(overhang);
+
+        // Merge it with the combined overflow
+        aScrolledFrameOverflowArea.UnionRect(aScrolledFrameOverflowArea,
+                                             overflow);
+      } else if (aCurrentFrame == aScrolledFrame) {
+        aScrolledFrameOverflowArea.UnionRect(
+            aScrolledFrameOverflowArea,
+            child->ScrollableOverflowRectRelativeToParent());
+      }
+    }
+  }
+}
+
+BaselineSharingGroup nsHTMLScrollFrame::GetDefaultBaselineSharingGroup() const {
+  return mScrolledFrame->GetDefaultBaselineSharingGroup();
+}
+
+nscoord nsHTMLScrollFrame::SynthesizeFallbackBaseline(
+    mozilla::WritingMode aWM, BaselineSharingGroup aBaselineGroup) const {
+  // Marign-end even for central baselines.
+  if (aWM.IsLineInverted()) {
+    return -GetLogicalUsedMargin(aWM).BStart(aWM);
+  }
+  return aBaselineGroup == BaselineSharingGroup::First
+             ? BSize(aWM) + GetLogicalUsedMargin(aWM).BEnd(aWM)
+             : -GetLogicalUsedMargin(aWM).BEnd(aWM);
+}
+
+Maybe<nscoord> nsHTMLScrollFrame::GetNaturalBaselineBOffset(
+    WritingMode aWM, BaselineSharingGroup aBaselineGroup,
+    BaselineExportContext aExportContext) const {
+  // Block containers that are scrollable always have a last baseline
+  // that are synthesized from block-end margin edge.
+  // Note(dshin): This behaviour is really only relevant to `inline-block`
+  // alignment context. In the context of table/flex/grid alignment, first/last
+  // baselines are calculated through `GetFirstLineBaseline`, which does
+  // calculations of its own.
+  // https://drafts.csswg.org/css-align/#baseline-export
+  if (aExportContext == BaselineExportContext::LineLayout &&
+      aBaselineGroup == BaselineSharingGroup::Last &&
+      mScrolledFrame->IsBlockFrameOrSubclass()) {
+    return Some(SynthesizeFallbackBaseline(aWM, aBaselineGroup));
+  }
+
+  if (StyleDisplay()->IsContainLayout()) {
+    return Nothing{};
+  }
+
+  // OK, here's where we defer to our scrolled frame.
+  return mScrolledFrame
+      ->GetNaturalBaselineBOffset(aWM, aBaselineGroup, aExportContext)
+      .map([this, aWM](nscoord aBaseline) {
+        // We have to add our border BStart thickness to whatever it returns, to
+        // produce an offset in our frame-rect's coordinate system. (We don't
+        // have to add padding, because the scrolled frame handles our padding.)
+        LogicalMargin border = GetLogicalUsedBorder(aWM);
+        const auto bSize = GetLogicalSize(aWM).BSize(aWM);
+        // Clamp the baseline to the border rect. See bug 1791069.
+        return std::clamp(border.BStart(aWM) + aBaseline, 0, bSize);
+      });
+}
+
+void nsHTMLScrollFrame::AdjustForPerspective(nsRect& aScrollableOverflow) {
+  // If we have perspective that is being applied to our children, then
+  // the effective transform on the child depends on the relative position
+  // of the child to us and changes during scrolling.
+  if (!ChildrenHavePerspective()) {
+    return;
+  }
+  aScrollableOverflow.SetEmpty();
+  GetScrollableOverflowForPerspective(mScrolledFrame, mScrolledFrame,
+                                      ScrollPort(), nsPoint(),
+                                      aScrollableOverflow);
+}
+
+void nsHTMLScrollFrame::Reflow(nsPresContext* aPresContext,
+                               ReflowOutput& aDesiredSize,
+                               const ReflowInput& aReflowInput,
+                               nsReflowStatus& aStatus) {
+  MarkInReflow();
+  DO_GLOBAL_REFLOW_COUNT("nsHTMLScrollFrame");
+  DISPLAY_REFLOW(aPresContext, this, aReflowInput, aDesiredSize, aStatus);
+  MOZ_ASSERT(aStatus.IsEmpty(), "Caller should pass a fresh reflow status!");
+
+  HandleScrollbarStyleSwitching();
+
+  ScrollReflowInput state(this, aReflowInput);
+
+  //------------ Handle Incremental Reflow -----------------
+  bool reflowHScrollbar = true;
+  bool reflowVScrollbar = true;
+  bool reflowScrollCorner = true;
+  if (!aReflowInput.ShouldReflowAllKids()) {
+    auto NeedsReflow = [](const nsIFrame* aFrame) {
+      return aFrame && aFrame->IsSubtreeDirty();
+    };
+
+    reflowHScrollbar = NeedsReflow(mHScrollbarBox);
+    reflowVScrollbar = NeedsReflow(mVScrollbarBox);
+    reflowScrollCorner =
+        NeedsReflow(mScrollCornerBox) || NeedsReflow(mResizerBox);
+  }
+
+  if (mIsRoot) {
+    reflowScrollCorner = false;
+  }
+
+  const nsRect oldScrollPort = ScrollPort();
+  nsRect oldScrolledAreaBounds =
+      mScrolledFrame->ScrollableOverflowRectRelativeToParent();
+  nsPoint oldScrollPosition = GetScrollPosition();
+
+  ReflowContents(state, aDesiredSize);
+
+  nsSize layoutSize =
+      mIsUsingMinimumScaleSize ? mMinimumScaleSize : state.mInsideBorderSize;
+  aDesiredSize.Width() = layoutSize.width + state.mComputedBorder.LeftRight();
+  aDesiredSize.Height() = layoutSize.height + state.mComputedBorder.TopBottom();
+
+  // Set the size of the frame now since computing the perspective-correct
+  // overflow (within PlaceScrollArea) can rely on it.
+  SetSize(aDesiredSize.GetWritingMode(),
+          aDesiredSize.Size(aDesiredSize.GetWritingMode()));
+
+  // Restore the old scroll position, for now, even if that's not valid anymore
+  // because we changed size. We'll fix it up in a post-reflow callback, because
+  // our current size may only be temporary (e.g. we're compute XUL desired
+  // sizes).
+  PlaceScrollArea(state, oldScrollPosition);
+  if (!mPostedReflowCallback) {
+    // Make sure we'll try scrolling to restored position
+    PresShell()->PostReflowCallback(this);
+    mPostedReflowCallback = true;
+  }
+
+  bool didOnlyHScrollbar = mOnlyNeedHScrollbarToScrollVVInsideLV;
+  bool didOnlyVScrollbar = mOnlyNeedVScrollbarToScrollVVInsideLV;
+  mOnlyNeedHScrollbarToScrollVVInsideLV =
+      state.mOnlyNeedHScrollbarToScrollVVInsideLV;
+  mOnlyNeedVScrollbarToScrollVVInsideLV =
+      state.mOnlyNeedVScrollbarToScrollVVInsideLV;
+
+  bool didHaveHScrollbar = mHasHorizontalScrollbar;
+  bool didHaveVScrollbar = mHasVerticalScrollbar;
+  mHasHorizontalScrollbar = state.mShowHScrollbar;
+  mHasVerticalScrollbar = state.mShowVScrollbar;
+  const nsRect& newScrollPort = ScrollPort();
+  nsRect newScrolledAreaBounds =
+      mScrolledFrame->ScrollableOverflowRectRelativeToParent();
+  if (mSkippedScrollbarLayout || reflowHScrollbar || reflowVScrollbar ||
+      reflowScrollCorner || HasAnyStateBits(NS_FRAME_IS_DIRTY) ||
+      didHaveHScrollbar != state.mShowHScrollbar ||
+      didHaveVScrollbar != state.mShowVScrollbar ||
+      didOnlyHScrollbar != mOnlyNeedHScrollbarToScrollVVInsideLV ||
+      didOnlyVScrollbar != mOnlyNeedVScrollbarToScrollVVInsideLV ||
+      !oldScrollPort.IsEqualEdges(newScrollPort) ||
+      !oldScrolledAreaBounds.IsEqualEdges(newScrolledAreaBounds)) {
+    if (!mSuppressScrollbarUpdate) {
+      mSkippedScrollbarLayout = false;
+      nsHTMLScrollFrame::SetScrollbarVisibility(mHScrollbarBox,
+                                                state.mShowHScrollbar);
+      nsHTMLScrollFrame::SetScrollbarVisibility(mVScrollbarBox,
+                                                state.mShowVScrollbar);
+      // place and reflow scrollbars
+      const nsRect insideBorderArea(
+          nsPoint(state.mComputedBorder.left, state.mComputedBorder.top),
+          layoutSize);
+      LayoutScrollbars(state, insideBorderArea, oldScrollPort);
+    } else {
+      mSkippedScrollbarLayout = true;
+    }
+  }
+  if (mIsRoot) {
+    if (RefPtr<MobileViewportManager> manager =
+            PresShell()->GetMobileViewportManager()) {
+      // Note that this runs during layout, and when we get here the root
+      // scrollframe has already been laid out. It may have added or removed
+      // scrollbars as a result of that layout, so we need to ensure the
+      // visual viewport is updated to account for that before we read the
+      // visual viewport size.
+      manager->UpdateVisualViewportSizeForPotentialScrollbarChange();
+    } else if (oldScrollPort.Size() != newScrollPort.Size()) {
+      // We want to make sure to send a visual viewport resize event if the
+      // scrollport changed sizes for root scroll frames. The
+      // MobileViewportManager will do that, but if we don't have one (ie we
+      // aren't a root content document for example) we have to send one
+      // ourselves.
+      if (auto* window = nsGlobalWindowInner::Cast(
+              aPresContext->Document()->GetInnerWindow())) {
+        window->VisualViewport()->PostResizeEvent();
+      }
+    }
+  }
+
+  // Note that we need to do this after the
+  // UpdateVisualViewportSizeForPotentialScrollbarChange call above because that
+  // is what updates the visual viewport size and we need it to be up to date.
+  if (mIsRoot && !state.OverlayScrollbars() &&
+      (didHaveHScrollbar != state.mShowHScrollbar ||
+       didHaveVScrollbar != state.mShowVScrollbar ||
+       didOnlyHScrollbar != mOnlyNeedHScrollbarToScrollVVInsideLV ||
+       didOnlyVScrollbar != mOnlyNeedVScrollbarToScrollVVInsideLV) &&
+      PresShell()->IsVisualViewportOffsetSet()) {
+    // Removing layout/classic scrollbars can make a previously valid vvoffset
+    // invalid. For example, if we are zoomed in on an overflow hidden document
+    // and then zoom back out, when apz reaches the initial resolution (ie 1.0)
+    // it won't know that we can remove the scrollbars, so the vvoffset can
+    // validly be upto the width/height of the scrollbars. After we reflow and
+    // remove the scrollbars the only valid vvoffset is (0,0). We could wait
+    // until we send the new frame metrics to apz and then have it reply with
+    // the new corrected vvoffset but having an inconsistent vvoffset causes
+    // problems so trigger the vvoffset to be re-set and re-clamped in
+    // ReflowFinished.
+    mReclampVVOffsetInReflowFinished = true;
+  }
+
+  aDesiredSize.SetOverflowAreasToDesiredBounds();
+
+  UpdateSticky();
+  FinishReflowWithAbsoluteFrames(aPresContext, aDesiredSize, aReflowInput,
+                                 aStatus);
+
+  if (!InInitialReflow() && !mHadNonInitialReflow) {
+    mHadNonInitialReflow = true;
+  }
+
+  if (mIsRoot && !oldScrolledAreaBounds.IsEqualEdges(newScrolledAreaBounds)) {
+    PostScrolledAreaEvent();
+  }
+
+  UpdatePrevScrolledRect();
+
+  aStatus.Reset();  // This type of frame can't be split.
+  PostOverflowEvent();
+}
+
+void nsHTMLScrollFrame::DidReflow(nsPresContext* aPresContext,
+                                  const ReflowInput* aReflowInput) {
+  nsContainerFrame::DidReflow(aPresContext, aReflowInput);
+  if (NeedsResnap()) {
+    PostPendingResnap();
+  } else {
+    PresShell()->PostPendingScrollAnchorAdjustment(Anchor());
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#ifdef DEBUG_FRAME_DUMP
+nsresult nsHTMLScrollFrame::GetFrameName(nsAString& aResult) const {
+  return MakeFrameName(u"HTMLScroll"_ns, aResult);
+}
+#endif
+
+#ifdef ACCESSIBILITY
+a11y::AccType nsHTMLScrollFrame::AccessibleType() {
+  if (IsTableCaption()) {
+    return GetRect().IsEmpty() ? a11y::eNoType : a11y::eHTMLCaptionType;
+  }
+
+  // Create an accessible regardless of focusable state because the state can be
+  // changed during frame life cycle without any notifications to accessibility.
+  if (mContent->IsRootOfNativeAnonymousSubtree() ||
+      GetScrollStyles().IsHiddenInBothDirections()) {
+    return a11y::eNoType;
+  }
+
+  return a11y::eHyperTextType;
+}
+#endif
+
+NS_QUERYFRAME_HEAD(nsHTMLScrollFrame)
+  NS_QUERYFRAME_ENTRY(nsIAnonymousContentCreator)
+  NS_QUERYFRAME_ENTRY(nsIScrollableFrame)
+  NS_QUERYFRAME_ENTRY(nsIStatefulFrame)
+  NS_QUERYFRAME_ENTRY(nsIScrollbarMediator)
+  NS_QUERYFRAME_ENTRY(nsHTMLScrollFrame)
+NS_QUERYFRAME_TAIL_INHERITING(nsContainerFrame)
+
+nsMargin nsHTMLScrollFrame::GetDesiredScrollbarSizes() const {
+  nsPresContext* pc = PresContext();
+  if (pc->UseOverlayScrollbars()) {
+    return {};
+  }
+
+  const auto& style = *nsLayoutUtils::StyleForScrollbar(this);
+  const auto scrollbarWidth = style.StyleUIReset()->ScrollbarWidth();
+  if (scrollbarWidth == StyleScrollbarWidth::None) {
+    return {};
+  }
+
+  ScrollStyles styles = GetScrollStyles();
+  nsMargin result(0, 0, 0, 0);
+
+  auto size = GetNonOverlayScrollbarSize(pc, scrollbarWidth);
+  if (styles.mVertical != StyleOverflow::Hidden) {
+    if (IsScrollbarOnRight()) {
+      result.right = size;
+    } else {
+      result.left = size;
+    }
+  }
+
+  if (styles.mHorizontal != StyleOverflow::Hidden) {
+    // We don't currently support any scripts that would require a scrollbar
+    // at the top. (Are there any?)
+    result.bottom = size;
+  }
+
+  return result;
+}
+
+nscoord nsHTMLScrollFrame::GetNonOverlayScrollbarSize(
+    const nsPresContext* aPc, StyleScrollbarWidth aScrollbarWidth) {
+  const auto size = aPc->Theme()->GetScrollbarSize(aPc, aScrollbarWidth,
+                                                   nsITheme::Overlay::No);
+  return aPc->DevPixelsToAppUnits(size);
+}
+
+void nsHTMLScrollFrame::HandleScrollbarStyleSwitching() {
+  // Check if we switched between scrollbar styles.
+  if (mScrollbarActivity && !UsesOverlayScrollbars()) {
+    mScrollbarActivity->Destroy();
+    mScrollbarActivity = nullptr;
+  } else if (!mScrollbarActivity && UsesOverlayScrollbars()) {
+    mScrollbarActivity = new ScrollbarActivity(do_QueryFrame(this));
+  }
+}
+
+#if defined(MOZ_WIDGET_ANDROID)
+static bool IsFocused(nsIContent* aContent) {
+  // Some content elements, like the GetContent() of a scroll frame
+  // for a text input field, are inside anonymous subtrees, but the focus
+  // manager always reports a non-anonymous element as the focused one, so
+  // walk up the tree until we reach a non-anonymous element.
+  while (aContent && aContent->IsInNativeAnonymousSubtree()) {
+    aContent = aContent->GetParent();
+  }
+
+  return aContent ? nsContentUtils::IsFocusedContent(aContent) : false;
+}
+#endif
+
+void nsHTMLScrollFrame::SetScrollableByAPZ(bool aScrollable) {
+  mScrollableByAPZ = aScrollable;
+}
+
+void nsHTMLScrollFrame::SetZoomableByAPZ(bool aZoomable) {
+  if (!nsLayoutUtils::UsesAsyncScrolling(this)) {
+    // If APZ is disabled on this window, then we're never actually going to
+    // do any zooming. So we don't need to do any of the setup for it. Note
+    // that this function gets called from ZoomConstraintsClient even if APZ
+    // is disabled to indicate the zoomability of content.
+    aZoomable = false;
+  }
+  if (mZoomableByAPZ != aZoomable) {
+    // We might be changing the result of DecideScrollableLayer() so schedule a
+    // paint to make sure we pick up the result of that change.
+    mZoomableByAPZ = aZoomable;
+    SchedulePaint();
+  }
+}
+
+void nsHTMLScrollFrame::SetHasOutOfFlowContentInsideFilter() {
+  mHasOutOfFlowContentInsideFilter = true;
+}
+
+bool nsHTMLScrollFrame::WantAsyncScroll() const {
+  ScrollStyles styles = GetScrollStyles();
+
+  // First, as an optimization because getting the scrollrange is
+  // relatively slow, check overflow hidden and not a zoomed scroll frame.
+  if (styles.mHorizontal == StyleOverflow::Hidden &&
+      styles.mVertical == StyleOverflow::Hidden) {
+    if (!mIsRoot || GetVisualViewportSize() == mScrollPort.Size()) {
+      return false;
+    }
+  }
+
+  nscoord oneDevPixel =
+      GetScrolledFrame()->PresContext()->AppUnitsPerDevPixel();
+  nsRect scrollRange = GetLayoutScrollRange();
+
+  bool isVScrollable = (scrollRange.height >= oneDevPixel) &&
+                       (styles.mVertical != StyleOverflow::Hidden);
+  bool isHScrollable = (scrollRange.width >= oneDevPixel) &&
+                       (styles.mHorizontal != StyleOverflow::Hidden);
+
+#if defined(MOZ_WIDGET_ANDROID)
+  // Mobile platforms need focus to scroll text inputs.
+  bool canScrollWithoutScrollbars =
+      !IsForTextControlWithNoScrollbars() || IsFocused(GetContent());
+#else
+  bool canScrollWithoutScrollbars = true;
+#endif
+
+  // The check for scroll bars was added in bug 825692 to prevent layerization
+  // of text inputs for performance reasons.
+  bool isVAsyncScrollable =
+      isVScrollable && (mVScrollbarBox || canScrollWithoutScrollbars);
+  bool isHAsyncScrollable =
+      isHScrollable && (mHScrollbarBox || canScrollWithoutScrollbars);
+  if (isVAsyncScrollable || isHAsyncScrollable) {
+    return true;
+  }
+
+  // If the page has a visual viewport size that's different from
+  // the layout viewport size at the current zoom level, we need to be
+  // able to scroll the visual viewport inside the layout viewport
+  // even if the page is not zoomable.
+  return mIsRoot && GetVisualViewportSize() != mScrollPort.Size() &&
+         !GetVisualScrollRange().IsEqualInterior(scrollRange);
+}
+
+static nsRect GetOnePixelRangeAroundPoint(const nsPoint& aPoint,
+                                          bool aIsHorizontal) {
+  nsRect allowedRange(aPoint, nsSize());
+  nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f);
+  if (aIsHorizontal) {
+    allowedRange.x = aPoint.x - halfPixel;
+    allowedRange.width = halfPixel * 2 - 1;
+  } else {
+    allowedRange.y = aPoint.y - halfPixel;
+    allowedRange.height = halfPixel * 2 - 1;
+  }
+  return allowedRange;
+}
+
+void nsHTMLScrollFrame::ScrollByPage(nsScrollbarFrame* aScrollbar,
+                                     int32_t aDirection,
+                                     ScrollSnapFlags aSnapFlags) {
+  ScrollByUnit(aScrollbar, ScrollMode::Smooth, aDirection, ScrollUnit::PAGES,
+               aSnapFlags);
+}
+
+void nsHTMLScrollFrame::ScrollByWhole(nsScrollbarFrame* aScrollbar,
+                                      int32_t aDirection,
+                                      ScrollSnapFlags aSnapFlags) {
+  ScrollByUnit(aScrollbar, ScrollMode::Instant, aDirection, ScrollUnit::WHOLE,
+               aSnapFlags);
+}
+
+void nsHTMLScrollFrame::ScrollByLine(nsScrollbarFrame* aScrollbar,
+                                     int32_t aDirection,
+                                     ScrollSnapFlags aSnapFlags) {
+  bool isHorizontal = aScrollbar->IsHorizontal();
+  nsIntPoint delta;
+  if (isHorizontal) {
+    const double kScrollMultiplier =
+        StaticPrefs::toolkit_scrollbox_horizontalScrollDistance();
+    delta.x = static_cast<int32_t>(aDirection * kScrollMultiplier);
+    if (GetLineScrollAmount().width * delta.x > GetPageScrollAmount().width) {
+      // The scroll frame is so small that the delta would be more
+      // than an entire page.  Scroll by one page instead to maintain
+      // context.
+      ScrollByPage(aScrollbar, aDirection);
+      return;
+    }
+  } else {
+    const double kScrollMultiplier =
+        StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+    delta.y = static_cast<int32_t>(aDirection * kScrollMultiplier);
+    if (GetLineScrollAmount().height * delta.y > GetPageScrollAmount().height) {
+      // The scroll frame is so small that the delta would be more
+      // than an entire page.  Scroll by one page instead to maintain
+      // context.
+      ScrollByPage(aScrollbar, aDirection);
+      return;
+    }
+  }
+
+  nsIntPoint overflow;
+  ScrollBy(delta, ScrollUnit::LINES, ScrollMode::Smooth, &overflow,
+           ScrollOrigin::Other, nsIScrollableFrame::NOT_MOMENTUM, aSnapFlags);
+}
+
+void nsHTMLScrollFrame::RepeatButtonScroll(nsScrollbarFrame* aScrollbar) {
+  aScrollbar->MoveToNewPosition(nsScrollbarFrame::ImplementsScrollByUnit::Yes);
+}
+
+void nsHTMLScrollFrame::ThumbMoved(nsScrollbarFrame* aScrollbar,
+                                   nscoord aOldPos, nscoord aNewPos) {
+  MOZ_ASSERT(aScrollbar != nullptr);
+  bool isHorizontal = aScrollbar->IsHorizontal();
+  nsPoint current = GetScrollPosition();
+  nsPoint dest = current;
+  if (isHorizontal) {
+    dest.x = IsPhysicalLTR() ? aNewPos : aNewPos - GetLayoutScrollRange().width;
+  } else {
+    dest.y = aNewPos;
+  }
+  nsRect allowedRange = GetOnePixelRangeAroundPoint(dest, isHorizontal);
+
+  // Don't try to scroll if we're already at an acceptable place.
+  // Don't call Contains here since Contains returns false when the point is
+  // on the bottom or right edge of the rectangle.
+  if (allowedRange.ClampPoint(current) == current) {
+    return;
+  }
+
+  ScrollToWithOrigin(
+      dest, &allowedRange,
+      ScrollOperationParams{ScrollMode::Instant, ScrollOrigin::Other});
+}
+
+void nsHTMLScrollFrame::ScrollbarReleased(nsScrollbarFrame* aScrollbar) {
+  // Scrollbar scrolling does not result in fling gestures, clear any
+  // accumulated velocity
+  mVelocityQueue.Reset();
+
+  // Perform scroll snapping, if needed.  Scrollbar movement uses the same
+  // smooth scrolling animation as keyboard scrolling.
+  ScrollSnap(mDestination, ScrollMode::Smooth);
+}
+
+void nsHTMLScrollFrame::ScrollByUnit(nsScrollbarFrame* aScrollbar,
+                                     ScrollMode aMode, int32_t aDirection,
+                                     ScrollUnit aUnit,
+                                     ScrollSnapFlags aSnapFlags) {
+  MOZ_ASSERT(aScrollbar != nullptr);
+  bool isHorizontal = aScrollbar->IsHorizontal();
+  nsIntPoint delta;
+  if (isHorizontal) {
+    delta.x = aDirection;
+  } else {
+    delta.y = aDirection;
+  }
+  nsIntPoint overflow;
+  ScrollBy(delta, aUnit, aMode, &overflow, ScrollOrigin::Other,
+           nsIScrollableFrame::NOT_MOMENTUM, aSnapFlags);
+}
+
+//-------------------- Helper ----------------------
+
+// AsyncSmoothMSDScroll has ref counting.
+class nsHTMLScrollFrame::AsyncSmoothMSDScroll final
+    : public nsARefreshObserver {
+ public:
+  AsyncSmoothMSDScroll(const nsPoint& aInitialPosition,
+                       const nsPoint& aInitialDestination,
+                       const nsSize& aInitialVelocity, const nsRect& aRange,
+                       const mozilla::TimeStamp& aStartTime,
+                       nsPresContext* aPresContext,
+                       UniquePtr<ScrollSnapTargetIds> aSnapTargetIds,
+                       ScrollTriggeredByScript aTriggeredByScript)
+      : mXAxisModel(aInitialPosition.x, aInitialDestination.x,
+                    aInitialVelocity.width,
+                    StaticPrefs::layout_css_scroll_behavior_spring_constant(),
+                    StaticPrefs::layout_css_scroll_behavior_damping_ratio()),
+        mYAxisModel(aInitialPosition.y, aInitialDestination.y,
+                    aInitialVelocity.height,
+                    StaticPrefs::layout_css_scroll_behavior_spring_constant(),
+                    StaticPrefs::layout_css_scroll_behavior_damping_ratio()),
+        mRange(aRange),
+        mStartPosition(aInitialPosition),
+        mLastRefreshTime(aStartTime),
+        mCallee(nullptr),
+        mOneDevicePixelInAppUnits(aPresContext->DevPixelsToAppUnits(1)),
+        mSnapTargetIds(std::move(aSnapTargetIds)),
+        mTriggeredByScript(aTriggeredByScript) {
+    Telemetry::SetHistogramRecordingEnabled(
+        Telemetry::FX_REFRESH_DRIVER_SYNC_SCROLL_FRAME_DELAY_MS, true);
+  }
+
+  NS_INLINE_DECL_REFCOUNTING(AsyncSmoothMSDScroll, override)
+
+  nsSize GetVelocity() {
+    // In nscoords per second
+    return nsSize(mXAxisModel.GetVelocity(), mYAxisModel.GetVelocity());
+  }
+
+  nsPoint GetPosition() {
+    // In nscoords
+    return nsPoint(NSToCoordRound(mXAxisModel.GetPosition()),
+                   NSToCoordRound(mYAxisModel.GetPosition()));
+  }
+
+  void SetDestination(const nsPoint& aDestination,
+                      ScrollTriggeredByScript aTriggeredByScript) {
+    mXAxisModel.SetDestination(static_cast<int32_t>(aDestination.x));
+    mYAxisModel.SetDestination(static_cast<int32_t>(aDestination.y));
+    mTriggeredByScript = aTriggeredByScript;
+  }
+
+  void SetRange(const nsRect& aRange) { mRange = aRange; }
+
+  nsRect GetRange() { return mRange; }
+
+  nsPoint GetStartPosition() { return mStartPosition; }
+
+  void Simulate(const TimeDuration& aDeltaTime) {
+    mXAxisModel.Simulate(aDeltaTime);
+    mYAxisModel.Simulate(aDeltaTime);
+
+    nsPoint desired = GetPosition();
+    nsPoint clamped = mRange.ClampPoint(desired);
+    if (desired.x != clamped.x) {
+      // The scroll has hit the "wall" at the left or right edge of the allowed
+      // scroll range.
+      // Absorb the impact to avoid bounceback effect.
+      mXAxisModel.SetVelocity(0.0);
+      mXAxisModel.SetPosition(clamped.x);
+    }
+
+    if (desired.y != clamped.y) {
+      // The scroll has hit the "wall" at the left or right edge of the allowed
+      // scroll range.
+      // Absorb the impact to avoid bounceback effect.
+      mYAxisModel.SetVelocity(0.0);
+      mYAxisModel.SetPosition(clamped.y);
+    }
+  }
+
+  bool IsFinished() {
+    return mXAxisModel.IsFinished(mOneDevicePixelInAppUnits) &&
+           mYAxisModel.IsFinished(mOneDevicePixelInAppUnits);
+  }
+
+  virtual void WillRefresh(mozilla::TimeStamp aTime) override {
+    mozilla::TimeDuration deltaTime = aTime - mLastRefreshTime;
+    mLastRefreshTime = aTime;
+
+    // The callback may release "this".
+    // We don't access members after returning, so no need for KungFuDeathGrip.
+    nsHTMLScrollFrame::AsyncSmoothMSDScrollCallback(mCallee, deltaTime);
+  }
+
+  /*
+   * Set a refresh observer for smooth scroll iterations (and start observing).
+   * Should be used at most once during the lifetime of this object.
+   */
+  void SetRefreshObserver(nsHTMLScrollFrame* aCallee) {
+    NS_ASSERTION(aCallee && !mCallee,
+                 "AsyncSmoothMSDScroll::SetRefreshObserver - Invalid usage.");
+
+    RefreshDriver(aCallee)->AddRefreshObserver(this, FlushType::Style,
+                                               "Smooth scroll (MSD) animation");
+    mCallee = aCallee;
+  }
+
+  /**
+   * The mCallee holds a strong ref to us since the refresh driver doesn't.
+   * Our dtor and mCallee's Destroy() method both call RemoveObserver() -
+   * whichever comes first removes us from the refresh driver.
+   */
+  void RemoveObserver() {
+    if (mCallee) {
+      RefreshDriver(mCallee)->RemoveRefreshObserver(this, FlushType::Style);
+      mCallee = nullptr;
+    }
+  }
+
+  UniquePtr<ScrollSnapTargetIds> TakeSnapTargetIds() {
+    return std::move(mSnapTargetIds);
+  }
+
+  bool WasTriggeredByScript() const {
+    return mTriggeredByScript == ScrollTriggeredByScript::Yes;
+  }
+
+ private:
+  // Private destructor, to discourage deletion outside of Release():
+  ~AsyncSmoothMSDScroll() {
+    RemoveObserver();
+    Telemetry::SetHistogramRecordingEnabled(
+        Telemetry::FX_REFRESH_DRIVER_SYNC_SCROLL_FRAME_DELAY_MS, false);
+  }
+
+  nsRefreshDriver* RefreshDriver(nsHTMLScrollFrame* aCallee) {
+    return aCallee->PresContext()->RefreshDriver();
+  }
+
+  mozilla::layers::AxisPhysicsMSDModel mXAxisModel, mYAxisModel;
+  nsRect mRange;
+  nsPoint mStartPosition;
+  mozilla::TimeStamp mLastRefreshTime;
+  nsHTMLScrollFrame* mCallee;
+  nscoord mOneDevicePixelInAppUnits;
+  UniquePtr<ScrollSnapTargetIds> mSnapTargetIds;
+  ScrollTriggeredByScript mTriggeredByScript;
+};
+
+// AsyncScroll has ref counting.
+class nsHTMLScrollFrame::AsyncScroll final : public nsARefreshObserver {
+ public:
+  typedef mozilla::TimeStamp TimeStamp;
+  typedef mozilla::TimeDuration TimeDuration;
+
+  explicit AsyncScroll(UniquePtr<ScrollSnapTargetIds> aSnapTargetIds,
+                       ScrollTriggeredByScript aTriggeredByScript)
+      : mOrigin(ScrollOrigin::NotSpecified),
+        mCallee(nullptr),
+        mSnapTargetIds(std::move(aSnapTargetIds)),
+        mTriggeredByScript(aTriggeredByScript) {
+    Telemetry::SetHistogramRecordingEnabled(
+        Telemetry::FX_REFRESH_DRIVER_SYNC_SCROLL_FRAME_DELAY_MS, true);
+  }
+
+ private:
+  // Private destructor, to discourage deletion outside of Release():
+  ~AsyncScroll() {
+    RemoveObserver();
+    Telemetry::SetHistogramRecordingEnabled(
+        Telemetry::FX_REFRESH_DRIVER_SYNC_SCROLL_FRAME_DELAY_MS, false);
+  }
+
+ public:
+  void InitSmoothScroll(TimeStamp aTime, nsPoint aInitialPosition,
+                        nsPoint aDestination, ScrollOrigin aOrigin,
+                        const nsRect& aRange, const nsSize& aCurrentVelocity);
+  void Init(nsPoint aInitialPosition, const nsRect& aRange) {
+    mAnimationPhysics = nullptr;
+    mRange = aRange;
+    mStartPosition = aInitialPosition;
+  }
+
+  bool IsSmoothScroll() { return mAnimationPhysics != nullptr; }
+
+  bool IsFinished(const TimeStamp& aTime) const {
+    MOZ_RELEASE_ASSERT(mAnimationPhysics);
+    return mAnimationPhysics->IsFinished(aTime);
+  }
+
+  nsPoint PositionAt(const TimeStamp& aTime) const {
+    MOZ_RELEASE_ASSERT(mAnimationPhysics);
+    return mAnimationPhysics->PositionAt(aTime);
+  }
+
+  nsSize VelocityAt(const TimeStamp& aTime) const {
+    MOZ_RELEASE_ASSERT(mAnimationPhysics);
+    return mAnimationPhysics->VelocityAt(aTime);
+  }
+
+  nsPoint GetStartPosition() const { return mStartPosition; }
+
+  // Most recent scroll origin.
+  ScrollOrigin mOrigin;
+
+  // Allowed destination positions around mDestination
+  nsRect mRange;
+
+  // Initial position where the async scroll was triggered.
+  nsPoint mStartPosition;
+
+ private:
+  void InitPreferences(TimeStamp aTime, nsAtom* aOrigin);
+
+  UniquePtr<ScrollAnimationPhysics> mAnimationPhysics;
+
+  // The next section is observer/callback management
+  // Bodies of WillRefresh and RefreshDriver contain nsHTMLScrollFrame specific
+  // code.
+ public:
+  NS_INLINE_DECL_REFCOUNTING(AsyncScroll, override)
+
+  /*
+   * Set a refresh observer for smooth scroll iterations (and start observing).
+   * Should be used at most once during the lifetime of this object.
+   */
+  void SetRefreshObserver(nsHTMLScrollFrame* aCallee) {
+    NS_ASSERTION(aCallee && !mCallee,
+                 "AsyncScroll::SetRefreshObserver - Invalid usage.");
+
+    RefreshDriver(aCallee)->AddRefreshObserver(this, FlushType::Style,
+                                               "Smooth scroll animation");
+    mCallee = aCallee;
+    auto* presShell = mCallee->PresShell();
+    MOZ_ASSERT(presShell);
+    presShell->SuppressDisplayport(true);
+  }
+
+  virtual void WillRefresh(mozilla::TimeStamp aTime) override {
+    // The callback may release "this".
+    // We don't access members after returning, so no need for KungFuDeathGrip.
+    nsHTMLScrollFrame::AsyncScrollCallback(mCallee, aTime);
+  }
+
+  /**
+   * The mCallee holds a strong ref to us since the refresh driver doesn't.
+   * Our dtor and mCallee's Destroy() method both call RemoveObserver() -
+   * whichever comes first removes us from the refresh driver.
+   */
+  void RemoveObserver() {
+    if (mCallee) {
+      RefreshDriver(mCallee)->RemoveRefreshObserver(this, FlushType::Style);
+      auto* presShell = mCallee->PresShell();
+      MOZ_ASSERT(presShell);
+      presShell->SuppressDisplayport(false);
+      mCallee = nullptr;
+    }
+  }
+
+  UniquePtr<ScrollSnapTargetIds> TakeSnapTargetIds() {
+    return std::move(mSnapTargetIds);
+  }
+
+  bool WasTriggeredByScript() const {
+    return mTriggeredByScript == ScrollTriggeredByScript::Yes;
+  }
+
+ private:
+  nsHTMLScrollFrame* mCallee;
+  UniquePtr<ScrollSnapTargetIds> mSnapTargetIds;
+  ScrollTriggeredByScript mTriggeredByScript;
+
+  nsRefreshDriver* RefreshDriver(nsHTMLScrollFrame* aCallee) {
+    return aCallee->PresContext()->RefreshDriver();
+  }
+};
+
+void nsHTMLScrollFrame::AsyncScroll::InitSmoothScroll(
+    TimeStamp aTime, nsPoint aInitialPosition, nsPoint aDestination,
+    ScrollOrigin aOrigin, const nsRect& aRange,
+    const nsSize& aCurrentVelocity) {
+  switch (aOrigin) {
+    case ScrollOrigin::NotSpecified:
+    case ScrollOrigin::Restore:
+    case ScrollOrigin::Relative:
+      // We don't have special prefs for "restore", just treat it as "other".
+      // "restore" scrolls are (for now) always instant anyway so unless
+      // something changes we should never have aOrigin ==
+      // ScrollOrigin::Restore here.
+      aOrigin = ScrollOrigin::Other;
+      break;
+    case ScrollOrigin::Apz:
+      // Likewise we should never get APZ-triggered scrolls here, and if that
+      // changes something is likely broken somewhere.
+      MOZ_ASSERT_UNREACHABLE(
+          "APZ scroll position updates should never be smooth");
+      break;
+    case ScrollOrigin::AnchorAdjustment:
+      MOZ_ASSERT_UNREACHABLE(
+          "scroll anchor adjustments should never be smooth");
+      break;
+    default:
+      break;
+  };
+
+  // Read preferences only on first iteration or for a different event origin.
+  if (!mAnimationPhysics || aOrigin != mOrigin) {
+    mOrigin = aOrigin;
+    if (StaticPrefs::general_smoothScroll_msdPhysics_enabled()) {
+      mAnimationPhysics =
+          MakeUnique<ScrollAnimationMSDPhysics>(aInitialPosition);
+    } else {
+      ScrollAnimationBezierPhysicsSettings settings =
+          layers::apz::ComputeBezierAnimationSettingsForOrigin(mOrigin);
+      mAnimationPhysics =
+          MakeUnique<ScrollAnimationBezierPhysics>(aInitialPosition, settings);
+    }
+  }
+
+  mStartPosition = aInitialPosition;
+  mRange = aRange;
+
+  mAnimationPhysics->Update(aTime, aDestination, aCurrentVelocity);
+}
+
+/*
+ * Callback function from AsyncSmoothMSDScroll, used in
+ * nsHTMLScrollFrame::ScrollTo
+ */
+void nsHTMLScrollFrame::AsyncSmoothMSDScrollCallback(
+    nsHTMLScrollFrame* aInstance, mozilla::TimeDuration aDeltaTime) {
+  NS_ASSERTION(aInstance != nullptr, "aInstance must not be null");
+  NS_ASSERTION(aInstance->mAsyncSmoothMSDScroll,
+               "Did not expect AsyncSmoothMSDScrollCallback without an active "
+               "MSD scroll.");
+
+  nsRect range = aInstance->mAsyncSmoothMSDScroll->GetRange();
+  aInstance->mAsyncSmoothMSDScroll->Simulate(aDeltaTime);
+
+  if (!aInstance->mAsyncSmoothMSDScroll->IsFinished()) {
+    nsPoint destination = aInstance->mAsyncSmoothMSDScroll->GetPosition();
+    // Allow this scroll operation to land on any pixel boundary within the
+    // allowed scroll range for this frame.
+    // If the MSD is under-dampened or the destination is changed rapidly,
+    // it is expected (and desired) that the scrolling may overshoot.
+    nsRect intermediateRange = nsRect(destination, nsSize()).UnionEdges(range);
+    aInstance->ScrollToImpl(destination, intermediateRange);
+    // 'aInstance' might be destroyed here
+    return;
+  }
+
+  aInstance->CompleteAsyncScroll(
+      aInstance->mAsyncSmoothMSDScroll->GetStartPosition(), range,
+      aInstance->mAsyncSmoothMSDScroll->TakeSnapTargetIds());
+}
+
+/*
+ * Callback function from AsyncScroll, used in nsHTMLScrollFrame::ScrollTo
+ */
+void nsHTMLScrollFrame::AsyncScrollCallback(nsHTMLScrollFrame* aInstance,
+                                            mozilla::TimeStamp aTime) {
+  MOZ_ASSERT(aInstance != nullptr, "aInstance must not be null");
+  MOZ_ASSERT(
+      aInstance->mAsyncScroll,
+      "Did not expect AsyncScrollCallback without an active async scroll.");
+
+  if (!aInstance || !aInstance->mAsyncScroll) {
+    return;  // XXX wallpaper bug 1107353 for now.
+  }
+
+  nsRect range = aInstance->mAsyncScroll->mRange;
+  if (aInstance->mAsyncScroll->IsSmoothScroll()) {
+    if (!aInstance->mAsyncScroll->IsFinished(aTime)) {
+      nsPoint destination = aInstance->mAsyncScroll->PositionAt(aTime);
+      // Allow this scroll operation to land on any pixel boundary between the
+      // current position and the final allowed range.  (We don't want
+      // intermediate steps to be more constrained than the final step!)
+      nsRect intermediateRange =
+          nsRect(aInstance->GetScrollPosition(), nsSize()).UnionEdges(range);
+      aInstance->ScrollToImpl(destination, intermediateRange);
+      // 'aInstance' might be destroyed here
+      return;
+    }
+  }
+
+  aInstance->CompleteAsyncScroll(aInstance->mAsyncScroll->GetStartPosition(),
+                                 range,
+                                 aInstance->mAsyncScroll->TakeSnapTargetIds());
+}
+
+void nsHTMLScrollFrame::SetTransformingByAPZ(bool aTransforming) {
+  if (mTransformingByAPZ && !aTransforming) {
+    PostScrollEndEvent();
+  }
+  mTransformingByAPZ = aTransforming;
+  if (!mozilla::css::TextOverflow::HasClippedTextOverflow(this) ||
+      mozilla::css::TextOverflow::HasBlockEllipsis(mScrolledFrame)) {
+    // If the block has some overflow marker stuff we should kick off a paint
+    // because we have special behaviour for it when APZ scrolling is active.
+    SchedulePaint();
+  }
+}
+
+void nsHTMLScrollFrame::CompleteAsyncScroll(
+    const nsPoint& aStartPosition, const nsRect& aRange,
+    UniquePtr<ScrollSnapTargetIds> aSnapTargetIds, ScrollOrigin aOrigin) {
+  SetLastSnapTargetIds(std::move(aSnapTargetIds));
+
+  bool scrollPositionChanged = mDestination != aStartPosition;
+  bool isNotHandledByApz =
+      nsLayoutUtils::CanScrollOriginClobberApz(aOrigin) ||
+      ScrollAnimationState().contains(AnimationState::MainThread);
+
+  // Apply desired destination range since this is the last step of scrolling.
+  RemoveObservers();
+  AutoWeakFrame weakFrame(this);
+  ScrollToImpl(mDestination, aRange, aOrigin);
+  if (!weakFrame.IsAlive()) {
+    return;
+  }
+  // We are done scrolling, set our destination to wherever we actually ended
+  // up scrolling to.
+  mDestination = GetScrollPosition();
+  // Post a `scrollend` event for scrolling not handled by APZ, including:
+  //
+  //  - programmatic instant scrolls
+  //  - the end of a smooth scroll animation running on the main thread
+  //
+  // For scrolling handled by APZ, the `scrollend` event is posted in
+  // SetTransformingByAPZ() when the APZC is transitioning from a transforming
+  // to a non-transforming state (e.g. a transition from PANNING to NOTHING).
+  // The scrollend event should not be fired for a scroll that does not
+  // result in a scroll position change.
+  if (isNotHandledByApz && scrollPositionChanged) {
+    PostScrollEndEvent();
+  }
+}
+
+bool nsHTMLScrollFrame::HasBgAttachmentLocal() const {
+  const nsStyleBackground* bg = StyleBackground();
+  return bg->HasLocalBackground();
+}
+
+void nsHTMLScrollFrame::ScrollToInternal(
+    nsPoint aScrollPosition, ScrollMode aMode, ScrollOrigin aOrigin,
+    const nsRect* aRange, ScrollSnapFlags aSnapFlags,
+    ScrollTriggeredByScript aTriggeredByScript) {
+  if (aOrigin == ScrollOrigin::NotSpecified) {
+    aOrigin = ScrollOrigin::Other;
+  }
+  ScrollToWithOrigin(
+      aScrollPosition, aRange,
+      ScrollOperationParams{aMode, aOrigin, aSnapFlags, aTriggeredByScript});
+}
+
+void nsHTMLScrollFrame::ScrollToCSSPixels(const CSSIntPoint& aScrollPosition,
+                                          ScrollMode aMode) {
+  CSSIntPoint currentCSSPixels = GetRoundedScrollPositionCSSPixels();
+  // Transmogrify this scroll to a relative one if there's any on-going
+  // animation in APZ triggered by __user__.
+  // Bug 1740164: We will apply it for cases there's no animation in APZ.
+
+  auto scrollAnimationState = ScrollAnimationState();
+  bool isScrollAnimating =
+      scrollAnimationState.contains(AnimationState::MainThread) ||
+      scrollAnimationState.contains(AnimationState::APZPending) ||
+      scrollAnimationState.contains(AnimationState::APZRequested);
+  if (mCurrentAPZScrollAnimationType ==
+          APZScrollAnimationType::TriggeredByUserInput &&
+      !isScrollAnimating) {
+    CSSIntPoint delta = aScrollPosition - currentCSSPixels;
+    // This transmogrification need to be an intended end position scroll
+    // operation.
+    ScrollByCSSPixelsInternal(delta, aMode,
+                              ScrollSnapFlags::IntendedEndPosition);
+    return;
+  }
+
+  nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f);
+  nsPoint pt = CSSPoint::ToAppUnits(aScrollPosition);
+  nsRect range(pt.x - halfPixel, pt.y - halfPixel, 2 * halfPixel - 1,
+               2 * halfPixel - 1);
+  // XXX I don't think the following blocks are needed anymore, now that
+  // ScrollToImpl simply tries to scroll an integer number of layer
+  // pixels from the current position
+  nsPoint current = GetScrollPosition();
+  if (currentCSSPixels.x == aScrollPosition.x) {
+    pt.x = current.x;
+    range.x = pt.x;
+    range.width = 0;
+  }
+  if (currentCSSPixels.y == aScrollPosition.y) {
+    pt.y = current.y;
+    range.y = pt.y;
+    range.height = 0;
+  }
+  ScrollToWithOrigin(
+      pt, &range,
+      ScrollOperationParams{
+          aMode, ScrollOrigin::Other,
+          // This ScrollToCSSPixels is used for Element.scrollTo,
+          // Element.scrollTop, Element.scrollLeft and for Window.scrollTo.
+          ScrollSnapFlags::IntendedEndPosition, ScrollTriggeredByScript::Yes});
+  // 'this' might be destroyed here
+}
+
+void nsHTMLScrollFrame::ScrollToCSSPixelsForApz(
+    const CSSPoint& aScrollPosition, ScrollSnapTargetIds&& aLastSnapTargetIds) {
+  nsPoint pt = CSSPoint::ToAppUnits(aScrollPosition);
+  nscoord halfRange = nsPresContext::CSSPixelsToAppUnits(1000);
+  nsRect range(pt.x - halfRange, pt.y - halfRange, 2 * halfRange - 1,
+               2 * halfRange - 1);
+  ScrollToWithOrigin(
+      pt, &range,
+      ScrollOperationParams{ScrollMode::Instant, ScrollOrigin::Apz,
+                            std::move(aLastSnapTargetIds)});
+  // 'this' might be destroyed here
+}
+
+CSSIntPoint nsHTMLScrollFrame::GetRoundedScrollPositionCSSPixels() {
+  return CSSIntPoint::FromAppUnitsRounded(GetScrollPosition());
+}
+
+/*
+ * this method wraps calls to ScrollToImpl(), either in one shot or
+ * incrementally, based on the setting of the smoothness scroll pref
+ */
+void nsHTMLScrollFrame::ScrollToWithOrigin(nsPoint aScrollPosition,
+                                           const nsRect* aRange,
+                                           ScrollOperationParams&& aParams) {
+  // None is never a valid scroll origin to be passed in.
+  MOZ_ASSERT(aParams.mOrigin != ScrollOrigin::None);
+
+  if (aParams.mOrigin != ScrollOrigin::Restore) {
+    // If we're doing a non-restore scroll, we don't want to later
+    // override it by restoring our saved scroll position.
+    SCROLLRESTORE_LOG("%p: Clearing mRestorePos (cur=%s, dst=%s)\n", this,
+                      ToString(GetScrollPosition()).c_str(),
+                      ToString(aScrollPosition).c_str());
+    mRestorePos.x = mRestorePos.y = -1;
+  }
+
+  Maybe<SnapDestination> snapDestination;
+  if (!aParams.IsScrollSnapDisabled()) {
+    snapDestination = GetSnapPointForDestination(ScrollUnit::DEVICE_PIXELS,
+                                                 aParams.mSnapFlags,
+                                                 mDestination, aScrollPosition);
+    if (snapDestination) {
+      aScrollPosition = snapDestination->mPosition;
+    }
+  }
+
+  nsRect scrollRange = GetLayoutScrollRange();
+  mDestination = scrollRange.ClampPoint(aScrollPosition);
+  if (mDestination != aScrollPosition &&
+      aParams.mOrigin == ScrollOrigin::Restore &&
+      GetPageLoadingState() != LoadingState::Loading) {
+    // If we're doing a restore but the scroll position is clamped, promote
+    // the origin from one that APZ can clobber to one that it can't clobber.
+    aParams.mOrigin = ScrollOrigin::Other;
+  }
+
+  nsRect range = aRange && snapDestination.isNothing()
+                     ? *aRange
+                     : nsRect(aScrollPosition, nsSize(0, 0));
+
+  UniquePtr<ScrollSnapTargetIds> snapTargetIds;
+  if (snapDestination) {
+    snapTargetIds =
+        MakeUnique<ScrollSnapTargetIds>(std::move(snapDestination->mTargetIds));
+  } else {
+    snapTargetIds =
+        MakeUnique<ScrollSnapTargetIds>(std::move(aParams.mTargetIds));
+  }
+  if (aParams.IsInstant()) {
+    // Asynchronous scrolling is not allowed, so we'll kill any existing
+    // async-scrolling process and do an instant scroll.
+    CompleteAsyncScroll(GetScrollPosition(), range, std::move(snapTargetIds),
+                        aParams.mOrigin);
+    mApzSmoothScrollDestination = Nothing();
+    return;
+  }
+
+  if (!aParams.IsSmoothMsd()) {
+    // If we get a non-smooth-scroll, reset the cached APZ scroll destination,
+    // so that we know to process the next smooth-scroll destined for APZ.
+    mApzSmoothScrollDestination = Nothing();
+  }
+
+  nsPresContext* presContext = PresContext();
+  TimeStamp now =
+      presContext->RefreshDriver()->IsTestControllingRefreshesEnabled()
+          ? presContext->RefreshDriver()->MostRecentRefresh()
+          : TimeStamp::Now();
+
+  nsSize currentVelocity(0, 0);
+
+  const bool canHandoffToApz =
+      nsLayoutUtils::AsyncPanZoomEnabled(this) && WantAsyncScroll() &&
+      CanApzScrollInTheseDirections(
+          DirectionsInDelta(mDestination - GetScrollPosition()));
+
+  if (aParams.IsSmoothMsd()) {
+    mIgnoreMomentumScroll = true;
+    if (!mAsyncSmoothMSDScroll) {
+      nsPoint sv = mVelocityQueue.GetVelocity();
+      currentVelocity.width = sv.x;
+      currentVelocity.height = sv.y;
+      if (mAsyncScroll) {
+        if (mAsyncScroll->IsSmoothScroll()) {
+          currentVelocity = mAsyncScroll->VelocityAt(now);
+        }
+        mAsyncScroll = nullptr;
+      }
+
+      if (canHandoffToApz) {
+        ApzSmoothScrollTo(mDestination, ScrollMode::SmoothMsd, aParams.mOrigin,
+                          aParams.mTriggeredByScript, std::move(snapTargetIds));
+        return;
+      }
+
+      mAsyncSmoothMSDScroll = new AsyncSmoothMSDScroll(
+          GetScrollPosition(), mDestination, currentVelocity,
+          GetLayoutScrollRange(), now, presContext, std::move(snapTargetIds),
+          aParams.mTriggeredByScript);
+
+      mAsyncSmoothMSDScroll->SetRefreshObserver(this);
+    } else {
+      // A previous smooth MSD scroll is still in progress, so we just need to
+      // update its range and destination.
+      mAsyncSmoothMSDScroll->SetRange(GetLayoutScrollRange());
+      mAsyncSmoothMSDScroll->SetDestination(mDestination,
+                                            aParams.mTriggeredByScript);
+    }
+
+    return;
+  }
+
+  if (mAsyncSmoothMSDScroll) {
+    currentVelocity = mAsyncSmoothMSDScroll->GetVelocity();
+    mAsyncSmoothMSDScroll = nullptr;
+  }
+
+  const bool isSmoothScroll =
+      aParams.IsSmooth() && nsLayoutUtils::IsSmoothScrollingEnabled();
+  if (!mAsyncScroll) {
+    if (isSmoothScroll && canHandoffToApz) {
+      ApzSmoothScrollTo(mDestination, ScrollMode::Smooth, aParams.mOrigin,
+                        aParams.mTriggeredByScript, std::move(snapTargetIds));
+      return;
+    }
+
+    mAsyncScroll =
+        new AsyncScroll(std::move(snapTargetIds), aParams.mTriggeredByScript);
+    mAsyncScroll->SetRefreshObserver(this);
+  }
+
+  if (isSmoothScroll) {
+    mAsyncScroll->InitSmoothScroll(now, GetScrollPosition(), mDestination,
+                                   aParams.mOrigin, range, currentVelocity);
+  } else {
+    mAsyncScroll->Init(GetScrollPosition(), range);
+  }
+}
+
+// We can't use nsContainerFrame::PositionChildViews here because
+// we don't want to invalidate views that have moved.
+static void AdjustViews(nsIFrame* aFrame) {
+  nsView* view = aFrame->GetView();
+  if (view) {
+    nsPoint pt;
+    aFrame->GetParent()->GetClosestView(&pt);
+    pt += aFrame->GetPosition();
+    view->SetPosition(pt.x, pt.y);
+
+    return;
+  }
+
+  if (!aFrame->HasAnyStateBits(NS_FRAME_HAS_CHILD_WITH_VIEW)) {
+    return;
+  }
+
+  // Call AdjustViews recursively for all child frames except the popup list as
+  // the views for popups are not scrolled.
+  for (const auto& [list, listID] : aFrame->ChildLists()) {
+    if (listID == FrameChildListID::Popup) {
+      continue;
+    }
+    for (nsIFrame* child : list) {
+      AdjustViews(child);
+    }
+  }
+}
+
+void nsHTMLScrollFrame::MarkScrollbarsDirtyForReflow() const {
+  auto* presShell = PresShell();
+  if (mVScrollbarBox) {
+    presShell->FrameNeedsReflow(mVScrollbarBox,
+                                IntrinsicDirty::FrameAncestorsAndDescendants,
+                                NS_FRAME_IS_DIRTY);
+  }
+  if (mHScrollbarBox) {
+    presShell->FrameNeedsReflow(mHScrollbarBox,
+                                IntrinsicDirty::FrameAncestorsAndDescendants,
+                                NS_FRAME_IS_DIRTY);
+  }
+}
+
+void nsHTMLScrollFrame::InvalidateScrollbars() const {
+  if (mHScrollbarBox) {
+    mHScrollbarBox->InvalidateFrameSubtree();
+  }
+  if (mVScrollbarBox) {
+    mVScrollbarBox->InvalidateFrameSubtree();
+  }
+}
+
+bool nsHTMLScrollFrame::IsAlwaysActive() const {
+  if (nsDisplayItem::ForceActiveLayers()) {
+    return true;
+  }
+
+  // Unless this is the root scrollframe for a non-chrome document
+  // which is the direct child of a chrome document, we default to not
+  // being "active".
+  if (!(mIsRoot && PresContext()->IsRootContentDocumentCrossProcess())) {
+    return false;
+  }
+
+  // If we have scrolled before, then we should stay active.
+  if (mHasBeenScrolled) {
+    return true;
+  }
+
+  // If we're overflow:hidden, then start as inactive until
+  // we get scrolled manually.
+  ScrollStyles styles = GetScrollStyles();
+  return (styles.mHorizontal != StyleOverflow::Hidden &&
+          styles.mVertical != StyleOverflow::Hidden);
+}
+
+static void RemoveDisplayPortCallback(nsITimer* aTimer, void* aClosure) {
+  nsHTMLScrollFrame* sf = static_cast<nsHTMLScrollFrame*>(aClosure);
+
+  // This function only ever gets called from the expiry timer, so it must
+  // be non-null here. Set it to null here so that we don't keep resetting
+  // it unnecessarily in MarkRecentlyScrolled().
+  MOZ_ASSERT(sf->mDisplayPortExpiryTimer);
+  sf->mDisplayPortExpiryTimer = nullptr;
+
+  if (!sf->AllowDisplayPortExpiration() || sf->mIsParentToActiveScrollFrames) {
+    // If this is a scroll parent for some other scrollable frame, don't
+    // expire the displayport because it would break scroll handoff. Once the
+    // descendant scrollframes have their displayports expired, they will
+    // trigger the displayport expiration on this scrollframe as well, and
+    // mIsParentToActiveScrollFrames will presumably be false when that kicks
+    // in.
+    return;
+  }
+
+  // Remove the displayport from this scrollframe if it's been a while
+  // since it's scrolled, except if it needs to be always active. Note that
+  // there is one scrollframe that doesn't fall under this general rule, and
+  // that is the one that nsLayoutUtils::MaybeCreateDisplayPort decides to put
+  // a displayport on (i.e. the first scrollframe that WantAsyncScroll()s).
+  // If that scrollframe is this one, we remove the displayport anyway, and
+  // as part of the next paint MaybeCreateDisplayPort will put another
+  // displayport back on it. Although the displayport will "flicker" off and
+  // back on, the layer itself should never disappear, because this all
+  // happens between actual painting. If the displayport is reset to a
+  // different position that's ok; this scrollframe hasn't been scrolled
+  // recently and so the reset should be correct.
+
+  nsIContent* content = sf->GetContent();
+
+  if (nsHTMLScrollFrame::ShouldActivateAllScrollFrames()) {
+    // If we are activating all scroll frames then we only want to remove the
+    // regular display port and downgrade to a minimal display port.
+    MOZ_ASSERT(!content->GetProperty(nsGkAtoms::MinimalDisplayPort));
+    content->SetProperty(nsGkAtoms::MinimalDisplayPort,
+                         reinterpret_cast<void*>(true));
+  } else {
+    content->RemoveProperty(nsGkAtoms::MinimalDisplayPort);
+    DisplayPortUtils::RemoveDisplayPort(content);
+    // Be conservative and unflag this this scrollframe as being scrollable by
+    // APZ. If it is still scrollable this will get flipped back soon enough.
+    sf->mScrollableByAPZ = false;
+  }
+
+  DisplayPortUtils::ExpireDisplayPortOnAsyncScrollableAncestor(sf);
+  sf->SchedulePaint();
+}
+
+void nsHTMLScrollFrame::MarkEverScrolled() {
+  // Mark this frame as having been scrolled. If this is the root
+  // scroll frame of a content document, then IsAlwaysActive()
+  // will return true from now on and MarkNotRecentlyScrolled() won't
+  // have any effect.
+  mHasBeenScrolled = true;
+}
+
+void nsHTMLScrollFrame::MarkNotRecentlyScrolled() {
+  if (!mHasBeenScrolledRecently) return;
+
+  mHasBeenScrolledRecently = false;
+  SchedulePaint();
+}
+
+void nsHTMLScrollFrame::MarkRecentlyScrolled() {
+  mHasBeenScrolledRecently = true;
+  if (IsAlwaysActive()) {
+    return;
+  }
+
+  if (mActivityExpirationState.IsTracked()) {
+    gScrollFrameActivityTracker->MarkUsed(this);
+  } else {
+    if (!gScrollFrameActivityTracker) {
+      gScrollFrameActivityTracker =
+          new ScrollFrameActivityTracker(GetMainThreadSerialEventTarget());
+    }
+    gScrollFrameActivityTracker->AddObject(this);
+  }
+
+  // If we just scrolled and there's a displayport expiry timer in place,
+  // reset the timer.
+  ResetDisplayPortExpiryTimer();
+}
+
+void nsHTMLScrollFrame::ResetDisplayPortExpiryTimer() {
+  if (mDisplayPortExpiryTimer) {
+    mDisplayPortExpiryTimer->InitWithNamedFuncCallback(
+        RemoveDisplayPortCallback, this,
+        StaticPrefs::apz_displayport_expiry_ms(), nsITimer::TYPE_ONE_SHOT,
+        "nsHTMLScrollFrame::ResetDisplayPortExpiryTimer");
+  }
+}
+
+bool nsHTMLScrollFrame::AllowDisplayPortExpiration() {
+  if (IsAlwaysActive()) {
+    return false;
+  }
+
+  if (mIsRoot && PresContext()->IsRoot()) {
+    return false;
+  }
+
+  // If this was the first scrollable frame found, this displayport should
+  // not expire.
+  if (IsFirstScrollableFrameSequenceNumber().isSome()) {
+    return false;
+  }
+
+  if (ShouldActivateAllScrollFrames() &&
+      GetContent()->GetProperty(nsGkAtoms::MinimalDisplayPort)) {
+    return false;
+  }
+  return true;
+}
+
+void nsHTMLScrollFrame::TriggerDisplayPortExpiration() {
+  if (!AllowDisplayPortExpiration()) {
+    return;
+  }
+
+  if (!StaticPrefs::apz_displayport_expiry_ms()) {
+    // a zero time disables the expiry
+    return;
+  }
+
+  if (!mDisplayPortExpiryTimer) {
+    mDisplayPortExpiryTimer = NS_NewTimer();
+  }
+  ResetDisplayPortExpiryTimer();
+}
+
+void nsHTMLScrollFrame::ScrollVisual() {
+  MarkEverScrolled();
+
+  AdjustViews(mScrolledFrame);
+  // We need to call this after fixing up the view positions
+  // to be consistent with the frame hierarchy.
+  MarkRecentlyScrolled();
+}
+
+/**
+ * Clamp desired scroll position aDesired and range [aDestLower, aDestUpper]
+ * to [aBoundLower, aBoundUpper] and then select the appunit value from among
+ * aBoundLower, aBoundUpper and those such that (aDesired - aCurrent) *
+ * aRes/aAppUnitsPerPixel is an integer (or as close as we can get
+ * modulo rounding to appunits) that is in [aDestLower, aDestUpper] and
+ * closest to aDesired.  If no such value exists, return the nearest in
+ * [aDestLower, aDestUpper].
+ */
+static nscoord ClampAndAlignWithPixels(nscoord aDesired, nscoord aBoundLower,
+                                       nscoord aBoundUpper, nscoord aDestLower,
+                                       nscoord aDestUpper,
+                                       nscoord aAppUnitsPerPixel, double aRes,
+                                       nscoord aCurrent) {
+  // Intersect scroll range with allowed range, by clamping the ends
+  // of aRange to be within bounds
+  nscoord destLower = clamped(aDestLower, aBoundLower, aBoundUpper);
+  nscoord destUpper = clamped(aDestUpper, aBoundLower, aBoundUpper);
+
+  nscoord desired = clamped(aDesired, destLower, destUpper);
+  if (StaticPrefs::layout_scroll_disable_pixel_alignment()) {
+    return desired;
+  }
+
+  double currentLayerVal = (aRes * aCurrent) / aAppUnitsPerPixel;
+  double desiredLayerVal = (aRes * desired) / aAppUnitsPerPixel;
+  double delta = desiredLayerVal - currentLayerVal;
+  double nearestLayerVal = NS_round(delta) + currentLayerVal;
+
+  // Convert back from PaintedLayer space to appunits relative to the top-left
+  // of the scrolled frame.
+  nscoord aligned =
+      aRes == 0.0
+          ? 0.0
+          : NSToCoordRoundWithClamp(nearestLayerVal * aAppUnitsPerPixel / aRes);
+
+  // Use a bound if it is within the allowed range and closer to desired than
+  // the nearest pixel-aligned value.
+  if (aBoundUpper == destUpper &&
+      static_cast<decltype(Abs(desired))>(aBoundUpper - desired) <
+          Abs(desired - aligned)) {
+    return aBoundUpper;
+  }
+
+  if (aBoundLower == destLower &&
+      static_cast<decltype(Abs(desired))>(desired - aBoundLower) <
+          Abs(aligned - desired)) {
+    return aBoundLower;
+  }
+
+  // Accept the nearest pixel-aligned value if it is within the allowed range.
+  if (aligned >= destLower && aligned <= destUpper) {
+    return aligned;
+  }
+
+  // Check if opposite pixel boundary fits into allowed range.
+  double oppositeLayerVal =
+      nearestLayerVal + ((nearestLayerVal < desiredLayerVal) ? 1.0 : -1.0);
+  nscoord opposite = aRes == 0.0
+                         ? 0.0
+                         : NSToCoordRoundWithClamp(oppositeLayerVal *
+                                                   aAppUnitsPerPixel / aRes);
+  if (opposite >= destLower && opposite <= destUpper) {
+    return opposite;
+  }
+
+  // No alignment available.
+  return desired;
+}
+
+/**
+ * Clamp desired scroll position aPt to aBounds and then snap
+ * it to the same layer pixel edges as aCurrent, keeping it within aRange
+ * during snapping. aCurrent is the current scroll position.
+ */
+static nsPoint ClampAndAlignWithLayerPixels(const nsPoint& aPt,
+                                            const nsRect& aBounds,
+                                            const nsRect& aRange,
+                                            const nsPoint& aCurrent,
+                                            nscoord aAppUnitsPerPixel,
+                                            const MatrixScales& aScale) {
+  return nsPoint(
+      ClampAndAlignWithPixels(aPt.x, aBounds.x, aBounds.XMost(), aRange.x,
+                              aRange.XMost(), aAppUnitsPerPixel, aScale.xScale,
+                              aCurrent.x),
+      ClampAndAlignWithPixels(aPt.y, aBounds.y, aBounds.YMost(), aRange.y,
+                              aRange.YMost(), aAppUnitsPerPixel, aScale.yScale,
+                              aCurrent.y));
+}
+
+/* static */
+void nsHTMLScrollFrame::ScrollActivityCallback(nsITimer* aTimer,
+                                               void* anInstance) {
+  nsHTMLScrollFrame* self = static_cast<nsHTMLScrollFrame*>(anInstance);
+
+  // Fire the synth mouse move.
+  self->mScrollActivityTimer->Cancel();
+  self->mScrollActivityTimer = nullptr;
+  self->PresShell()->SynthesizeMouseMove(true);
+}
+
+void nsHTMLScrollFrame::ScheduleSyntheticMouseMove() {
+  if (!mScrollActivityTimer) {
+    mScrollActivityTimer = NS_NewTimer(GetMainThreadSerialEventTarget());
+    if (!mScrollActivityTimer) {
+      return;
+    }
+  }
+
+  mScrollActivityTimer->InitWithNamedFuncCallback(
+      ScrollActivityCallback, this, 100, nsITimer::TYPE_ONE_SHOT,
+      "nsHTMLScrollFrame::ScheduleSyntheticMouseMove");
+}
+
+void nsHTMLScrollFrame::NotifyApproximateFrameVisibilityUpdate(
+    bool aIgnoreDisplayPort) {
+  mLastUpdateFramesPos = GetScrollPosition();
+  if (aIgnoreDisplayPort) {
+    mHadDisplayPortAtLastFrameUpdate = false;
+    mDisplayPortAtLastFrameUpdate = nsRect();
+  } else {
+    mHadDisplayPortAtLastFrameUpdate = DisplayPortUtils::GetDisplayPort(
+        GetContent(), &mDisplayPortAtLastFrameUpdate);
+  }
+}
+
+bool nsHTMLScrollFrame::GetDisplayPortAtLastApproximateFrameVisibilityUpdate(
+    nsRect* aDisplayPort) {
+  if (mHadDisplayPortAtLastFrameUpdate) {
+    *aDisplayPort = mDisplayPortAtLastFrameUpdate;
+  }
+  return mHadDisplayPortAtLastFrameUpdate;
+}
+
+/* aIncludeCSSTransform controls if we include CSS transforms that are in this
+ * process (the BrowserChild EffectsInfo mTransformToAncestorScale will include
+ * CSS transforms in ancestor processes in all cases). */
+MatrixScales GetPaintedLayerScaleForFrame(nsIFrame* aFrame,
+                                          bool aIncludeCSSTransform) {
+  MOZ_ASSERT(aFrame, "need a frame");
+
+  nsPresContext* presCtx = aFrame->PresContext()->GetRootPresContext();
+
+  if (!presCtx) {
+    presCtx = aFrame->PresContext();
+    MOZ_ASSERT(presCtx);
+  }
+
+  ParentLayerToScreenScale2D transformToAncestorScale;
+  if (aIncludeCSSTransform) {
+    transformToAncestorScale =
+        nsLayoutUtils::GetTransformToAncestorScaleCrossProcessForFrameMetrics(
+            aFrame);
+  } else {
+    if (BrowserChild* browserChild =
+            BrowserChild::GetFrom(aFrame->PresShell())) {
+      transformToAncestorScale =
+          browserChild->GetEffectsInfo().mTransformToAncestorScale;
+    }
+  }
+  transformToAncestorScale =
+      ParentLayerToParentLayerScale(
+          presCtx->PresShell()->GetCumulativeResolution()) *
+      transformToAncestorScale;
+
+  return transformToAncestorScale.ToUnknownScale();
+}
+
+void nsHTMLScrollFrame::ScrollToImpl(
+    nsPoint aPt, const nsRect& aRange, ScrollOrigin aOrigin,
+    ScrollTriggeredByScript aTriggeredByScript) {
+  // None is never a valid scroll origin to be passed in.
+  MOZ_ASSERT(aOrigin != ScrollOrigin::None);
+
+  // Figure out the effective origin for this scroll request.
+  if (aOrigin == ScrollOrigin::NotSpecified) {
+    // If no origin was specified, we still want to set it to something that's
+    // non-unknown, so that we can use eUnknown to distinguish if the frame was
+    // scrolled at all. Default it to some generic placeholder.
+    aOrigin = ScrollOrigin::Other;
+  }
+
+  // If this scroll is |relative|, but we've already had a user scroll that
+  // was not relative, promote this origin to |other|. This ensures that we
+  // may only transmit a relative update to APZ if all scrolls since the last
+  // transaction or repaint request have been relative.
+  if (aOrigin == ScrollOrigin::Relative &&
+      (mLastScrollOrigin != ScrollOrigin::None &&
+       mLastScrollOrigin != ScrollOrigin::NotSpecified &&
+       mLastScrollOrigin != ScrollOrigin::Relative &&
+       mLastScrollOrigin != ScrollOrigin::AnchorAdjustment &&
+       mLastScrollOrigin != ScrollOrigin::Apz)) {
+    aOrigin = ScrollOrigin::Other;
+  }
+
+  // If the origin is a downgrade, and downgrades are allowed, process the
+  // downgrade even if we're going to early-exit because we're already at
+  // the correct scroll position. This ensures that if there wasn't a main-
+  // thread scroll update pending before a frame reconstruction (as indicated
+  // by mAllowScrollOriginDowngrade=true), then after the frame reconstruction
+  // the origin is downgraded to "restore" even if the layout scroll offset to
+  // be restored is (0,0) (which will take the early-exit below). This is
+  // important so that restoration of a *visual* scroll offset (which might be
+  // to something other than (0,0)) isn't clobbered.
+  bool isScrollOriginDowngrade =
+      nsLayoutUtils::CanScrollOriginClobberApz(mLastScrollOrigin) &&
+      !nsLayoutUtils::CanScrollOriginClobberApz(aOrigin);
+  bool allowScrollOriginChange =
+      mAllowScrollOriginDowngrade && isScrollOriginDowngrade;
+
+  if (allowScrollOriginChange) {
+    mLastScrollOrigin = aOrigin;
+    mAllowScrollOriginDowngrade = false;
+  }
+
+  nsPresContext* presContext = PresContext();
+  nscoord appUnitsPerDevPixel = presContext->AppUnitsPerDevPixel();
+  // 'scale' is our estimate of the scale factor that will be applied
+  // when rendering the scrolled content to its own PaintedLayer.
+  MatrixScales scale = GetPaintedLayerScaleForFrame(
+      mScrolledFrame, /* aIncludeCSSTransform = */ true);
+  nsPoint curPos = GetScrollPosition();
+
+  // Try to align aPt with curPos so they have an integer number of layer
+  // pixels between them. This gives us the best chance of scrolling without
+  // having to invalidate due to changes in subpixel rendering.
+  // Note that when we actually draw into a PaintedLayer, the coordinates
+  // that get mapped onto the layer buffer pixels are from the display list,
+  // which are relative to the display root frame's top-left increasing down,
+  // whereas here our coordinates are scroll positions which increase upward
+  // and are relative to the scrollport top-left. This difference doesn't
+  // actually matter since all we are about is that there be an integer number
+  // of layer pixels between pt and curPos.
+  nsPoint pt = ClampAndAlignWithLayerPixels(aPt, GetLayoutScrollRange(), aRange,
+                                            curPos, appUnitsPerDevPixel, scale);
+  if (pt == curPos) {
+    // Even if we are bailing out due to no-op main-thread scroll position
+    // change, we might need to cancel an APZ smooth scroll that we already
+    // kicked off. It might be reasonable to eventually remove the
+    // mApzSmoothScrollDestination clause from this if statement, as that
+    // may simplify this a bit and should be fine from the APZ side.
+    if (mApzSmoothScrollDestination && aOrigin != ScrollOrigin::Clamp) {
+      if (aOrigin == ScrollOrigin::Relative) {
+        AppendScrollUpdate(ScrollPositionUpdate::NewRelativeScroll(
+            // Clamp |mApzScrollPos| here. See the comment for this clamping
+            // reason below NewRelativeScroll call.
+            GetLayoutScrollRange().ClampPoint(mApzScrollPos), pt));
+        mApzScrollPos = pt;
+      } else if (aOrigin != ScrollOrigin::Apz) {
+        ScrollOrigin origin =
+            (mAllowScrollOriginDowngrade || !isScrollOriginDowngrade)
+                ? aOrigin
+                : mLastScrollOrigin;
+        AppendScrollUpdate(ScrollPositionUpdate::NewScroll(origin, pt));
+      }
+    }
+    return;
+  }
+
+  // If we are scrolling the RCD-RSF, and a visual scroll update is pending,
+  // cancel it; otherwise, it will clobber this scroll.
+  if (IsRootScrollFrameOfDocument() &&
+      presContext->IsRootContentDocumentCrossProcess()) {
+    auto* ps = presContext->GetPresShell();
+    if (const auto& visualScrollUpdate = ps->GetPendingVisualScrollUpdate()) {
+      if (visualScrollUpdate->mVisualScrollOffset != aPt) {
+        // Only clobber if the scroll was originated by the main thread.
+        // Respect the priority of origins (an "eRestore" layout scroll should
+        // not clobber an "eMainThread" visual scroll.)
+        bool shouldClobber =
+            aOrigin == ScrollOrigin::Other ||
+            (aOrigin == ScrollOrigin::Restore &&
+             visualScrollUpdate->mUpdateType == FrameMetrics::eRestore);
+        if (shouldClobber) {
+          ps->AcknowledgePendingVisualScrollUpdate();
+          ps->ClearPendingVisualScrollUpdate();
+        }
+      }
+    }
+  }
+
+  bool needFrameVisibilityUpdate = mLastUpdateFramesPos == nsPoint(-1, -1);
+
+  nsPoint dist(std::abs(pt.x - mLastUpdateFramesPos.x),
+               std::abs(pt.y - mLastUpdateFramesPos.y));
+  nsSize visualViewportSize = GetVisualViewportSize();
+  nscoord horzAllowance = std::max(
+      visualViewportSize.width /
+          std::max(
+              StaticPrefs::
+                  layout_framevisibility_amountscrollbeforeupdatehorizontal(),
+              1),
+      AppUnitsPerCSSPixel());
+  nscoord vertAllowance = std::max(
+      visualViewportSize.height /
+          std::max(
+              StaticPrefs::
+                  layout_framevisibility_amountscrollbeforeupdatevertical(),
+              1),
+      AppUnitsPerCSSPixel());
+  if (dist.x >= horzAllowance || dist.y >= vertAllowance) {
+    needFrameVisibilityUpdate = true;
+  }
+
+  // notify the listeners.
+  for (uint32_t i = 0; i < mListeners.Length(); i++) {
+    mListeners[i]->ScrollPositionWillChange(pt.x, pt.y);
+  }
+
+  nsRect oldDisplayPort;
+  nsIContent* content = GetContent();
+  DisplayPortUtils::GetDisplayPort(content, &oldDisplayPort);
+  oldDisplayPort.MoveBy(-mScrolledFrame->GetPosition());
+
+  // Update frame position for scrolling
+  mScrolledFrame->SetPosition(mScrollPort.TopLeft() - pt);
+
+  // If |mLastScrollOrigin| is already set to something that can clobber APZ's
+  // scroll offset, then we don't want to change it to something that can't.
+  // If we allowed this, then we could end up in a state where APZ ignores
+  // legitimate scroll offset updates because the origin has been masked by
+  // a later change within the same refresh driver tick.
+  allowScrollOriginChange =
+      (mAllowScrollOriginDowngrade || !isScrollOriginDowngrade);
+
+  if (allowScrollOriginChange) {
+    mLastScrollOrigin = aOrigin;
+    mAllowScrollOriginDowngrade = false;
+  }
+
+  if (aOrigin == ScrollOrigin::Relative) {
+    MOZ_ASSERT(!isScrollOriginDowngrade);
+    MOZ_ASSERT(mLastScrollOrigin == ScrollOrigin::Relative);
+    AppendScrollUpdate(ScrollPositionUpdate::NewRelativeScroll(
+        // It's possible that |mApzScrollPos| is no longer within the scroll
+        // range, we need to clamp it to the current scroll range, otherwise
+        // calculating a relative scroll distance from the outside point will
+        // result a point far from the desired point.
+        GetLayoutScrollRange().ClampPoint(mApzScrollPos), pt));
+    mApzScrollPos = pt;
+  } else if (aOrigin == ScrollOrigin::AnchorAdjustment) {
+    AppendScrollUpdate(ScrollPositionUpdate::NewMergeableScroll(aOrigin, pt));
+    mApzScrollPos = pt;
+  } else if (aOrigin != ScrollOrigin::Apz) {
+    AppendScrollUpdate(ScrollPositionUpdate::NewScroll(mLastScrollOrigin, pt));
+  }
+
+  if (mLastScrollOrigin == ScrollOrigin::Apz) {
+    mApzScrollPos = GetScrollPosition();
+  }
+
+  ScrollVisual();
+  mAnchor.UserScrolled();
+
+  // Only report user-triggered scrolling interactions
+  bool jsOnStack = nsContentUtils::GetCurrentJSContext() != nullptr;
+  bool scrollingToAnchor = ScrollingInteractionContext::IsScrollingToAnchor();
+  if (!jsOnStack && !scrollingToAnchor) {
+    nsPoint distanceScrolled(std::abs(pt.x - curPos.x),
+                             std::abs(pt.y - curPos.y));
+    ScrollingMetrics::OnScrollingInteraction(
+        CSSPoint::FromAppUnits(distanceScrolled).Length());
+  }
+
+  bool schedulePaint = true;
+  if (nsLayoutUtils::AsyncPanZoomEnabled(this) &&
+      !nsLayoutUtils::ShouldDisableApzForElement(content) &&
+      !content->GetProperty(nsGkAtoms::MinimalDisplayPort) &&
+      StaticPrefs::apz_paint_skipping_enabled()) {
+    // If APZ is enabled with paint-skipping, there are certain conditions in
+    // which we can skip paints:
+    // 1) If APZ triggered this scroll, and the tile-aligned displayport is
+    //    unchanged.
+    // 2) If non-APZ triggered this scroll, but we can handle it by just asking
+    //    APZ to update the scroll position. Again we make this conditional on
+    //    the tile-aligned displayport being unchanged.
+    // We do the displayport check first since it's common to all scenarios,
+    // and then if the displayport is unchanged, we check if APZ triggered,
+    // or can handle, this scroll. If so, we set schedulePaint to false and
+    // skip the paint.
+    // Because of bug 1264297, we also don't do paint-skipping for elements with
+    // perspective, because the displayport may not have captured everything
+    // that needs to be painted. So even if the final tile-aligned displayport
+    // is the same, we force a repaint for these elements. Bug 1254260 tracks
+    // fixing this properly.
+    nsRect displayPort;
+    bool usingDisplayPort =
+        DisplayPortUtils::GetDisplayPort(content, &displayPort);
+    displayPort.MoveBy(-mScrolledFrame->GetPosition());
+
+    PAINT_SKIP_LOG(
+        "New scrollpos %s usingDP %d dpEqual %d scrollableByApz "
+        "%d perspective %d bglocal %d filter %d\n",
+        ToString(CSSPoint::FromAppUnits(GetScrollPosition())).c_str(),
+        usingDisplayPort, displayPort.IsEqualEdges(oldDisplayPort),
+        mScrollableByAPZ, HasPerspective(), HasBgAttachmentLocal(),
+        mHasOutOfFlowContentInsideFilter);
+    if (usingDisplayPort && displayPort.IsEqualEdges(oldDisplayPort) &&
+        !HasPerspective() && !HasBgAttachmentLocal() &&
+        !mHasOutOfFlowContentInsideFilter) {
+      bool haveScrollLinkedEffects =
+          content->GetComposedDoc()->HasScrollLinkedEffect();
+      bool apzDisabled = haveScrollLinkedEffects &&
+                         StaticPrefs::apz_disable_for_scroll_linked_effects();
+      if (!apzDisabled) {
+        if (LastScrollOrigin() == ScrollOrigin::Apz) {
+          schedulePaint = false;
+          PAINT_SKIP_LOG("Skipping due to APZ scroll\n");
+        } else if (mScrollableByAPZ) {
+          nsIWidget* widget = presContext->GetNearestWidget();
+          WindowRenderer* renderer =
+              widget ? widget->GetWindowRenderer() : nullptr;
+          if (renderer) {
+            mozilla::layers::ScrollableLayerGuid::ViewID id;
+            bool success = nsLayoutUtils::FindIDFor(content, &id);
+            MOZ_ASSERT(success);  // we have a displayport, we better have an ID
+
+            // Schedule an empty transaction to carry over the scroll offset
+            // update, instead of a full transaction. This empty transaction
+            // might still get squashed into a full transaction if something
+            // happens to trigger one.
+            MOZ_ASSERT(!mScrollUpdates.IsEmpty());
+            success = renderer->AddPendingScrollUpdateForNextTransaction(
+                id, mScrollUpdates.LastElement());
+            if (success) {
+              schedulePaint = false;
+              SchedulePaint(nsIFrame::PAINT_COMPOSITE_ONLY);
+              PAINT_SKIP_LOG(
+                  "Skipping due to APZ-forwarded main-thread scroll\n");
+            } else {
+              PAINT_SKIP_LOG(
+                  "Failed to set pending scroll update on layer manager\n");
+            }
+          }
+        }
+      }
+    }
+  }
+
+  // If the new scroll offset is going to clobber APZ's scroll offset, for
+  // the RCD-RSF this will have the effect of updating the visual viewport
+  // offset in a way that keeps the relative offset between the layout and
+  // visual viewports constant. This will cause APZ to send us a new visual
+  // viewport offset, but instead of waiting for  that, just set the value
+  // we expect APZ will set ourselves, to minimize the chances of
+  // inconsistencies from querying a stale value.
+  if (mIsRoot && nsLayoutUtils::CanScrollOriginClobberApz(aOrigin)) {
+    AutoWeakFrame weakFrame(this);
+    AutoScrollbarRepaintSuppression repaintSuppression(this, weakFrame,
+                                                       !schedulePaint);
+
+    nsPoint visualViewportOffset = curPos;
+    if (presContext->PresShell()->IsVisualViewportOffsetSet()) {
+      visualViewportOffset =
+          presContext->PresShell()->GetVisualViewportOffset();
+    }
+    nsPoint relativeOffset = visualViewportOffset - curPos;
+
+    presContext->PresShell()->SetVisualViewportOffset(pt + relativeOffset,
+                                                      curPos);
+    if (!weakFrame.IsAlive()) {
+      return;
+    }
+  }
+
+  if (schedulePaint) {
+    SchedulePaint();
+
+    if (needFrameVisibilityUpdate) {
+      presContext->PresShell()->ScheduleApproximateFrameVisibilityUpdateNow();
+    }
+  }
+
+  if (ChildrenHavePerspective()) {
+    // The overflow areas of descendants may depend on the scroll position,
+    // so ensure they get updated.
+
+    // First we recompute the overflow areas of the transformed children
+    // that use the perspective. FinishAndStoreOverflow only calls this
+    // if the size changes, so we need to do it manually.
+    RecomputePerspectiveChildrenOverflow(this);
+
+    // Update the overflow for the scrolled frame to take any changes from the
+    // children into account.
+    mScrolledFrame->UpdateOverflow();
+
+    // Update the overflow for the outer so that we recompute scrollbars.
+    UpdateOverflow();
+  }
+
+  ScheduleSyntheticMouseMove();
+
+  nsAutoScriptBlocker scriptBlocker;
+  PresShell::AutoAssertNoFlush noFlush(*PresShell());
+
+  {  // scope the AutoScrollbarRepaintSuppression
+    AutoWeakFrame weakFrame(this);
+    AutoScrollbarRepaintSuppression repaintSuppression(this, weakFrame,
+                                                       !schedulePaint);
+    UpdateScrollbarPosition();
+    if (!weakFrame.IsAlive()) {
+      return;
+    }
+  }
+
+  presContext->RecordInteractionTime(
+      nsPresContext::InteractionType::ScrollInteraction, TimeStamp::Now());
+
+  PostScrollEvent();
+  // If this is a viewport scroll, this could affect the relative offset
+  // between layout and visual viewport, so we might have to fire a visual
+  // viewport scroll event as well.
+  if (mIsRoot) {
+    if (auto* window = nsGlobalWindowInner::Cast(
+            PresContext()->Document()->GetInnerWindow())) {
+      window->VisualViewport()->PostScrollEvent(
+          presContext->PresShell()->GetVisualViewportOffset(), curPos);
+    }
+  }
+
+  // Schedule the scroll-timelines linked to its scrollable frame.
+  // if `pt == curPos`, we early return, so the position must be changed at
+  // this moment. Therefore, we can schedule scroll animations directly.
+  ScheduleScrollAnimations();
+
+  // notify the listeners.
+  for (uint32_t i = 0; i < mListeners.Length(); i++) {
+    mListeners[i]->ScrollPositionDidChange(pt.x, pt.y);
+  }
+
+  if (nsCOMPtr<nsIDocShell> docShell = presContext->GetDocShell()) {
+    docShell->NotifyScrollObservers();
+  }
+}
+
+// Finds the max z-index of the items in aList that meet the following
+// conditions
+//   1) have z-index auto or z-index >= 0, and
+//   2) aFrame is a proper ancestor of the item's frame.
+// Returns Nothing() if there is no such item.
+static Maybe<int32_t> MaxZIndexInListOfItemsContainedInFrame(
+    nsDisplayList* aList, nsIFrame* aFrame) {
+  Maybe<int32_t> maxZIndex = Nothing();
+  for (nsDisplayItem* item : *aList) {
+    int32_t zIndex = item->ZIndex();
+    if (zIndex < 0 ||
+        !nsLayoutUtils::IsProperAncestorFrame(aFrame, item->Frame())) {
+      continue;
+    }
+    if (!maxZIndex) {
+      maxZIndex = Some(zIndex);
+    } else {
+      maxZIndex = Some(std::max(maxZIndex.value(), zIndex));
+    }
+  }
+  return maxZIndex;
+}
+
+template <class T>
+static void AppendInternalItemToTop(const nsDisplayListSet& aLists, T* aItem,
+                                    const Maybe<int32_t>& aZIndex) {
+  if (aZIndex) {
+    aItem->SetOverrideZIndex(aZIndex.value());
+    aLists.PositionedDescendants()->AppendToTop(aItem);
+  } else {
+    aLists.Content()->AppendToTop(aItem);
+  }
+}
+
+static const uint32_t APPEND_OWN_LAYER = 0x1;
+static const uint32_t APPEND_POSITIONED = 0x2;
+static const uint32_t APPEND_SCROLLBAR_CONTAINER = 0x4;
+static const uint32_t APPEND_OVERLAY = 0x8;
+static const uint32_t APPEND_TOP = 0x10;
+
+static void AppendToTop(nsDisplayListBuilder* aBuilder,
+                        const nsDisplayListSet& aLists, nsDisplayList* aSource,
+                        nsIFrame* aSourceFrame, nsIFrame* aScrollFrame,
+                        uint32_t aFlags) {
+  if (aSource->IsEmpty()) {
+    return;
+  }
+
+  nsDisplayWrapList* newItem;
+  const ActiveScrolledRoot* asr = aBuilder->CurrentActiveScrolledRoot();
+  if (aFlags & APPEND_OWN_LAYER) {
+    ScrollbarData scrollbarData;
+    if (aFlags & APPEND_SCROLLBAR_CONTAINER) {
+      scrollbarData = ScrollbarData::CreateForScrollbarContainer(
+          aBuilder->GetCurrentScrollbarDirection(),
+          aBuilder->GetCurrentScrollbarTarget());
+      // Direction should be set
+      MOZ_ASSERT(scrollbarData.mDirection.isSome());
+    }
+
+    newItem = MakeDisplayItemWithIndex<nsDisplayOwnLayer>(
+        aBuilder, aSourceFrame,
+        /* aIndex = */ nsDisplayOwnLayer::OwnLayerForScrollbar, aSource, asr,
+        nsDisplayOwnLayerFlags::None, scrollbarData, true, false);
+  } else {
+    // Build the wrap list with an index of 1, since the scrollbar frame itself
+    // might have already built an nsDisplayWrapList.
+    newItem = MakeDisplayItemWithIndex<nsDisplayWrapper>(
+        aBuilder, aSourceFrame, 1, aSource, asr, false);
+  }
+  if (!newItem) {
+    return;
+  }
+
+  if (aFlags & APPEND_POSITIONED) {
+    // We want overlay scrollbars to always be on top of the scrolled content,
+    // but we don't want them to unnecessarily cover overlapping elements from
+    // outside our scroll frame.
+    Maybe<int32_t> zIndex = Nothing();
+    if (aFlags & APPEND_TOP) {
+      zIndex = Some(INT32_MAX);
+    } else if (aFlags & APPEND_OVERLAY) {
+      zIndex = MaxZIndexInListOfItemsContainedInFrame(
+          aLists.PositionedDescendants(), aScrollFrame);
+    } else if (aSourceFrame->StylePosition()->mZIndex.IsInteger()) {
+      zIndex = Some(aSourceFrame->StylePosition()->mZIndex.integer._0);
+    }
+    AppendInternalItemToTop(aLists, newItem, zIndex);
+  } else {
+    aLists.BorderBackground()->AppendToTop(newItem);
+  }
+}
+
+struct HoveredStateComparator {
+  static bool Hovered(const nsIFrame* aFrame) {
+    return aFrame->GetContent()->IsElement() &&
+           aFrame->GetContent()->AsElement()->HasAttr(nsGkAtoms::hover);
+  }
+
+  bool Equals(nsIFrame* A, nsIFrame* B) const {
+    return Hovered(A) == Hovered(B);
+  }
+
+  bool LessThan(nsIFrame* A, nsIFrame* B) const {
+    return !Hovered(A) && Hovered(B);
+  }
+};
+
+void nsHTMLScrollFrame::AppendScrollPartsTo(nsDisplayListBuilder* aBuilder,
+                                            const nsDisplayListSet& aLists,
+                                            bool aCreateLayer,
+                                            bool aPositioned) {
+  const bool overlayScrollbars = UsesOverlayScrollbars();
+
+  AutoTArray<nsIFrame*, 3> scrollParts;
+  for (nsIFrame* kid : PrincipalChildList()) {
+    if (kid == mScrolledFrame ||
+        (kid->IsAbsPosContainingBlock() || overlayScrollbars) != aPositioned) {
+      continue;
+    }
+
+    scrollParts.AppendElement(kid);
+  }
+  if (scrollParts.IsEmpty()) {
+    return;
+  }
+
+  // We can't check will-change budget during display list building phase.
+  // This means that we will build scroll bar layers for out of budget
+  // will-change: scroll position.
+  const mozilla::layers::ScrollableLayerGuid::ViewID scrollTargetId =
+      IsScrollingActive()
+          ? nsLayoutUtils::FindOrCreateIDFor(mScrolledFrame->GetContent())
+          : mozilla::layers::ScrollableLayerGuid::NULL_SCROLL_ID;
+
+  scrollParts.Sort(HoveredStateComparator());
+
+  DisplayListClipState::AutoSaveRestore clipState(aBuilder);
+  // Don't let scrollparts extent outside our frame's border-box, if these are
+  // viewport scrollbars. They would create layerization problems. This wouldn't
+  // normally be an issue but themes can add overflow areas to scrollbar parts.
+  if (mIsRoot) {
+    nsRect scrollPartsClip(aBuilder->ToReferenceFrame(this),
+                           TrueOuterSize(aBuilder));
+    clipState.ClipContentDescendants(scrollPartsClip);
+  }
+
+  for (uint32_t i = 0; i < scrollParts.Length(); ++i) {
+    MOZ_ASSERT(scrollParts[i]);
+    Maybe<ScrollDirection> scrollDirection;
+    uint32_t appendToTopFlags = 0;
+    if (scrollParts[i] == mVScrollbarBox) {
+      scrollDirection.emplace(ScrollDirection::eVertical);
+      appendToTopFlags |= APPEND_SCROLLBAR_CONTAINER;
+    }
+    if (scrollParts[i] == mHScrollbarBox) {
+      MOZ_ASSERT(!scrollDirection.isSome());
+      scrollDirection.emplace(ScrollDirection::eHorizontal);
+      appendToTopFlags |= APPEND_SCROLLBAR_CONTAINER;
+    }
+
+    // The display port doesn't necessarily include the scrollbars, so just
+    // include all of the scrollbars if we are in a RCD-RSF. We only do
+    // this for the root scrollframe of the root content document, which is
+    // zoomable, and where the scrollbar sizes are bounded by the widget.
+    const nsRect visible =
+        mIsRoot && PresContext()->IsRootContentDocumentCrossProcess()
+            ? scrollParts[i]->InkOverflowRectRelativeToParent()
+            : aBuilder->GetVisibleRect();
+    if (visible.IsEmpty()) {
+      continue;
+    }
+    const nsRect dirty =
+        mIsRoot && PresContext()->IsRootContentDocumentCrossProcess()
+            ? scrollParts[i]->InkOverflowRectRelativeToParent()
+            : aBuilder->GetDirtyRect();
+
+    // Always create layers for overlay scrollbars so that we don't create a
+    // giant layer covering the whole scrollport if both scrollbars are visible.
+    const bool isOverlayScrollbar =
+        scrollDirection.isSome() && overlayScrollbars;
+    const bool createLayer =
+        aCreateLayer || isOverlayScrollbar ||
+        StaticPrefs::layout_scrollbars_always_layerize_track();
+
+    nsDisplayListCollection partList(aBuilder);
+    {
+      nsDisplayListBuilder::AutoBuildingDisplayList buildingForChild(
+          aBuilder, this, visible, dirty);
+
+      nsDisplayListBuilder::AutoCurrentScrollbarInfoSetter infoSetter(
+          aBuilder, scrollTargetId, scrollDirection, createLayer);
+      BuildDisplayListForChild(
+          aBuilder, scrollParts[i], partList,
+          nsIFrame::DisplayChildFlag::ForceStackingContext);
+    }
+
+    // DisplayChildFlag::ForceStackingContext put everything into
+    // partList.PositionedDescendants().
+    if (partList.PositionedDescendants()->IsEmpty()) {
+      continue;
+    }
+
+    if (createLayer) {
+      appendToTopFlags |= APPEND_OWN_LAYER;
+    }
+    if (aPositioned) {
+      appendToTopFlags |= APPEND_POSITIONED;
+    }
+
+    if (isOverlayScrollbar || scrollParts[i] == mResizerBox) {
+      if (isOverlayScrollbar && mIsRoot) {
+        appendToTopFlags |= APPEND_TOP;
+      } else {
+        appendToTopFlags |= APPEND_OVERLAY;
+        aBuilder->SetDisablePartialUpdates(true);
+      }
+    }
+
+    {
+      nsDisplayListBuilder::AutoBuildingDisplayList buildingForChild(
+          aBuilder, scrollParts[i], visible + GetOffsetTo(scrollParts[i]),
+          dirty + GetOffsetTo(scrollParts[i]));
+      if (scrollParts[i]->IsTransformed()) {
+        nsPoint toOuterReferenceFrame;
+        const nsIFrame* outerReferenceFrame = aBuilder->FindReferenceFrameFor(
+            scrollParts[i]->GetParent(), &toOuterReferenceFrame);
+        toOuterReferenceFrame += scrollParts[i]->GetPosition();
+
+        buildingForChild.SetReferenceFrameAndCurrentOffset(
+            outerReferenceFrame, toOuterReferenceFrame);
+      }
+      nsDisplayListBuilder::AutoCurrentScrollbarInfoSetter infoSetter(
+          aBuilder, scrollTargetId, scrollDirection, createLayer);
+
+      ::AppendToTop(aBuilder, aLists, partList.PositionedDescendants(),
+                    scrollParts[i], this, appendToTopFlags);
+    }
+  }
+}
+
+nsRect nsHTMLScrollFrame::ExpandRectToNearlyVisible(const nsRect& aRect) const {
+  // We don't want to expand a rect in a direction that we can't scroll, so we
+  // check the scroll range.
+  nsRect scrollRange = GetLayoutScrollRange();
+  nsPoint scrollPos = GetScrollPosition();
+  nsMargin expand(0, 0, 0, 0);
+
+  nscoord vertShift =
+      StaticPrefs::layout_framevisibility_numscrollportheights() * aRect.height;
+  if (scrollRange.y < scrollPos.y) {
+    expand.top = vertShift;
+  }
+  if (scrollPos.y < scrollRange.YMost()) {
+    expand.bottom = vertShift;
+  }
+
+  nscoord horzShift =
+      StaticPrefs::layout_framevisibility_numscrollportwidths() * aRect.width;
+  if (scrollRange.x < scrollPos.x) {
+    expand.left = horzShift;
+  }
+  if (scrollPos.x < scrollRange.XMost()) {
+    expand.right = horzShift;
+  }
+
+  nsRect rect = aRect;
+  rect.Inflate(expand);
+  return rect;
+}
+
+static bool ShouldBeClippedByFrame(nsIFrame* aClipFrame,
+                                   nsIFrame* aClippedFrame) {
+  return nsLayoutUtils::IsProperAncestorFrame(aClipFrame, aClippedFrame);
+}
+
+static void ClipItemsExceptCaret(
+    nsDisplayList* aList, nsDisplayListBuilder* aBuilder, nsIFrame* aClipFrame,
+    const DisplayItemClipChain* aExtraClip,
+    nsTHashMap<nsPtrHashKey<const DisplayItemClipChain>,
+               const DisplayItemClipChain*>& aCache) {
+  for (nsDisplayItem* i : *aList) {
+    if (!ShouldBeClippedByFrame(aClipFrame, i->Frame())) {
+      continue;
+    }
+
+    const DisplayItemType type = i->GetType();
+    if (type != DisplayItemType::TYPE_CARET &&
+        type != DisplayItemType::TYPE_CONTAINER) {
+      const DisplayItemClipChain* clip = i->GetClipChain();
+      const DisplayItemClipChain* intersection = nullptr;
+      if (aCache.Get(clip, &intersection)) {
+        i->SetClipChain(intersection, true);
+      } else {
+        i->IntersectClip(aBuilder, aExtraClip, true);
+        aCache.InsertOrUpdate(clip, i->GetClipChain());
+      }
+    }
+    nsDisplayList* children = i->GetSameCoordinateSystemChildren();
+    if (children) {
+      ClipItemsExceptCaret(children, aBuilder, aClipFrame, aExtraClip, aCache);
+    }
+  }
+}
+
+static void ClipListsExceptCaret(nsDisplayListCollection* aLists,
+                                 nsDisplayListBuilder* aBuilder,
+                                 nsIFrame* aClipFrame,
+                                 const DisplayItemClipChain* aExtraClip) {
+  nsTHashMap<nsPtrHashKey<const DisplayItemClipChain>,
+             const DisplayItemClipChain*>
+      cache;
+  ClipItemsExceptCaret(aLists->BorderBackground(), aBuilder, aClipFrame,
+                       aExtraClip, cache);
+  ClipItemsExceptCaret(aLists->BlockBorderBackgrounds(), aBuilder, aClipFrame,
+                       aExtraClip, cache);
+  ClipItemsExceptCaret(aLists->Floats(), aBuilder, aClipFrame, aExtraClip,
+                       cache);
+  ClipItemsExceptCaret(aLists->PositionedDescendants(), aBuilder, aClipFrame,
+                       aExtraClip, cache);
+  ClipItemsExceptCaret(aLists->Outlines(), aBuilder, aClipFrame, aExtraClip,
+                       cache);
+  ClipItemsExceptCaret(aLists->Content(), aBuilder, aClipFrame, aExtraClip,
+                       cache);
+}
+
+// This is similar to a "save-restore" RAII class for
+// DisplayListBuilder::ContainsBlendMode(), with a slight enhancement.
+// If this class is put on the stack and then unwound, the DL builder's
+// ContainsBlendMode flag will be effectively the same as if this class wasn't
+// put on the stack. However, if the CaptureContainsBlendMode method is called,
+// there will be a difference - the blend mode in the descendant display lists
+// will be "captured" and extracted.
+// The main goal here is to allow conditionally capturing the flag that
+// indicates whether or not a blend mode was encountered in the descendant part
+// of the display list.
+class MOZ_RAII AutoContainsBlendModeCapturer {
+  nsDisplayListBuilder& mBuilder;
+  bool mSavedContainsBlendMode;
+
+ public:
+  explicit AutoContainsBlendModeCapturer(nsDisplayListBuilder& aBuilder)
+      : mBuilder(aBuilder),
+        mSavedContainsBlendMode(aBuilder.ContainsBlendMode()) {
+    mBuilder.SetContainsBlendMode(false);
+  }
+
+  bool CaptureContainsBlendMode() {
+    // "Capture" the flag by extracting and clearing the ContainsBlendMode flag
+    // on the builder.
+    bool capturedBlendMode = mBuilder.ContainsBlendMode();
+    mBuilder.SetContainsBlendMode(false);
+    return capturedBlendMode;
+  }
+
+  ~AutoContainsBlendModeCapturer() {
+    // If CaptureContainsBlendMode() was called, the descendant blend mode was
+    // "captured" and so uncapturedContainsBlendMode will be false. If
+    // CaptureContainsBlendMode() wasn't called, then no capture occurred, and
+    // uncapturedContainsBlendMode may be true if there was a descendant blend
+    // mode. In that case, we set the flag on the DL builder so that we restore
+    // state to what it would have been without this RAII class on the stack.
+    bool uncapturedContainsBlendMode = mBuilder.ContainsBlendMode();
+    mBuilder.SetContainsBlendMode(mSavedContainsBlendMode ||
+                                  uncapturedContainsBlendMode);
+  }
+};
+
+void nsHTMLScrollFrame::MaybeCreateTopLayerAndWrapRootItems(
+    nsDisplayListBuilder* aBuilder, nsDisplayListCollection& aSet,
+    bool aCreateAsyncZoom,
+    AutoContainsBlendModeCapturer* aAsyncZoomBlendCapture,
+    const nsRect& aAsyncZoomClipRect, nscoord* aRadii) {
+  if (!mIsRoot) {
+    return;
+  }
+
+  // Create any required items for the 'top layer' and check if they'll be
+  // opaque over the entire area of the viewport. If they are, then we can
+  // skip building display items for the rest of the page.
+  if (ViewportFrame* viewport = do_QueryFrame(GetParent())) {
+    bool topLayerIsOpaque = false;
+    if (nsDisplayWrapList* topLayerWrapList =
+            viewport->BuildDisplayListForTopLayer(aBuilder,
+                                                  &topLayerIsOpaque)) {
+      // If the top layer content is opaque, and we're the root content document
+      // in the process, we can drop the display items behind it. We only
+      // support doing this for the root content document in the process, since
+      // the top layer content might have fixed position items that have a
+      // scrolltarget referencing the APZ data for the document. APZ builds this
+      // data implicitly for the root content document in the process, but
+      // subdocuments etc need their display items to generate it, so we can't
+      // cull those.
+      if (topLayerIsOpaque && PresContext()->IsRootContentDocumentInProcess()) {
+        aSet.DeleteAll(aBuilder);
+      }
+      aSet.PositionedDescendants()->AppendToTop(topLayerWrapList);
+    }
+  }
+
+  nsDisplayList rootResultList(aBuilder);
+
+  bool serializedList = false;
+  auto SerializeList = [&] {
+    if (!serializedList) {
+      serializedList = true;
+      aSet.SerializeWithCorrectZOrder(&rootResultList, GetContent());
+    }
+  };
+
+  if (nsIFrame* rootStyleFrame = GetFrameForStyle()) {
+    bool usingBackdropFilter =
+        rootStyleFrame->StyleEffects()->HasBackdropFilters() &&
+        rootStyleFrame->IsVisibleForPainting();
+
+    if (rootStyleFrame->StyleEffects()->HasFilters() &&
+        !aBuilder->IsForGenerateGlyphMask()) {
+      SerializeList();
+      rootResultList.AppendNewToTop<nsDisplayFilters>(
+          aBuilder, this, &rootResultList, rootStyleFrame, usingBackdropFilter);
+    }
+
+    if (usingBackdropFilter) {
+      SerializeList();
+      DisplayListClipState::AutoSaveRestore clipState(aBuilder);
+      nsRect backdropRect =
+          GetRectRelativeToSelf() + aBuilder->ToReferenceFrame(this);
+      rootResultList.AppendNewToTop<nsDisplayBackdropFilters>(
+          aBuilder, this, &rootResultList, backdropRect, rootStyleFrame);
+    }
+  }
+
+  if (aCreateAsyncZoom) {
+    MOZ_ASSERT(mIsRoot);
+
+    // Wrap all our scrolled contents in an nsDisplayAsyncZoom. This will be
+    // the layer that gets scaled for APZ zooming. It does not have the
+    // scrolled ASR, but it does have the composition bounds clip applied to
+    // it. The children have the layout viewport clip applied to them (above).
+    // Effectively we are double clipping to the viewport, at potentially
+    // different async scales.
+    SerializeList();
+
+    if (aAsyncZoomBlendCapture->CaptureContainsBlendMode()) {
+      // The async zoom contents contain a mix-blend mode, so let's wrap all
+      // those contents into a blend container, and then wrap the blend
+      // container in the async zoom container. Otherwise the blend container
+      // ends up outside the zoom container which results in blend failure for
+      // WebRender.
+      nsDisplayItem* blendContainer =
+          nsDisplayBlendContainer::CreateForMixBlendMode(
+              aBuilder, this, &rootResultList,
+              aBuilder->CurrentActiveScrolledRoot());
+      rootResultList.AppendToTop(blendContainer);
+
+      // Blend containers can be created or omitted during partial updates
+      // depending on the dirty rect. So we basically can't do partial updates
+      // if there's a blend container involved. There is equivalent code to this
+      // in the BuildDisplayListForStackingContext function as well, with a more
+      // detailed comment explaining things better.
+      if (aBuilder->IsRetainingDisplayList()) {
+        if (aBuilder->IsPartialUpdate()) {
+          aBuilder->SetPartialBuildFailed(true);
+        } else {
+          aBuilder->SetDisablePartialUpdates(true);
+        }
+      }
+    }
+
+    mozilla::layers::FrameMetrics::ViewID viewID =
+        nsLayoutUtils::FindOrCreateIDFor(mScrolledFrame->GetContent());
+
+    DisplayListClipState::AutoSaveRestore clipState(aBuilder);
+    clipState.ClipContentDescendants(aAsyncZoomClipRect, aRadii);
+
+    rootResultList.AppendNewToTop<nsDisplayAsyncZoom>(
+        aBuilder, this, &rootResultList, aBuilder->CurrentActiveScrolledRoot(),
+        viewID);
+  }
+
+  if (serializedList) {
+    aSet.Content()->AppendToTop(&rootResultList);
+  }
+}
+
+void nsHTMLScrollFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
+                                         const nsDisplayListSet& aLists) {
+  SetAndNullOnExit<const nsIFrame> tmpBuilder(
+      mReferenceFrameDuringPainting, aBuilder->GetCurrentReferenceFrame());
+  if (aBuilder->IsForFrameVisibility()) {
+    NotifyApproximateFrameVisibilityUpdate(false);
+  }
+
+  DisplayBorderBackgroundOutline(aBuilder, aLists);
+
+  const bool isRootContent =
+      mIsRoot && PresContext()->IsRootContentDocumentCrossProcess();
+
+  nsRect effectiveScrollPort = mScrollPort;
+  if (isRootContent && PresContext()->HasDynamicToolbar()) {
+    // Expand the scroll port to the size including the area covered by dynamic
+    // toolbar in the case where the dynamic toolbar is being used since
+    // position:fixed elements attached to this root scroller might be taller
+    // than its scroll port (e.g 100vh). Even if the dynamic toolbar covers the
+    // taller area, it doesn't mean the area is clipped by the toolbar because
+    // the dynamic toolbar is laid out outside of our topmost window and it
+    // transitions without changing our topmost window size.
+    effectiveScrollPort.SizeTo(nsLayoutUtils::ExpandHeightForDynamicToolbar(
+        PresContext(), effectiveScrollPort.Size()));
+  }
+
+  // It's safe to get this value before the DecideScrollableLayer call below
+  // because that call cannot create a displayport for root scroll frames,
+  // and hence it cannot create an ignore scroll frame.
+  const bool ignoringThisScrollFrame = aBuilder->GetIgnoreScrollFrame() == this;
+
+  // Overflow clipping can never clip frames outside our subtree, so there
+  // is no need to worry about whether we are a moving frame that might clip
+  // non-moving frames.
+  // Not all our descendants will be clipped by overflow clipping, but all
+  // the ones that aren't clipped will be out of flow frames that have already
+  // had dirty rects saved for them by their parent frames calling
+  // MarkOutOfFlowChildrenForDisplayList, so it's safe to restrict our
+  // dirty rect here.
+  nsRect visibleRect = aBuilder->GetVisibleRect();
+  nsRect dirtyRect = aBuilder->GetDirtyRect();
+  if (!ignoringThisScrollFrame) {
+    visibleRect = visibleRect.Intersect(effectiveScrollPort);
+    dirtyRect = dirtyRect.Intersect(effectiveScrollPort);
+  }
+
+  bool dirtyRectHasBeenOverriden = false;
+  Unused << DecideScrollableLayer(aBuilder, &visibleRect, &dirtyRect,
+                                  /* aSetBase = */ !mIsRoot,
+                                  &dirtyRectHasBeenOverriden);
+
+  if (aBuilder->IsForFrameVisibility()) {
+    // We expand the dirty rect to catch frames just outside of the scroll port.
+    // We use the dirty rect instead of the whole scroll port to prevent
+    // too much expansion in the presence of very large (bigger than the
+    // viewport) scroll ports.
+    dirtyRect = ExpandRectToNearlyVisible(dirtyRect);
+    visibleRect = dirtyRect;
+  }
+
+  // We put non-overlay scrollbars in their own layers when this is the root
+  // scroll frame and we are a toplevel content document. In this situation,
+  // the scrollbar(s) would normally be assigned their own layer anyway, since
+  // they're not scrolled with the rest of the document. But when both
+  // scrollbars are visible, the layer's visible rectangle would be the size
+  // of the viewport, so most layer implementations would create a layer buffer
+  // that's much larger than necessary. Creating independent layers for each
+  // scrollbar works around the problem.
+  const bool createLayersForScrollbars = isRootContent;
+
+  nsDisplayListCollection set(aBuilder);
+
+  if (ignoringThisScrollFrame) {
+    // If we are a root scroll frame that has a display port we want to add
+    // scrollbars, they will be children of the scrollable layer, but they get
+    // adjusted by the APZC automatically.
+    bool addScrollBars =
+        mIsRoot && mWillBuildScrollableLayer && aBuilder->IsPaintingToWindow();
+
+    if (addScrollBars) {
+      // Add classic scrollbars.
+      AppendScrollPartsTo(aBuilder, set, createLayersForScrollbars, false);
+    }
+
+    {
+      nsDisplayListBuilder::AutoBuildingDisplayList building(
+          aBuilder, this, visibleRect, dirtyRect);
+
+      // Don't clip the scrolled child, and don't paint scrollbars/scrollcorner.
+      // The scrolled frame shouldn't have its own background/border, so we
+      // can just pass aLists directly.
+      BuildDisplayListForChild(aBuilder, mScrolledFrame, set);
+    }
+
+    MaybeCreateTopLayerAndWrapRootItems(aBuilder, set,
+                                        /* aCreateAsyncZoom = */ false, nullptr,
+                                        nsRect(), nullptr);
+
+    if (addScrollBars) {
+      // Add overlay scrollbars.
+      AppendScrollPartsTo(aBuilder, set, createLayersForScrollbars, true);
+    }
+
+    set.MoveTo(aLists);
+    return;
+  }
+
+  // Whether we might want to build a scrollable layer for this scroll frame
+  // at some point in the future. This controls whether we add the information
+  // to the layer tree (a scroll info layer if necessary, and add the right
+  // area to the dispatch to content layer event regions) necessary to activate
+  // a scroll frame so it creates a scrollable layer.
+  const bool couldBuildLayer = [&] {
+    if (!aBuilder->IsPaintingToWindow()) {
+      return false;
+    }
+    if (mWillBuildScrollableLayer) {
+      return true;
+    }
+    return StyleVisibility()->IsVisible() &&
+           nsLayoutUtils::AsyncPanZoomEnabled(this) && WantAsyncScroll();
+  }();
+
+  // Now display the scrollbars and scrollcorner. These parts are drawn
+  // in the border-background layer, on top of our own background and
+  // borders and underneath borders and backgrounds of later elements
+  // in the tree.
+  // Note that this does not apply for overlay scrollbars; those are drawn
+  // in the positioned-elements layer on top of everything else by the call
+  // to AppendScrollPartsTo(..., true) further down.
+  AppendScrollPartsTo(aBuilder, aLists, createLayersForScrollbars, false);
+
+  const nsStyleDisplay* disp = StyleDisplay();
+  if (aBuilder->IsForPainting() &&
+      disp->mWillChange.bits & StyleWillChangeBits::SCROLL) {
+    aBuilder->AddToWillChangeBudget(this, GetVisualViewportSize());
+  }
+
+  mScrollParentID = aBuilder->GetCurrentScrollParentId();
+
+  Maybe<nsRect> contentBoxClip;
+  Maybe<const DisplayItemClipChain*> extraContentBoxClipForNonCaretContent;
+  if (MOZ_UNLIKELY(
+          disp->mOverflowClipBoxBlock == StyleOverflowClipBox::ContentBox ||
+          disp->mOverflowClipBoxInline == StyleOverflowClipBox::ContentBox)) {
+    WritingMode wm = mScrolledFrame->GetWritingMode();
+    bool cbH = (wm.IsVertical() ? disp->mOverflowClipBoxBlock
+                                : disp->mOverflowClipBoxInline) ==
+               StyleOverflowClipBox::ContentBox;
+    bool cbV = (wm.IsVertical() ? disp->mOverflowClipBoxInline
+                                : disp->mOverflowClipBoxBlock) ==
+               StyleOverflowClipBox::ContentBox;
+    // We only clip if there is *scrollable* overflow, to avoid clipping
+    // *ink* overflow unnecessarily.
+    nsRect clipRect = effectiveScrollPort + aBuilder->ToReferenceFrame(this);
+    nsMargin padding = GetUsedPadding();
+    if (!cbH) {
+      padding.left = padding.right = nscoord(0);
+    }
+    if (!cbV) {
+      padding.top = padding.bottom = nscoord(0);
+    }
+    clipRect.Deflate(padding);
+
+    nsRect so = mScrolledFrame->ScrollableOverflowRect();
+    if ((cbH && (clipRect.width != so.width || so.x < 0)) ||
+        (cbV && (clipRect.height != so.height || so.y < 0))) {
+      // The non-inflated clip needs to be set on all non-caret items.
+      // We prepare an extra DisplayItemClipChain here that will be intersected
+      // with those items after they've been created.
+      const ActiveScrolledRoot* asr = aBuilder->CurrentActiveScrolledRoot();
+
+      DisplayItemClip newClip;
+      newClip.SetTo(clipRect);
+
+      const DisplayItemClipChain* extraClip =
+          aBuilder->AllocateDisplayItemClipChain(newClip, asr, nullptr);
+
+      extraContentBoxClipForNonCaretContent = Some(extraClip);
+
+      nsIFrame* caretFrame = aBuilder->GetCaretFrame();
+      // Avoid clipping it in a zero-height line box (heuristic only).
+      if (caretFrame && caretFrame->GetRect().height != 0) {
+        nsRect caretRect = aBuilder->GetCaretRect();
+        // Allow the caret to stick out of the content box clip by half the
+        // caret height on the top, and its full width on the right.
+        nsRect inflatedClip = clipRect;
+        inflatedClip.Inflate(
+            nsMargin(caretRect.height / 2, caretRect.width, 0, 0));
+        contentBoxClip = Some(inflatedClip);
+      }
+    }
+  }
+
+  AutoContainsBlendModeCapturer blendCapture(*aBuilder);
+
+  const bool willBuildAsyncZoomContainer =
+      mWillBuildScrollableLayer && aBuilder->ShouldBuildAsyncZoomContainer() &&
+      isRootContent;
+
+  nsRect scrollPortClip =
+      effectiveScrollPort + aBuilder->ToReferenceFrame(this);
+  nsRect clipRect = scrollPortClip;
+  // Our override of GetBorderRadii ensures we never have a radius at
+  // the corners where we have a scrollbar.
+  nscoord radii[8];
+  const bool haveRadii = GetPaddingBoxBorderRadii(radii);
+  if (mIsRoot) {
+    clipRect.SizeTo(nsLayoutUtils::CalculateCompositionSizeForFrame(
+        this, true /* aSubtractScrollbars */,
+        nullptr /* aOverrideScrollPortSize */,
+        // With the dynamic toolbar, this CalculateCompositionSizeForFrame call
+        // basically expands the region being covered up by the dynamic toolbar,
+        // but if the root scroll container is not scrollable, e.g. the root
+        // element has `overflow: hidden` or `position: fixed`, the function
+        // doesn't expand the region since expanding the region in such cases
+        // will prevent the content from restoring zooming to 1.0 zoom level
+        // such as bug 1652190. That said, this `clipRect` which will be used
+        // for the async zoom container needs to be expanded because zoomed-in
+        // contents can be scrollable __visually__ so that the region under the
+        // dynamic toolbar area will be revealed.
+        nsLayoutUtils::IncludeDynamicToolbar::Force));
+
+    // The composition size is essentially in visual coordinates.
+    // If we are hit-testing in layout coordinates, transform the clip rect
+    // to layout coordinates to match.
+    if (aBuilder->IsRelativeToLayoutViewport() && isRootContent) {
+      clipRect = ViewportUtils::VisualToLayout(clipRect, PresShell());
+    }
+  }
+
+  {
+    DisplayListClipState::AutoSaveRestore clipState(aBuilder);
+
+    // If we're building an async zoom container, clip the contents inside
+    // to the layout viewport (scrollPortClip). The composition bounds clip
+    // (clipRect) will be applied to the zoom container itself in
+    // MaybeCreateTopLayerAndWrapRootItems.
+    nsRect clipRectForContents =
+        willBuildAsyncZoomContainer ? scrollPortClip : clipRect;
+    if (mIsRoot) {
+      clipState.ClipContentDescendants(clipRectForContents,
+                                       haveRadii ? radii : nullptr);
+    } else {
+      clipState.ClipContainingBlockDescendants(clipRectForContents,
+                                               haveRadii ? radii : nullptr);
+    }
+
+    Maybe<DisplayListClipState::AutoSaveRestore> contentBoxClipState;
+    if (contentBoxClip) {
+      contentBoxClipState.emplace(aBuilder);
+      if (mIsRoot) {
+        contentBoxClipState->ClipContentDescendants(*contentBoxClip);
+      } else {
+        contentBoxClipState->ClipContainingBlockDescendants(*contentBoxClip);
+      }
+    }
+
+    nsDisplayListBuilder::AutoCurrentActiveScrolledRootSetter asrSetter(
+        aBuilder);
+
+    if (mWillBuildScrollableLayer && aBuilder->IsPaintingToWindow()) {
+      // If this scroll frame has a first scrollable frame sequence number,
+      // ensure that it matches the current paint sequence number. If it does
+      // not, reset it so that we can expire the displayport. The stored
+      // sequence number will not match that of the current paint if the dom
+      // was mutated in some way that alters the order of scroll frames.
+      if (IsFirstScrollableFrameSequenceNumber().isSome() &&
+          *IsFirstScrollableFrameSequenceNumber() !=
+              nsDisplayListBuilder::GetPaintSequenceNumber()) {
+        SetIsFirstScrollableFrameSequenceNumber(Nothing());
+      }
+      asrSetter.EnterScrollFrame(this);
+    }
+
+    if (couldBuildLayer && mScrolledFrame->GetContent()) {
+      asrSetter.SetCurrentScrollParentId(
+          nsLayoutUtils::FindOrCreateIDFor(mScrolledFrame->GetContent()));
+    }
+
+    if (mWillBuildScrollableLayer && aBuilder->BuildCompositorHitTestInfo()) {
+      // Create a hit test info item for the scrolled content that's not
+      // clipped to the displayport. This ensures that within the bounds
+      // of the scroll frame, the scrolled content is always hit, even
+      // if we are checkerboarding.
+      CompositorHitTestInfo info =
+          mScrolledFrame->GetCompositorHitTestInfo(aBuilder);
+
+      if (info != CompositorHitTestInvisibleToHit) {
+        auto* hitInfo =
+            MakeDisplayItemWithIndex<nsDisplayCompositorHitTestInfo>(
+                aBuilder, mScrolledFrame, 1);
+        if (hitInfo) {
+          aBuilder->SetCompositorHitTestInfo(info);
+          set.BorderBackground()->AppendToTop(hitInfo);
+        }
+      }
+    }
+
+    {
+      // Clip our contents to the unsnapped scrolled rect. This makes sure
+      // that we don't have display items over the subpixel seam at the edge
+      // of the scrolled area.
+      DisplayListClipState::AutoSaveRestore scrolledRectClipState(aBuilder);
+      nsRect scrolledRectClip =
+          GetUnsnappedScrolledRectInternal(
+              mScrolledFrame->ScrollableOverflowRect(), mScrollPort.Size()) +
+          mScrolledFrame->GetPosition();
+      bool clippedToDisplayPort = false;
+      if (mWillBuildScrollableLayer && aBuilder->IsPaintingToWindow()) {
+        // Clip the contents to the display port.
+        // The dirty rect already acts kind of like a clip, in that
+        // FrameLayerBuilder intersects item bounds and opaque regions with
+        // it, but it doesn't have the consistent snapping behavior of a
+        // true clip.
+        // For a case where this makes a difference, imagine the following
+        // scenario: The display port has an edge that falls on a fractional
+        // layer pixel, and there's an opaque display item that covers the
+        // whole display port up until that fractional edge, and there is a
+        // transparent display item that overlaps the edge. We want to prevent
+        // this transparent item from enlarging the scrolled layer's visible
+        // region beyond its opaque region. The dirty rect doesn't do that -
+        // it gets rounded out, whereas a true clip gets rounded to nearest
+        // pixels.
+        // If there is no display port, we don't need this because the clip
+        // from the scroll port is still applied.
+        scrolledRectClip = scrolledRectClip.Intersect(visibleRect);
+        clippedToDisplayPort = scrolledRectClip.IsEqualEdges(visibleRect);
+      }
+      scrolledRectClipState.ClipContainingBlockDescendants(
+          scrolledRectClip + aBuilder->ToReferenceFrame(this));
+      if (clippedToDisplayPort) {
+        // We have to do this after the ClipContainingBlockDescendants call
+        // above, otherwise that call will clobber the flag set by this call
+        // to SetClippedToDisplayPort.
+        scrolledRectClipState.SetClippedToDisplayPort();
+      }
+
+      nsRect visibleRectForChildren = visibleRect;
+      nsRect dirtyRectForChildren = dirtyRect;
+
+      // If we are entering the RCD-RSF, we are crossing the async zoom
+      // container boundary, and need to convert from visual to layout
+      // coordinates.
+      if (willBuildAsyncZoomContainer && aBuilder->IsForEventDelivery()) {
+        MOZ_ASSERT(ViewportUtils::IsZoomedContentRoot(mScrolledFrame));
+        visibleRectForChildren =
+            ViewportUtils::VisualToLayout(visibleRectForChildren, PresShell());
+        dirtyRectForChildren =
+            ViewportUtils::VisualToLayout(dirtyRectForChildren, PresShell());
+      }
+
+      nsDisplayListBuilder::AutoBuildingDisplayList building(
+          aBuilder, this, visibleRectForChildren, dirtyRectForChildren);
+
+      BuildDisplayListForChild(aBuilder, mScrolledFrame, set);
+
+      if (dirtyRectHasBeenOverriden &&
+          StaticPrefs::layout_display_list_show_rebuild_area()) {
+        nsDisplaySolidColor* color = MakeDisplayItem<nsDisplaySolidColor>(
+            aBuilder, this,
+            dirtyRect + aBuilder->GetCurrentFrameOffsetToReferenceFrame(),
+            NS_RGBA(0, 0, 255, 64), false);
+        if (color) {
+          color->SetOverrideZIndex(INT32_MAX);
+          set.PositionedDescendants()->AppendToTop(color);
+        }
+      }
+    }
+
+    if (extraContentBoxClipForNonCaretContent) {
+      // The items were built while the inflated content box clip was in
+      // effect, so that the caret wasn't clipped unnecessarily. We apply
+      // the non-inflated clip to the non-caret items now, by intersecting
+      // it with their existing clip.
+      ClipListsExceptCaret(&set, aBuilder, mScrolledFrame,
+                           *extraContentBoxClipForNonCaretContent);
+    }
+
+    if (aBuilder->IsPaintingToWindow()) {
+      mIsParentToActiveScrollFrames =
+          ShouldActivateAllScrollFrames()
+              ? asrSetter.GetContainsNonMinimalDisplayPort()
+              : asrSetter.ShouldForceLayerForScrollParent();
+    }
+
+    if (asrSetter.ShouldForceLayerForScrollParent()) {
+      // Note that forcing layerization of scroll parents follows the scroll
+      // handoff chain which is subject to the out-of-flow-frames caveat noted
+      // above (where the asrSetter variable is created).
+      MOZ_ASSERT(couldBuildLayer && mScrolledFrame->GetContent() &&
+                 aBuilder->IsPaintingToWindow());
+      if (!mWillBuildScrollableLayer) {
+        // Set a displayport so next paint we don't have to force layerization
+        // after the fact. It's ok to pass DoNotRepaint here, since we've
+        // already painted the change and we're just optimizing it to be
+        // detected earlier. We also won't confuse RetainedDisplayLists
+        // with the silent change, since we explicitly request partial updates
+        // to be disabled on the next paint.
+        DisplayPortUtils::SetDisplayPortMargins(
+            GetContent(), PresShell(), DisplayPortMargins::Empty(GetContent()),
+            DisplayPortUtils::ClearMinimalDisplayPortProperty::Yes, 0,
+            DisplayPortUtils::RepaintMode::DoNotRepaint);
+        // Call DecideScrollableLayer to recompute mWillBuildScrollableLayer
+        // and recompute the current animated geometry root if needed. It's
+        // too late to change the dirty rect so pass a copy.
+        nsRect copyOfDirtyRect = dirtyRect;
+        nsRect copyOfVisibleRect = visibleRect;
+        Unused << DecideScrollableLayer(aBuilder, &copyOfVisibleRect,
+                                        &copyOfDirtyRect,
+                                        /* aSetBase = */ false, nullptr);
+        if (mWillBuildScrollableLayer) {
+#ifndef MOZ_WIDGET_ANDROID
+          gfxCriticalNoteOnce << "inserted scroll frame";
+#endif
+          asrSetter.InsertScrollFrame(this);
+          aBuilder->SetDisablePartialUpdates(true);
+        }
+      }
+    }
+  }
+
+  if (mWillBuildScrollableLayer && aBuilder->IsPaintingToWindow()) {
+    aBuilder->ForceLayerForScrollParent();
+  }
+
+  MaybeCreateTopLayerAndWrapRootItems(
+      aBuilder, set, willBuildAsyncZoomContainer, &blendCapture, clipRect,
+      haveRadii ? radii : nullptr);
+
+  // We want to call SetContainsNonMinimalDisplayPort if
+  // mWillBuildScrollableLayer is true for any reason other than having a
+  // minimal display port.
+  if (aBuilder->IsPaintingToWindow()) {
+    if (DisplayPortUtils::HasNonMinimalDisplayPort(GetContent()) ||
+        mZoomableByAPZ || nsContentUtils::HasScrollgrab(GetContent())) {
+      aBuilder->SetContainsNonMinimalDisplayPort();
+    }
+  }
+
+  if (couldBuildLayer) {
+    CompositorHitTestInfo info(CompositorHitTestFlags::eVisibleToHitTest,
+                               CompositorHitTestFlags::eInactiveScrollframe);
+    // If the scroll frame has non-default overscroll-behavior, instruct
+    // APZ to require a target confirmation before processing events that
+    // hit this scroll frame (that is, to drop the events if a
+    // confirmation does not arrive within the timeout period). Otherwise,
+    // APZ's fallback behaviour of scrolling the enclosing scroll frame
+    // would violate the specified overscroll-behavior.
+    auto overscroll = GetOverscrollBehaviorInfo();
+    if (overscroll.mBehaviorX != OverscrollBehavior::Auto ||
+        overscroll.mBehaviorY != OverscrollBehavior::Auto) {
+      info += CompositorHitTestFlags::eRequiresTargetConfirmation;
+    }
+
+    nsRect area = effectiveScrollPort + aBuilder->ToReferenceFrame(this);
+
+    // Make sure that APZ will dispatch events back to content so we can
+    // create a displayport for this frame. We'll add the item later on.
+    if (!mWillBuildScrollableLayer && aBuilder->BuildCompositorHitTestInfo()) {
+      // Make sure the z-index of the inactive item is at least zero.
+      // Otherwise, it will end up behind non-positioned items in the scrolled
+      // content.
+      int32_t zIndex = MaxZIndexInListOfItemsContainedInFrame(
+                           set.PositionedDescendants(), this)
+                           .valueOr(0);
+      if (aBuilder->IsPartialUpdate()) {
+        for (nsDisplayItem* item : mScrolledFrame->DisplayItems()) {
+          if (item->GetType() ==
+              DisplayItemType::TYPE_COMPOSITOR_HITTEST_INFO) {
+            auto* hitTestItem =
+                static_cast<nsDisplayCompositorHitTestInfo*>(item);
+            if (hitTestItem->GetHitTestInfo().Info().contains(
+                    CompositorHitTestFlags::eInactiveScrollframe)) {
+              zIndex = std::max(zIndex, hitTestItem->ZIndex());
+              item->SetCantBeReused();
+            }
+          }
+        }
+      }
+      nsDisplayCompositorHitTestInfo* hitInfo =
+          MakeDisplayItemWithIndex<nsDisplayCompositorHitTestInfo>(
+              aBuilder, mScrolledFrame, 1, area, info);
+      if (hitInfo) {
+        AppendInternalItemToTop(set, hitInfo, Some(zIndex));
+        aBuilder->SetCompositorHitTestInfo(info);
+      }
+    }
+
+    if (aBuilder->ShouldBuildScrollInfoItemsForHoisting()) {
+      aBuilder->AppendNewScrollInfoItemForHoisting(
+          MakeDisplayItem<nsDisplayScrollInfoLayer>(aBuilder, mScrolledFrame,
+                                                    this, info, area));
+    }
+  }
+
+  // Now display overlay scrollbars and the resizer, if we have one.
+  AppendScrollPartsTo(aBuilder, set, createLayersForScrollbars, true);
+
+  set.MoveTo(aLists);
+}
+
+nsRect nsHTMLScrollFrame::RestrictToRootDisplayPort(
+    const nsRect& aDisplayportBase) {
+  // This function clips aDisplayportBase so that it is no larger than the
+  // root frame's displayport (or the root composition bounds, if we can't
+  // obtain the root frame's displayport). This is useful for ensuring that
+  // the displayport of a tall scrollframe doesn't gobble up all the memory.
+
+  nsPresContext* pc = PresContext();
+  const nsPresContext* rootPresContext =
+      pc->GetInProcessRootContentDocumentPresContext();
+  if (!rootPresContext) {
+    rootPresContext = pc->GetRootPresContext();
+  }
+  if (!rootPresContext) {
+    return aDisplayportBase;
+  }
+  const mozilla::PresShell* const rootPresShell = rootPresContext->PresShell();
+  nsIFrame* rootFrame = rootPresShell->GetRootScrollFrame();
+  if (!rootFrame) {
+    rootFrame = rootPresShell->GetRootFrame();
+  }
+  if (!rootFrame) {
+    return aDisplayportBase;
+  }
+
+  // Make sure we aren't trying to restrict to our own displayport, which is a
+  // circular dependency.
+  MOZ_ASSERT(!mIsRoot || rootPresContext != pc);
+
+  nsRect rootDisplayPort;
+  bool hasDisplayPort =
+      rootFrame->GetContent() && DisplayPortUtils::GetDisplayPort(
+                                     rootFrame->GetContent(), &rootDisplayPort);
+  if (hasDisplayPort) {
+    // The display port of the root frame already factors in it's callback
+    // transform, so subtract it out here, the GetCumulativeApzCallbackTransform
+    // call below will add it back.
+    MOZ_LOG(sDisplayportLog, LogLevel::Verbose,
+            ("RestrictToRootDisplayPort: Existing root displayport is %s\n",
+             ToString(rootDisplayPort).c_str()));
+    if (nsIContent* content = rootFrame->GetContent()) {
+      if (void* property =
+              content->GetProperty(nsGkAtoms::apzCallbackTransform)) {
+        rootDisplayPort -=
+            CSSPoint::ToAppUnits(*static_cast<CSSPoint*>(property));
+      }
+    }
+  } else {
+    // If we don't have a display port on the root frame let's fall back to
+    // the root composition bounds instead.
+    nsRect rootCompBounds =
+        nsRect(nsPoint(0, 0),
+               nsLayoutUtils::CalculateCompositionSizeForFrame(rootFrame));
+
+    // If rootFrame is the RCD-RSF then
+    // CalculateCompositionSizeForFrame did not take the document's
+    // resolution into account, so we must.
+    if (rootPresContext->IsRootContentDocumentCrossProcess() &&
+        rootFrame == rootPresShell->GetRootScrollFrame()) {
+      MOZ_LOG(
+          sDisplayportLog, LogLevel::Verbose,
+          ("RestrictToRootDisplayPort: Removing resolution %f from root "
+           "composition bounds %s\n",
+           rootPresShell->GetResolution(), ToString(rootCompBounds).c_str()));
+      rootCompBounds =
+          rootCompBounds.RemoveResolution(rootPresShell->GetResolution());
+    }
+
+    rootDisplayPort = rootCompBounds;
+  }
+  MOZ_LOG(sDisplayportLog, LogLevel::Verbose,
+          ("RestrictToRootDisplayPort: Intermediate root displayport %s\n",
+           ToString(rootDisplayPort).c_str()));
+
+  // We want to convert the root display port from the
+  // coordinate space of |rootFrame| to the coordinate space of
+  // |this|. We do that with the TransformRect call below.
+  // However, since we care about the root display port
+  // relative to what the user is actually seeing, we also need to
+  // incorporate the APZ callback transforms into this. Most of the
+  // time those transforms are negligible, but in some cases (e.g.
+  // when a zoom is applied on an overflow:hidden document) it is
+  // not (see bug 1280013).
+  // XXX: Eventually we may want to create a modified version of
+  // TransformRect that includes the APZ callback transforms
+  // directly.
+  nsLayoutUtils::TransformRect(rootFrame, this, rootDisplayPort);
+  MOZ_LOG(sDisplayportLog, LogLevel::Verbose,
+          ("RestrictToRootDisplayPort: Transformed root displayport %s\n",
+           ToString(rootDisplayPort).c_str()));
+  rootDisplayPort += CSSPoint::ToAppUnits(
+      nsLayoutUtils::GetCumulativeApzCallbackTransform(this));
+  MOZ_LOG(sDisplayportLog, LogLevel::Verbose,
+          ("RestrictToRootDisplayPort: Final root displayport %s\n",
+           ToString(rootDisplayPort).c_str()));
+
+  // We want to limit aDisplayportBase to be no larger than
+  // rootDisplayPort on either axis, but we don't want to just
+  // blindly intersect the two, because rootDisplayPort might be
+  // offset from where aDisplayportBase is (see bug 1327095 comment
+  // 8). Instead, we translate rootDisplayPort so as to maximize the
+  // overlap with aDisplayportBase, and *then* do the intersection.
+  if (rootDisplayPort.x > aDisplayportBase.x &&
+      rootDisplayPort.XMost() > aDisplayportBase.XMost()) {
+    // rootDisplayPort is at a greater x-position for both left and
+    // right, so translate it such that the XMost() values are the
+    // same. This will line up the right edge of the two rects, and
+    // might mean that rootDisplayPort.x is smaller than
+    // aDisplayportBase.x. We can avoid that by taking the min of the
+    // x delta and XMost() delta, but it doesn't really matter
+    // because the intersection between the two rects below will end
+    // up the same.
+    rootDisplayPort.x -= (rootDisplayPort.XMost() - aDisplayportBase.XMost());
+  } else if (rootDisplayPort.x < aDisplayportBase.x &&
+             rootDisplayPort.XMost() < aDisplayportBase.XMost()) {
+    // Analaogous code for when the rootDisplayPort is at a smaller
+    // x-position.
+    rootDisplayPort.x = aDisplayportBase.x;
+  }
+  // Do the same for y-axis
+  if (rootDisplayPort.y > aDisplayportBase.y &&
+      rootDisplayPort.YMost() > aDisplayportBase.YMost()) {
+    rootDisplayPort.y -= (rootDisplayPort.YMost() - aDisplayportBase.YMost());
+  } else if (rootDisplayPort.y < aDisplayportBase.y &&
+             rootDisplayPort.YMost() < aDisplayportBase.YMost()) {
+    rootDisplayPort.y = aDisplayportBase.y;
+  }
+  MOZ_LOG(
+      sDisplayportLog, LogLevel::Verbose,
+      ("RestrictToRootDisplayPort: Root displayport translated to %s to "
+       "better enclose %s\n",
+       ToString(rootDisplayPort).c_str(), ToString(aDisplayportBase).c_str()));
+
+  // Now we can do the intersection
+  return aDisplayportBase.Intersect(rootDisplayPort);
+}
+
+/* static */ bool nsHTMLScrollFrame::ShouldActivateAllScrollFrames() {
+  return (StaticPrefs::apz_wr_activate_all_scroll_frames() ||
+          (StaticPrefs::apz_wr_activate_all_scroll_frames_when_fission() &&
+           FissionAutostart()));
+}
+
+bool nsHTMLScrollFrame::DecideScrollableLayer(
+    nsDisplayListBuilder* aBuilder, nsRect* aVisibleRect, nsRect* aDirtyRect,
+    bool aSetBase, bool* aDirtyRectHasBeenOverriden) {
+  nsIContent* content = GetContent();
+  bool hasDisplayPort = DisplayPortUtils::HasDisplayPort(content);
+  // For hit testing purposes with fission we want to create a
+  // minimal display port for every scroll frame that could be active. (We only
+  // do this when aSetBase is true because we only want to do this the first
+  // time this function is called for the same scroll frame.)
+  if (aSetBase && !hasDisplayPort && aBuilder->IsPaintingToWindow() &&
+      ShouldActivateAllScrollFrames() &&
+      nsLayoutUtils::AsyncPanZoomEnabled(this) && WantAsyncScroll()) {
+    // SetDisplayPortMargins calls TriggerDisplayPortExpiration which starts a
+    // display port expiry timer for display ports that do expire. However
+    // minimal display ports do not expire, so the display port has to be
+    // marked before the SetDisplayPortMargins call so the expiry timer
+    // doesn't get started.
+    content->SetProperty(nsGkAtoms::MinimalDisplayPort,
+                         reinterpret_cast<void*>(true));
+
+    DisplayPortUtils::SetDisplayPortMargins(
+        content, PresShell(), DisplayPortMargins::Empty(content),
+        DisplayPortUtils::ClearMinimalDisplayPortProperty::No, 0,
+        DisplayPortUtils::RepaintMode::DoNotRepaint);
+    hasDisplayPort = true;
+  }
+
+  if (aBuilder->IsPaintingToWindow()) {
+    if (aSetBase) {
+      nsRect displayportBase = *aVisibleRect;
+      nsPresContext* pc = PresContext();
+
+      bool isChromeRootDoc =
+          !pc->Document()->IsContentDocument() && !pc->GetParentPresContext();
+
+      if (mIsRoot &&
+          (pc->IsRootContentDocumentCrossProcess() || isChromeRootDoc)) {
+        displayportBase =
+            nsRect(nsPoint(0, 0),
+                   nsLayoutUtils::CalculateCompositionSizeForFrame(this));
+      } else {
+        // Make the displayport base equal to the visible rect restricted to
+        // the scrollport and the root composition bounds, relative to the
+        // scrollport.
+        displayportBase = aVisibleRect->Intersect(mScrollPort);
+
+        mozilla::layers::ScrollableLayerGuid::ViewID viewID =
+            mozilla::layers::ScrollableLayerGuid::NULL_SCROLL_ID;
+        if (MOZ_LOG_TEST(sDisplayportLog, LogLevel::Verbose)) {
+          nsLayoutUtils::FindIDFor(GetContent(), &viewID);
+          MOZ_LOG(
+              sDisplayportLog, LogLevel::Verbose,
+              ("Scroll id %" PRIu64 " has visible rect %s, scroll port %s\n",
+               viewID, ToString(*aVisibleRect).c_str(),
+               ToString(mScrollPort).c_str()));
+        }
+
+        // Only restrict to the root displayport bounds if necessary,
+        // as the required coordinate transformation is expensive.
+        // And don't call RestrictToRootDisplayPort if we would be trying to
+        // restrict to our own display port, which doesn't make sense (ie if we
+        // are a root scroll frame in a process root prescontext).
+        if (hasDisplayPort && (!mIsRoot || pc->GetParentPresContext()) &&
+            !DisplayPortUtils::WillUseEmptyDisplayPortMargins(content)) {
+          displayportBase = RestrictToRootDisplayPort(displayportBase);
+          MOZ_LOG(sDisplayportLog, LogLevel::Verbose,
+                  ("Scroll id %" PRIu64 " has restricted base %s\n", viewID,
+                   ToString(displayportBase).c_str()));
+        }
+        displayportBase -= mScrollPort.TopLeft();
+      }
+
+      DisplayPortUtils::SetDisplayPortBase(GetContent(), displayportBase);
+    }
+
+    // If we don't have aSetBase == true then should have already
+    // been called with aSetBase == true which should have set a
+    // displayport base.
+    MOZ_ASSERT(content->GetProperty(nsGkAtoms::DisplayPortBase));
+    nsRect displayPort;
+    hasDisplayPort = DisplayPortUtils::GetDisplayPort(
+        content, &displayPort,
+        DisplayPortOptions().With(DisplayportRelativeTo::ScrollFrame));
+
+    auto OverrideDirtyRect = [&](const nsRect& aRect) {
+      *aDirtyRect = aRect;
+      if (aDirtyRectHasBeenOverriden) {
+        *aDirtyRectHasBeenOverriden = true;
+      }
+    };
+
+    if (hasDisplayPort) {
+      // Override the dirty rectangle if the displayport has been set.
+      *aVisibleRect = displayPort;
+      if (aBuilder->IsReusingStackingContextItems() ||
+          !aBuilder->IsPartialUpdate() || aBuilder->InInvalidSubtree() ||
+          IsFrameModified()) {
+        OverrideDirtyRect(displayPort);
+      } else if (HasOverrideDirtyRegion()) {
+        nsRect* rect = GetProperty(
+            nsDisplayListBuilder::DisplayListBuildingDisplayPortRect());
+        if (rect) {
+          OverrideDirtyRect(*rect);
+        }
+      }
+    } else if (mIsRoot) {
+      // The displayPort getter takes care of adjusting for resolution. So if
+      // we have resolution but no displayPort then we need to adjust for
+      // resolution here.
+      auto* presShell = PresShell();
+      *aVisibleRect =
+          aVisibleRect->RemoveResolution(presShell->GetResolution());
+      *aDirtyRect = aDirtyRect->RemoveResolution(presShell->GetResolution());
+    }
+  }
+
+  // Since making new layers is expensive, only create a scrollable layer
+  // for some scroll frames.
+  // When a displayport is being used, force building of a layer so that
+  // the compositor can find the scrollable layer for async scrolling.
+  // If the element is marked 'scrollgrab', also force building of a layer
+  // so that APZ can implement scroll grabbing.
+  mWillBuildScrollableLayer = hasDisplayPort ||
+                              nsContentUtils::HasScrollgrab(content) ||
+                              mZoomableByAPZ;
+  return mWillBuildScrollableLayer;
+}
+
+void nsHTMLScrollFrame::NotifyApzTransaction() {
+  mAllowScrollOriginDowngrade = true;
+  mApzScrollPos = GetScrollPosition();
+  mApzAnimationRequested = IsLastScrollUpdateAnimating();
+  mApzAnimationTriggeredByScriptRequested =
+      IsLastScrollUpdateTriggeredByScriptAnimating();
+  mScrollUpdates.Clear();
+  if (mIsRoot) {
+    PresShell()->SetResolutionUpdated(false);
+  }
+}
+
+Maybe<ScrollMetadata> nsHTMLScrollFrame::ComputeScrollMetadata(
+    WebRenderLayerManager* aLayerManager, const nsIFrame* aItemFrame,
+    const nsPoint& aOffsetToReferenceFrame) const {
+  if (!mWillBuildScrollableLayer) {
+    return Nothing();
+  }
+
+  bool isRootContent =
+      mIsRoot && PresContext()->IsRootContentDocumentCrossProcess();
+
+  MOZ_ASSERT(mScrolledFrame->GetContent());
+
+  return Some(nsLayoutUtils::ComputeScrollMetadata(
+      mScrolledFrame, this, GetContent(), aItemFrame, aOffsetToReferenceFrame,
+      aLayerManager, mScrollParentID, mScrollPort.Size(), isRootContent));
+}
+
+bool nsHTMLScrollFrame::IsRectNearlyVisible(const nsRect& aRect) const {
+  // Use the right rect depending on if a display port is set.
+  nsRect displayPort;
+  bool usingDisplayport = DisplayPortUtils::GetDisplayPort(
+      GetContent(), &displayPort,
+      DisplayPortOptions().With(DisplayportRelativeTo::ScrollFrame));
+
+  if (mIsRoot && !usingDisplayport &&
+      PresContext()->IsRootContentDocumentInProcess() &&
+      !PresContext()->IsRootContentDocumentCrossProcess()) {
+    // In the case of the root scroller of OOP iframes, there are cases where
+    // any display port value isn't set, e.g. the iframe element is out of view
+    // in the parent document. In such cases we'd consider the iframe is not
+    // visible.
+    return false;
+  }
+
+  return aRect.Intersects(
+      ExpandRectToNearlyVisible(usingDisplayport ? displayPort : mScrollPort));
+}
+
+OverscrollBehaviorInfo nsHTMLScrollFrame::GetOverscrollBehaviorInfo() const {
+  nsIFrame* frame = GetFrameForStyle();
+  if (!frame) {
+    return {};
+  }
+
+  auto& disp = *frame->StyleDisplay();
+  return OverscrollBehaviorInfo::FromStyleConstants(disp.mOverscrollBehaviorX,
+                                                    disp.mOverscrollBehaviorY);
+}
+
+ScrollStyles nsHTMLScrollFrame::GetScrollStyles() const {
+  nsPresContext* presContext = PresContext();
+  if (!presContext->IsDynamic() &&
+      !(mIsRoot && presContext->HasPaginatedScrolling())) {
+    return ScrollStyles(StyleOverflow::Hidden, StyleOverflow::Hidden);
+  }
+
+  if (!mIsRoot) {
+    return ScrollStyles(*StyleDisplay(),
+                        ScrollStyles::MapOverflowToValidScrollStyle);
+  }
+
+  ScrollStyles result = presContext->GetViewportScrollStylesOverride();
+  if (nsDocShell* ds = presContext->GetDocShell()) {
+    switch (ds->ScrollbarPreference()) {
+      case ScrollbarPreference::Auto:
+        break;
+      case ScrollbarPreference::Never:
+        result.mHorizontal = result.mVertical = StyleOverflow::Hidden;
+        break;
+    }
+  }
+  return result;
+}
+
+nsRect nsHTMLScrollFrame::GetLayoutScrollRange() const {
+  return GetScrollRange(mScrollPort.width, mScrollPort.height);
+}
+
+nsRect nsHTMLScrollFrame::GetScrollRange(nscoord aWidth,
+                                         nscoord aHeight) const {
+  nsRect range = GetScrolledRect();
+  range.width = std::max(range.width - aWidth, 0);
+  range.height = std::max(range.height - aHeight, 0);
+  return range;
+}
+
+nsRect nsHTMLScrollFrame::GetVisualScrollRange() const {
+  nsSize visualViewportSize = GetVisualViewportSize();
+  return GetScrollRange(visualViewportSize.width, visualViewportSize.height);
+}
+
+nsSize nsHTMLScrollFrame::GetVisualViewportSize() const {
+  auto* presShell = PresShell();
+  if (mIsRoot && presShell->IsVisualViewportSizeSet()) {
+    return presShell->GetVisualViewportSize();
+  }
+  return mScrollPort.Size();
+}
+
+nsPoint nsHTMLScrollFrame::GetVisualViewportOffset() const {
+  if (mIsRoot) {
+    auto* presShell = PresShell();
+    if (auto pendingUpdate = presShell->GetPendingVisualScrollUpdate()) {
+      // The pending visual scroll update on the PresShell contains a raw,
+      // unclamped offset (basically, whatever was passed to ScrollToVisual()).
+      // It will be clamped on the APZ side, but if we use it as the
+      // main-thread's visual viewport offset we need to clamp it ourselves.
+      // Use GetScrollRangeForUserInputEvents() to do the clamping because this
+      // the scroll range that APZ will use.
+      return GetScrollRangeForUserInputEvents().ClampPoint(
+          pendingUpdate->mVisualScrollOffset);
+    }
+    return presShell->GetVisualViewportOffset();
+  }
+  return GetScrollPosition();
+}
+
+bool nsHTMLScrollFrame::SetVisualViewportOffset(const nsPoint& aOffset,
+                                                bool aRepaint) {
+  MOZ_ASSERT(mIsRoot);
+  AutoWeakFrame weakFrame(this);
+  AutoScrollbarRepaintSuppression repaintSuppression(this, weakFrame,
+                                                     !aRepaint);
+
+  bool retVal =
+      PresShell()->SetVisualViewportOffset(aOffset, GetScrollPosition());
+  if (!weakFrame.IsAlive()) {
+    return false;
+  }
+  return retVal;
+}
+
+nsRect nsHTMLScrollFrame::GetVisualOptimalViewingRect() const {
+  auto* presShell = PresShell();
+  nsRect rect = mScrollPort;
+  if (mIsRoot && presShell->IsVisualViewportSizeSet() &&
+      presShell->IsVisualViewportOffsetSet()) {
+    rect = nsRect(mScrollPort.TopLeft() - GetScrollPosition() +
+                      presShell->GetVisualViewportOffset(),
+                  presShell->GetVisualViewportSize());
+  }
+  // NOTE: We intentionally resolve scroll-padding percentages against the
+  // scrollport even when the visual viewport is set, see
+  // https://github.com/w3c/csswg-drafts/issues/4393.
+  rect.Deflate(GetScrollPadding());
+  return rect;
+}
+
+static void AdjustDestinationForWholeDelta(const nsIntPoint& aDelta,
+                                           const nsRect& aScrollRange,
+                                           nsPoint& aPoint) {
+  if (aDelta.x < 0) {
+    aPoint.x = aScrollRange.X();
+  } else if (aDelta.x > 0) {
+    aPoint.x = aScrollRange.XMost();
+  }
+  if (aDelta.y < 0) {
+    aPoint.y = aScrollRange.Y();
+  } else if (aDelta.y > 0) {
+    aPoint.y = aScrollRange.YMost();
+  }
+}
+
+/**
+ * Calculate lower/upper scrollBy range in given direction.
+ * @param aDelta specifies scrollBy direction, if 0 then range will be 0 size
+ * @param aPos desired destination in AppUnits
+ * @param aNeg/PosTolerance defines relative range distance
+ *   below and above of aPos point
+ * @param aMultiplier used for conversion of tolerance into appUnis
+ */
+static void CalcRangeForScrollBy(int32_t aDelta, nscoord aPos,
+                                 float aNegTolerance, float aPosTolerance,
+                                 nscoord aMultiplier, nscoord* aLower,
+                                 nscoord* aUpper) {
+  if (!aDelta) {
+    *aLower = *aUpper = aPos;
+    return;
+  }
+  *aLower = aPos - NSToCoordRound(aMultiplier *
+                                  (aDelta > 0 ? aNegTolerance : aPosTolerance));
+  *aUpper = aPos + NSToCoordRound(aMultiplier *
+                                  (aDelta > 0 ? aPosTolerance : aNegTolerance));
+}
+
+void nsHTMLScrollFrame::ScrollBy(nsIntPoint aDelta, ScrollUnit aUnit,
+                                 ScrollMode aMode, nsIntPoint* aOverflow,
+                                 ScrollOrigin aOrigin,
+                                 nsIScrollableFrame::ScrollMomentum aMomentum,
+                                 ScrollSnapFlags aSnapFlags) {
+  // When a smooth scroll is being processed on a frame, mouse wheel and
+  // trackpad momentum scroll event updates must notcancel the SMOOTH or
+  // SMOOTH_MSD scroll animations, enabling Javascript that depends on them to
+  // be responsive without forcing the user to wait for the fling animations to
+  // completely stop.
+  switch (aMomentum) {
+    case nsIScrollableFrame::NOT_MOMENTUM:
+      mIgnoreMomentumScroll = false;
+      break;
+    case nsIScrollableFrame::SYNTHESIZED_MOMENTUM_EVENT:
+      if (mIgnoreMomentumScroll) {
+        return;
+      }
+      break;
+  }
+
+  if (mAsyncSmoothMSDScroll != nullptr) {
+    // When CSSOM-View scroll-behavior smooth scrolling is interrupted,
+    // the scroll is not completed to avoid non-smooth snapping to the
+    // prior smooth scroll's destination.
+    mDestination = GetScrollPosition();
+  }
+
+  nsSize deltaMultiplier;
+  float negativeTolerance;
+  float positiveTolerance;
+  if (aOrigin == ScrollOrigin::NotSpecified) {
+    aOrigin = ScrollOrigin::Other;
+  }
+  bool isGenericOrigin = (aOrigin == ScrollOrigin::Other);
+
+  bool askApzToDoTheScroll = false;
+  if ((aSnapFlags == ScrollSnapFlags::Disabled || !NeedsScrollSnap()) &&
+      gfxPlatform::UseDesktopZoomingScrollbars() &&
+      nsLayoutUtils::AsyncPanZoomEnabled(this) &&
+      !nsLayoutUtils::ShouldDisableApzForElement(GetContent()) &&
+      (WantAsyncScroll() || mZoomableByAPZ) &&
+      CanApzScrollInTheseDirections(DirectionsInDelta(aDelta))) {
+    askApzToDoTheScroll = true;
+  }
+
+  switch (aUnit) {
+    case ScrollUnit::DEVICE_PIXELS: {
+      nscoord appUnitsPerDevPixel = PresContext()->AppUnitsPerDevPixel();
+      deltaMultiplier = nsSize(appUnitsPerDevPixel, appUnitsPerDevPixel);
+      if (isGenericOrigin) {
+        aOrigin = ScrollOrigin::Pixels;
+      }
+      negativeTolerance = positiveTolerance = 0.5f;
+      break;
+    }
+    case ScrollUnit::LINES: {
+      deltaMultiplier = GetLineScrollAmount();
+      if (isGenericOrigin) {
+        aOrigin = ScrollOrigin::Lines;
+      }
+      negativeTolerance = positiveTolerance = 0.1f;
+      break;
+    }
+    case ScrollUnit::PAGES: {
+      deltaMultiplier = GetPageScrollAmount();
+      if (isGenericOrigin) {
+        aOrigin = ScrollOrigin::Pages;
+      }
+      negativeTolerance = 0.05f;
+      positiveTolerance = 0;
+      break;
+    }
+    case ScrollUnit::WHOLE: {
+      if (askApzToDoTheScroll) {
+        MOZ_ASSERT(aDelta.x >= -1 && aDelta.x <= 1 && aDelta.y >= -1 &&
+                   aDelta.y <= 1);
+        deltaMultiplier = GetScrollRangeForUserInputEvents().Size();
+        break;
+      } else {
+        nsPoint pos = GetScrollPosition();
+        AdjustDestinationForWholeDelta(aDelta, GetLayoutScrollRange(), pos);
+        ScrollToWithOrigin(
+            pos, nullptr /* range */,
+            ScrollOperationParams{aMode, ScrollOrigin::Other, aSnapFlags,
+                                  ScrollTriggeredByScript::No});
+        // 'this' might be destroyed here
+        if (aOverflow) {
+          *aOverflow = nsIntPoint(0, 0);
+        }
+        return;
+      }
+    }
+    default:
+      NS_ERROR("Invalid scroll mode");
+      return;
+  }
+
+  if (askApzToDoTheScroll) {
+    nsPoint delta(
+        NSCoordSaturatingNonnegativeMultiply(aDelta.x, deltaMultiplier.width),
+        NSCoordSaturatingNonnegativeMultiply(aDelta.y, deltaMultiplier.height));
+
+    AppendScrollUpdate(
+        ScrollPositionUpdate::NewPureRelativeScroll(aOrigin, aMode, delta));
+
+    nsIContent* content = GetContent();
+    if (!DisplayPortUtils::HasNonMinimalNonZeroDisplayPort(content)) {
+      if (MOZ_LOG_TEST(sDisplayportLog, LogLevel::Debug)) {
+        mozilla::layers::ScrollableLayerGuid::ViewID viewID =
+            mozilla::layers::ScrollableLayerGuid::NULL_SCROLL_ID;
+        nsLayoutUtils::FindIDFor(content, &viewID);
+        MOZ_LOG(
+            sDisplayportLog, LogLevel::Debug,
+            ("ScrollBy setting displayport on scrollId=%" PRIu64 "\n", viewID));
+      }
+
+      DisplayPortUtils::CalculateAndSetDisplayPortMargins(
+          GetScrollTargetFrame(), DisplayPortUtils::RepaintMode::Repaint);
+      nsIFrame* frame = do_QueryFrame(GetScrollTargetFrame());
+      DisplayPortUtils::SetZeroMarginDisplayPortOnAsyncScrollableAncestors(
+          frame);
+    }
+
+    SchedulePaint();
+    return;
+  }
+
+  nsPoint newPos(NSCoordSaturatingAdd(mDestination.x,
+                                      NSCoordSaturatingNonnegativeMultiply(
+                                          aDelta.x, deltaMultiplier.width)),
+                 NSCoordSaturatingAdd(mDestination.y,
+                                      NSCoordSaturatingNonnegativeMultiply(
+                                          aDelta.y, deltaMultiplier.height)));
+
+  Maybe<SnapDestination> snapDestination;
+  if (aSnapFlags != ScrollSnapFlags::Disabled) {
+    if (NeedsScrollSnap()) {
+      nscoord appUnitsPerDevPixel = PresContext()->AppUnitsPerDevPixel();
+      deltaMultiplier = nsSize(appUnitsPerDevPixel, appUnitsPerDevPixel);
+      negativeTolerance = 0.1f;
+      positiveTolerance = 0;
+      ScrollUnit snapUnit = aUnit;
+      if (aOrigin == ScrollOrigin::MouseWheel) {
+        // When using a clicky scroll wheel, snap point selection works the same
+        // as keyboard up/down/left/right navigation, but with varying amounts
+        // of scroll delta.
+        snapUnit = ScrollUnit::LINES;
+      }
+      snapDestination = GetSnapPointForDestination(snapUnit, aSnapFlags,
+                                                   mDestination, newPos);
+      if (snapDestination) {
+        newPos = snapDestination->mPosition;
+      }
+    }
+  }
+
+  // Calculate desired range values.
+  nscoord rangeLowerX, rangeUpperX, rangeLowerY, rangeUpperY;
+  CalcRangeForScrollBy(aDelta.x, newPos.x, negativeTolerance, positiveTolerance,
+                       deltaMultiplier.width, &rangeLowerX, &rangeUpperX);
+  CalcRangeForScrollBy(aDelta.y, newPos.y, negativeTolerance, positiveTolerance,
+                       deltaMultiplier.height, &rangeLowerY, &rangeUpperY);
+  nsRect range(rangeLowerX, rangeLowerY, rangeUpperX - rangeLowerX,
+               rangeUpperY - rangeLowerY);
+  AutoWeakFrame weakFrame(this);
+  ScrollToWithOrigin(
+      newPos, &range,
+      snapDestination
+          ? ScrollOperationParams{aMode, aOrigin,
+                                  std::move(snapDestination->mTargetIds)}
+          : ScrollOperationParams{aMode, aOrigin});
+  if (!weakFrame.IsAlive()) {
+    return;
+  }
+
+  if (aOverflow) {
+    nsPoint clampAmount = newPos - mDestination;
+    float appUnitsPerDevPixel = PresContext()->AppUnitsPerDevPixel();
+    *aOverflow =
+        nsIntPoint(NSAppUnitsToIntPixels(clampAmount.x, appUnitsPerDevPixel),
+                   NSAppUnitsToIntPixels(clampAmount.y, appUnitsPerDevPixel));
+  }
+
+  if (aUnit == ScrollUnit::DEVICE_PIXELS &&
+      !nsLayoutUtils::AsyncPanZoomEnabled(this)) {
+    // When APZ is disabled, we must track the velocity
+    // on the main thread; otherwise, the APZC will manage this.
+    mVelocityQueue.Sample(GetScrollPosition());
+  }
+}
+
+void nsHTMLScrollFrame::ScrollByCSSPixelsInternal(const CSSIntPoint& aDelta,
+                                                  ScrollMode aMode,
+                                                  ScrollSnapFlags aSnapFlags) {
+  nsPoint current = GetScrollPosition();
+  // `current` value above might be a value which was aligned to in
+  // layer-pixels, so starting from such points will make the difference between
+  // the given position in script (e.g. scrollTo) and the aligned position
+  // larger, in the worst case the difference can be observed in CSS pixels.
+  // To avoid it, we use the current position in CSS pixels as the start
+  // position.  Hopefully it exactly matches the position where it was given by
+  // the previous scrolling operation, but there may be some edge cases where
+  // the current position in CSS pixels differs from the given position, the
+  // cases should be fixed in bug 1556685.
+  CSSPoint currentCSSPixels;
+  if (StaticPrefs::layout_scroll_disable_pixel_alignment()) {
+    currentCSSPixels = GetScrollPositionCSSPixels();
+  } else {
+    currentCSSPixels = GetRoundedScrollPositionCSSPixels();
+  }
+  nsPoint pt = CSSPoint::ToAppUnits(currentCSSPixels + aDelta);
+
+  nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f);
+  nsRect range(pt.x - halfPixel, pt.y - halfPixel, 2 * halfPixel - 1,
+               2 * halfPixel - 1);
+  // XXX I don't think the following blocks are needed anymore, now that
+  // ScrollToImpl simply tries to scroll an integer number of layer
+  // pixels from the current position
+  if (aDelta.x == 0.0f) {
+    pt.x = current.x;
+    range.x = pt.x;
+    range.width = 0;
+  }
+  if (aDelta.y == 0.0f) {
+    pt.y = current.y;
+    range.y = pt.y;
+    range.height = 0;
+  }
+  ScrollToWithOrigin(
+      pt, &range,
+      ScrollOperationParams{aMode, ScrollOrigin::Relative, aSnapFlags,
+                            ScrollTriggeredByScript::Yes});
+  // 'this' might be destroyed here
+}
+
+void nsHTMLScrollFrame::ScrollSnap(ScrollMode aMode) {
+  float flingSensitivity =
+      StaticPrefs::layout_css_scroll_snap_prediction_sensitivity();
+  int maxVelocity =
+      StaticPrefs::layout_css_scroll_snap_prediction_max_velocity();
+  maxVelocity = nsPresContext::CSSPixelsToAppUnits(maxVelocity);
+  int maxOffset = maxVelocity * flingSensitivity;
+  nsPoint velocity = mVelocityQueue.GetVelocity();
+  // Multiply each component individually to avoid integer multiply
+  nsPoint predictedOffset =
+      nsPoint(velocity.x * flingSensitivity, velocity.y * flingSensitivity);
+  predictedOffset.Clamp(maxOffset);
+  nsPoint pos = GetScrollPosition();
+  nsPoint destinationPos = pos + predictedOffset;
+  ScrollSnap(destinationPos, aMode);
+}
+
+void nsHTMLScrollFrame::ScrollSnap(const nsPoint& aDestination,
+                                   ScrollMode aMode) {
+  nsRect scrollRange = GetLayoutScrollRange();
+  nsPoint pos = GetScrollPosition();
+  nsPoint destination = scrollRange.ClampPoint(aDestination);
+  ScrollSnapFlags snapFlags = ScrollSnapFlags::IntendedEndPosition;
+  if (mVelocityQueue.GetVelocity() != nsPoint()) {
+    snapFlags |= ScrollSnapFlags::IntendedDirection;
+  }
+
+  // Bug 1776624 : Consider using mDestination as |aStartPos| argument for this
+  // GetSnapPointForDestination call, this function call is the only one call
+  // site using `GetScrollPosition()` as |aStartPos|.
+  if (auto snapDestination = GetSnapPointForDestination(
+          ScrollUnit::DEVICE_PIXELS, snapFlags, pos, destination)) {
+    destination = snapDestination->mPosition;
+    ScrollToWithOrigin(
+        destination, nullptr /* range */,
+        ScrollOperationParams{aMode, ScrollOrigin::Other,
+                              std::move(snapDestination->mTargetIds)});
+  }
+}
+
+nsSize nsHTMLScrollFrame::GetLineScrollAmount() const {
+  RefPtr<nsFontMetrics> fm =
+      nsLayoutUtils::GetInflatedFontMetricsForFrame(this);
+  NS_ASSERTION(fm, "FontMetrics is null, assuming fontHeight == 1 appunit");
+  int32_t appUnitsPerDevPixel = PresContext()->AppUnitsPerDevPixel();
+  nscoord minScrollAmountInAppUnits =
+      std::max(1, StaticPrefs::mousewheel_min_line_scroll_amount()) *
+      appUnitsPerDevPixel;
+  nscoord horizontalAmount = fm ? fm->AveCharWidth() : 0;
+  nscoord verticalAmount = fm ? fm->MaxHeight() : 0;
+  return nsSize(std::max(horizontalAmount, minScrollAmountInAppUnits),
+                std::max(verticalAmount, minScrollAmountInAppUnits));
+}
+
+/**
+ * Compute the scrollport size excluding any fixed-pos and sticky-pos (that are
+ * stuck) headers and footers. A header or footer is an box that spans that
+ * entire width of the viewport and touches the top (or bottom, respectively) of
+ * the viewport. We also want to consider fixed/sticky elements that stack or
+ * overlap to effectively create a larger header or footer. Headers and footers
+ * that cover more than a third of the the viewport are ignored since they
+ * probably aren't true headers and footers and we don't want to restrict
+ * scrolling too much in such cases. This is a bit conservative --- some
+ * pages use elements as headers or footers that don't span the entire width
+ * of the viewport --- but it should be a good start.
+ *
+ * If aViewportFrame is non-null then the scroll frame is the root scroll
+ * frame and we should consider fixed-pos items.
+ */
+struct TopAndBottom {
+  TopAndBottom(nscoord aTop, nscoord aBottom) : top(aTop), bottom(aBottom) {}
+
+  nscoord top, bottom;
+};
+struct TopComparator {
+  bool Equals(const TopAndBottom& A, const TopAndBottom& B) const {
+    return A.top == B.top;
+  }
+  bool LessThan(const TopAndBottom& A, const TopAndBottom& B) const {
+    return A.top < B.top;
+  }
+};
+struct ReverseBottomComparator {
+  bool Equals(const TopAndBottom& A, const TopAndBottom& B) const {
+    return A.bottom == B.bottom;
+  }
+  bool LessThan(const TopAndBottom& A, const TopAndBottom& B) const {
+    return A.bottom > B.bottom;
+  }
+};
+
+static void AddToListIfHeaderFooter(nsIFrame* aFrame,
+                                    nsIFrame* aScrollPortFrame,
+                                    const nsRect& aScrollPort,
+                                    nsTArray<TopAndBottom>& aList) {
+  nsRect r = aFrame->GetRectRelativeToSelf();
+  r = nsLayoutUtils::TransformFrameRectToAncestor(aFrame, r, aScrollPortFrame);
+  r = r.Intersect(aScrollPort);
+  if ((r.width >= aScrollPort.width / 2 ||
+       r.width >= NSIntPixelsToAppUnits(800, AppUnitsPerCSSPixel())) &&
+      r.height <= aScrollPort.height / 3) {
+    aList.AppendElement(TopAndBottom(r.y, r.YMost()));
+  }
+}
+
+static nsSize GetScrollPortSizeExcludingHeadersAndFooters(
+    nsIFrame* aScrollFrame, nsIFrame* aViewportFrame,
+    const nsRect& aScrollPort) {
+  AutoTArray<TopAndBottom, 10> list;
+  if (aViewportFrame) {
+    for (nsIFrame* f : aViewportFrame->GetChildList(FrameChildListID::Fixed)) {
+      AddToListIfHeaderFooter(f, aViewportFrame, aScrollPort, list);
+    }
+  }
+
+  // Add sticky frames that are currently in "fixed" positions
+  StickyScrollContainer* ssc =
+      StickyScrollContainer::GetStickyScrollContainerForScrollFrame(
+          aScrollFrame);
+  if (ssc) {
+    const nsTArray<nsIFrame*>& stickyFrames = ssc->GetFrames();
+    for (nsIFrame* f : stickyFrames) {
+      // If it's acting like fixed position.
+      if (ssc->IsStuckInYDirection(f)) {
+        AddToListIfHeaderFooter(f, aScrollFrame, aScrollPort, list);
+      }
+    }
+  }
+
+  list.Sort(TopComparator());
+  nscoord headerBottom = 0;
+  for (uint32_t i = 0; i < list.Length(); ++i) {
+    if (list[i].top <= headerBottom) {
+      headerBottom = std::max(headerBottom, list[i].bottom);
+    }
+  }
+
+  list.Sort(ReverseBottomComparator());
+  nscoord footerTop = aScrollPort.height;
+  for (uint32_t i = 0; i < list.Length(); ++i) {
+    if (list[i].bottom >= footerTop) {
+      footerTop = std::min(footerTop, list[i].top);
+    }
+  }
+
+  headerBottom = std::min(aScrollPort.height / 3, headerBottom);
+  footerTop = std::max(aScrollPort.height - aScrollPort.height / 3, footerTop);
+
+  return nsSize(aScrollPort.width, footerTop - headerBottom);
+}
+
+nsSize nsHTMLScrollFrame::GetPageScrollAmount() const {
+  nsSize effectiveScrollPortSize;
+
+  if (GetVisualViewportSize() != mScrollPort.Size()) {
+    // We want to use the visual viewport size if one is set.
+    // The headers/footers adjustment is too complicated to do if there is a
+    // visual viewport that differs from the layout viewport, this is probably
+    // okay.
+    effectiveScrollPortSize = GetVisualViewportSize();
+  } else {
+    // Reduce effective scrollport height by the height of any
+    // fixed-pos/sticky-pos headers or footers
+    effectiveScrollPortSize = GetScrollPortSizeExcludingHeadersAndFooters(
+        const_cast<nsHTMLScrollFrame*>(this),
+        mIsRoot ? PresShell()->GetRootFrame() : nullptr, mScrollPort);
+  }
+
+  nsSize lineScrollAmount = GetLineScrollAmount();
+
+  // The page increment is the size of the page, minus the smaller of
+  // 10% of the size or 2 lines.
+  return nsSize(effectiveScrollPortSize.width -
+                    std::min(effectiveScrollPortSize.width / 10,
+                             2 * lineScrollAmount.width),
+                effectiveScrollPortSize.height -
+                    std::min(effectiveScrollPortSize.height / 10,
+                             2 * lineScrollAmount.height));
+}
+
+/**
+ * this code is resposible for restoring the scroll position back to some
+ * saved position. if the user has not moved the scroll position manually
+ * we keep scrolling down until we get to our original position. keep in
+ * mind that content could incrementally be coming in. we only want to stop
+ * when we reach our new position.
+ */
+void nsHTMLScrollFrame::ScrollToRestoredPosition() {
+  if (mRestorePos.y == -1 || mLastPos.x == -1 || mLastPos.y == -1) {
+    return;
+  }
+  // make sure our scroll position did not change for where we last put
+  // it. if it does then the user must have moved it, and we no longer
+  // need to restore.
+  //
+  // In the RTL case, we check whether the scroll position changed using the
+  // logical scroll position, but we scroll to the physical scroll position in
+  // all cases
+
+  // The layout offset we want to restore is the same as the visual offset
+  // (for now, may change in bug 1499210), but clamped to the layout scroll
+  // range (which can be a subset of the visual scroll range).
+  // Note that we can't do the clamping when initializing mRestorePos in
+  // RestoreState(), since the scrollable rect (which the clamping depends
+  // on) can change over the course of the restoration process.
+  nsPoint layoutRestorePos = GetLayoutScrollRange().ClampPoint(mRestorePos);
+  nsPoint visualRestorePos = GetVisualScrollRange().ClampPoint(mRestorePos);
+
+  // Continue restoring until both the layout and visual scroll positions
+  // reach the destination. (Note that the two can only be different for
+  // the root content document's root scroll frame, and when zoomed in).
+  // This is necessary to avoid situations where the two offsets get stuck
+  // at different values and nothing reconciles them (see bug 1519621 comment
+  // 8).
+  nsPoint logicalLayoutScrollPos = GetLogicalScrollPosition();
+
+  SCROLLRESTORE_LOG(
+      "%p: ScrollToRestoredPosition (mRestorePos=%s, mLastPos=%s, "
+      "layoutRestorePos=%s, visualRestorePos=%s, "
+      "logicalLayoutScrollPos=%s, "
+      "GetLogicalVisualViewportOffset()=%s)\n",
+      this, ToString(mRestorePos).c_str(), ToString(mLastPos).c_str(),
+      ToString(layoutRestorePos).c_str(), ToString(visualRestorePos).c_str(),
+      ToString(logicalLayoutScrollPos).c_str(),
+      ToString(GetLogicalVisualViewportOffset()).c_str());
+
+  // if we didn't move, we still need to restore
+  if (GetLogicalVisualViewportOffset() == mLastPos ||
+      logicalLayoutScrollPos == mLastPos) {
+    // if our desired position is different to the scroll position, scroll.
+    // remember that we could be incrementally loading so we may enter
+    // and scroll many times.
+    if (mRestorePos != mLastPos /* GetLogicalVisualViewportOffset() */ ||
+        layoutRestorePos != logicalLayoutScrollPos) {
+      LoadingState state = GetPageLoadingState();
+      if (state == LoadingState::Stopped && !IsSubtreeDirty()) {
+        return;
+      }
+      nsPoint visualScrollToPos = visualRestorePos;
+      nsPoint layoutScrollToPos = layoutRestorePos;
+      if (!IsPhysicalLTR()) {
+        // convert from logical to physical scroll position
+        visualScrollToPos.x -=
+            (GetVisualViewportSize().width - mScrolledFrame->GetRect().width);
+        layoutScrollToPos.x -=
+            (GetVisualViewportSize().width - mScrolledFrame->GetRect().width);
+      }
+      AutoWeakFrame weakFrame(this);
+      // It's very important to pass ScrollOrigin::Restore here, so
+      // ScrollToWithOrigin won't clear out mRestorePos.
+      ScrollToWithOrigin(
+          layoutScrollToPos, nullptr,
+          ScrollOperationParams{ScrollMode::Instant, ScrollOrigin::Restore});
+      if (!weakFrame.IsAlive()) {
+        return;
+      }
+      if (mIsRoot) {
+        PresShell()->ScrollToVisual(visualScrollToPos, FrameMetrics::eRestore,
+                                    ScrollMode::Instant);
+      }
+      if (state == LoadingState::Loading || IsSubtreeDirty()) {
+        // If we're trying to do a history scroll restore, then we want to
+        // keep trying this until we succeed, because the page can be loading
+        // incrementally. So re-get the scroll position for the next iteration,
+        // it might not be exactly equal to mRestorePos due to rounding and
+        // clamping.
+        mLastPos = GetLogicalVisualViewportOffset();
+        return;
+      }
+    }
+    // If we get here, either we reached the desired position (mLastPos ==
+    // mRestorePos) or we're not trying to do a history scroll restore, so
+    // we can stop after the scroll attempt above.
+    mRestorePos.y = -1;
+    mLastPos.x = -1;
+    mLastPos.y = -1;
+  } else {
+    // user moved the position, so we won't need to restore
+    mLastPos.x = -1;
+    mLastPos.y = -1;
+  }
+}
+
+auto nsHTMLScrollFrame::GetPageLoadingState() -> LoadingState {
+  bool loadCompleted = false, stopped = false;
+  nsCOMPtr<nsIDocShell> ds = GetContent()->GetComposedDoc()->GetDocShell();
+  if (ds) {
+    nsCOMPtr<nsIDocumentViewer> viewer;
+    ds->GetDocViewer(getter_AddRefs(viewer));
+    if (viewer) {
+      loadCompleted = viewer->GetLoadCompleted();
+      stopped = viewer->GetIsStopped();
+    }
+  }
+  return loadCompleted
+             ? (stopped ? LoadingState::Stopped : LoadingState::Loaded)
+             : LoadingState::Loading;
+}
+
+nsHTMLScrollFrame::OverflowState nsHTMLScrollFrame::GetOverflowState() const {
+  nsSize scrollportSize = mScrollPort.Size();
+  nsSize childSize = GetScrolledRect().Size();
+
+  OverflowState result = OverflowState::None;
+
+  if (childSize.height > scrollportSize.height) {
+    result |= OverflowState::Vertical;
+  }
+
+  if (childSize.width > scrollportSize.width) {
+    result |= OverflowState::Horizontal;
+  }
+
+  return result;
+}
+
+nsresult nsHTMLScrollFrame::FireScrollPortEvent() {
+  mAsyncScrollPortEvent.Forget();
+
+  // TODO(emilio): why do we need the whole WillPaintObserver infrastructure and
+  // can't use AddScriptRunner & co? I guess it made sense when we used
+  // WillPaintObserver for scroll events too, or when this used to flush.
+  //
+  // Should we remove this?
+
+  OverflowState overflowState = GetOverflowState();
+
+  bool newVerticalOverflow = !!(overflowState & OverflowState::Vertical);
+  bool vertChanged = mVerticalOverflow != newVerticalOverflow;
+
+  bool newHorizontalOverflow = !!(overflowState & OverflowState::Horizontal);
+  bool horizChanged = mHorizontalOverflow != newHorizontalOverflow;
+
+  if (!vertChanged && !horizChanged) {
+    return NS_OK;
+  }
+
+  // If both either overflowed or underflowed then we dispatch only one
+  // DOM event.
+  bool both = vertChanged && horizChanged &&
+              newVerticalOverflow == newHorizontalOverflow;
+  InternalScrollPortEvent::OrientType orient;
+  if (both) {
+    orient = InternalScrollPortEvent::eBoth;
+    mHorizontalOverflow = newHorizontalOverflow;
+    mVerticalOverflow = newVerticalOverflow;
+  } else if (vertChanged) {
+    orient = InternalScrollPortEvent::eVertical;
+    mVerticalOverflow = newVerticalOverflow;
+    if (horizChanged) {
+      // We need to dispatch a separate horizontal DOM event. Do that the next
+      // time around since dispatching the vertical DOM event might destroy
+      // the frame.
+      PostOverflowEvent();
+    }
+  } else {
+    orient = InternalScrollPortEvent::eHorizontal;
+    mHorizontalOverflow = newHorizontalOverflow;
+  }
+
+  InternalScrollPortEvent event(
+      true,
+      (orient == InternalScrollPortEvent::eHorizontal ? mHorizontalOverflow
+                                                      : mVerticalOverflow)
+          ? eScrollPortOverflow
+          : eScrollPortUnderflow,
+      nullptr);
+  event.mOrient = orient;
+
+  RefPtr<nsIContent> content = GetContent();
+  RefPtr<nsPresContext> presContext = PresContext();
+  return EventDispatcher::Dispatch(content, presContext, &event);
+}
+
+void nsHTMLScrollFrame::PostScrollEndEvent() {
+  if (mScrollEndEvent) {
+    return;
+  }
+
+  // The ScrollEndEvent constructor registers itself with the refresh driver.
+  mScrollEndEvent = new ScrollEndEvent(this);
+}
+
+void nsHTMLScrollFrame::FireScrollEndEvent() {
+  MOZ_ASSERT(GetContent());
+  MOZ_ASSERT(mScrollEndEvent);
+
+  RefPtr<nsPresContext> presContext = PresContext();
+  mScrollEndEvent->Revoke();
+  mScrollEndEvent = nullptr;
+
+  nsEventStatus status = nsEventStatus_eIgnore;
+  WidgetGUIEvent event(true, eScrollend, nullptr);
+  event.mFlags.mBubbles = mIsRoot;
+  event.mFlags.mCancelable = false;
+  RefPtr<nsINode> target =
+      mIsRoot ? static_cast<nsINode*>(presContext->Document()) : GetContent();
+  EventDispatcher::Dispatch(target, presContext, &event, nullptr, &status);
+}
+
+void nsHTMLScrollFrame::ReloadChildFrames() {
+  mScrolledFrame = nullptr;
+  mHScrollbarBox = nullptr;
+  mVScrollbarBox = nullptr;
+  mScrollCornerBox = nullptr;
+  mResizerBox = nullptr;
+
+  for (nsIFrame* frame : PrincipalChildList()) {
+    nsIContent* content = frame->GetContent();
+    if (content == GetContent()) {
+      NS_ASSERTION(!mScrolledFrame, "Already found the scrolled frame");
+      mScrolledFrame = frame;
+    } else {
+      nsAutoString value;
+      if (content->IsElement()) {
+        content->AsElement()->GetAttr(nsGkAtoms::orient, value);
+      }
+      if (!value.IsEmpty()) {
+        // probably a scrollbar then
+        if (value.LowerCaseEqualsLiteral("horizontal")) {
+          NS_ASSERTION(!mHScrollbarBox,
+                       "Found multiple horizontal scrollbars?");
+          mHScrollbarBox = do_QueryFrame(frame);
+          MOZ_ASSERT(mHScrollbarBox, "Not a scrollbar?");
+        } else {
+          NS_ASSERTION(!mVScrollbarBox, "Found multiple vertical scrollbars?");
+          mVScrollbarBox = do_QueryFrame(frame);
+          MOZ_ASSERT(mVScrollbarBox, "Not a scrollbar?");
+        }
+      } else if (content->IsXULElement(nsGkAtoms::resizer)) {
+        NS_ASSERTION(!mResizerBox, "Found multiple resizers");
+        mResizerBox = frame;
+      } else if (content->IsXULElement(nsGkAtoms::scrollcorner)) {
+        // probably a scrollcorner
+        NS_ASSERTION(!mScrollCornerBox, "Found multiple scrollcorners");
+        mScrollCornerBox = frame;
+      }
+    }
+  }
+}
+
+already_AddRefed<Element> nsHTMLScrollFrame::MakeScrollbar(
+    NodeInfo* aNodeInfo, bool aVertical, AnonymousContentKey& aKey) {
+  MOZ_ASSERT(aNodeInfo);
+  MOZ_ASSERT(
+      aNodeInfo->Equals(nsGkAtoms::scrollbar, nullptr, kNameSpaceID_XUL));
+
+  static constexpr nsLiteralString kOrientValues[2] = {
+      u"horizontal"_ns,
+      u"vertical"_ns,
+  };
+
+  aKey = AnonymousContentKey::Type_Scrollbar;
+  if (aVertical) {
+    aKey |= AnonymousContentKey::Flag_Vertical;
+  }
+
+  RefPtr<Element> e;
+  NS_TrustedNewXULElement(getter_AddRefs(e), do_AddRef(aNodeInfo));
+
+#ifdef DEBUG
+  // Scrollbars can get restyled by theme changes.  Whether such a restyle
+  // will actually reconstruct them correctly if it involves a frame
+  // reconstruct... I don't know.  :(
+  e->SetProperty(nsGkAtoms::restylableAnonymousNode,
+                 reinterpret_cast<void*>(true));
+#endif  // DEBUG
+
+  e->SetAttr(kNameSpaceID_None, nsGkAtoms::orient, kOrientValues[aVertical],
+             false);
+
+  if (mIsRoot) {
+    e->SetProperty(nsGkAtoms::docLevelNativeAnonymousContent,
+                   reinterpret_cast<void*>(true));
+    e->SetAttr(kNameSpaceID_None, nsGkAtoms::root_, u"true"_ns, false);
+
+    // Don't bother making style caching take [root="true"] styles into account.
+    aKey = AnonymousContentKey::None;
+  }
+
+  return e.forget();
+}
+
+bool nsHTMLScrollFrame::IsForTextControlWithNoScrollbars() const {
+  // FIXME(emilio): we should probably make the scroller inside <input> an
+  // internal pseudo-element, and then this would be simpler.
+  //
+  // Also, this could just use scrollbar-width these days.
+  auto* content = GetContent();
+  if (!content) {
+    return false;
+  }
+  auto* input = content->GetClosestNativeAnonymousSubtreeRootParentOrHost();
+  return input && input->IsHTMLElement(nsGkAtoms::input);
+}
+
+auto nsHTMLScrollFrame::GetCurrentAnonymousContent() const
+    -> EnumSet<AnonymousContentType> {
+  EnumSet<AnonymousContentType> result;
+  if (mHScrollbarContent) {
+    result += AnonymousContentType::HorizontalScrollbar;
+  }
+  if (mVScrollbarContent) {
+    result += AnonymousContentType::VerticalScrollbar;
+  }
+  if (mResizerContent) {
+    result += AnonymousContentType::Resizer;
+  }
+  return result;
+}
+
+auto nsHTMLScrollFrame::GetNeededAnonymousContent() const
+    -> EnumSet<AnonymousContentType> {
+  nsPresContext* pc = PresContext();
+
+  // Don't create scrollbars if we're an SVG document being used as an image,
+  // or if we're printing/print previewing.
+  // (In the printing case, we allow scrollbars if this is the child of the
+  // viewport & paginated scrolling is enabled, because then we must be the
+  // scroll frame for the print preview window, & that does need scrollbars.)
+  if (pc->Document()->IsBeingUsedAsImage() ||
+      (!pc->IsDynamic() && !(mIsRoot && pc->HasPaginatedScrolling()))) {
+    return {};
+  }
+
+  if (IsForTextControlWithNoScrollbars()) {
+    return {};
+  }
+
+  EnumSet<AnonymousContentType> result;
+  // If we're the scrollframe for the root, then we want to construct our
+  // scrollbar frames no matter what.  That way later dynamic changes to
+  // propagated overflow styles will show or hide scrollbars on the viewport
+  // without requiring frame reconstruction of the viewport (good!).
+  //
+  // TODO(emilio): Figure out if we can remove this special-case now that we
+  // have more targeted optimizations.
+  if (mIsRoot) {
+    result += AnonymousContentType::HorizontalScrollbar;
+    result += AnonymousContentType::VerticalScrollbar;
+    // If scrollbar-width is none, don't generate scrollbars.
+  } else if (StyleUIReset()->ScrollbarWidth() != StyleScrollbarWidth::None) {
+    ScrollStyles styles = GetScrollStyles();
+    if (styles.mHorizontal != StyleOverflow::Hidden) {
+      result += AnonymousContentType::HorizontalScrollbar;
+    }
+    if (styles.mVertical != StyleOverflow::Hidden) {
+      result += AnonymousContentType::VerticalScrollbar;
+    }
+  }
+
+  // Check if the frame is resizable. Note:
+  // "The effect of the resize property on generated content is undefined.
+  //  Implementations should not apply the resize property to generated
+  //  content." [1]
+  // For info on what is generated content, see [2].
+  // [1]: https://drafts.csswg.org/css-ui/#resize
+  // [2]: https://www.w3.org/TR/CSS2/generate.html#content
+  auto resizeStyle = StyleDisplay()->mResize;
+  if (resizeStyle != StyleResize::None &&
+      !HasAnyStateBits(NS_FRAME_GENERATED_CONTENT)) {
+    result += AnonymousContentType::Resizer;
+  }
+
+  return result;
+}
+
+nsresult nsHTMLScrollFrame::CreateAnonymousContent(
+    nsTArray<nsIAnonymousContentCreator::ContentInfo>& aElements) {
+  typedef nsIAnonymousContentCreator::ContentInfo ContentInfo;
+
+  nsPresContext* presContext = PresContext();
+  nsNodeInfoManager* nodeInfoManager =
+      presContext->Document()->NodeInfoManager();
+
+  auto neededAnonContent = GetNeededAnonymousContent();
+  if (neededAnonContent.isEmpty()) {
+    return NS_OK;
+  }
+
+  {
+    RefPtr<NodeInfo> nodeInfo = nodeInfoManager->GetNodeInfo(
+        nsGkAtoms::scrollbar, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
+    NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
+
+    if (neededAnonContent.contains(AnonymousContentType::HorizontalScrollbar)) {
+      AnonymousContentKey key;
+      mHScrollbarContent = MakeScrollbar(nodeInfo, /* aVertical */ false, key);
+      aElements.AppendElement(ContentInfo(mHScrollbarContent, key));
+    }
+
+    if (neededAnonContent.contains(AnonymousContentType::VerticalScrollbar)) {
+      AnonymousContentKey key;
+      mVScrollbarContent = MakeScrollbar(nodeInfo, /* aVertical */ true, key);
+      aElements.AppendElement(ContentInfo(mVScrollbarContent, key));
+    }
+  }
+
+  if (neededAnonContent.contains(AnonymousContentType::Resizer)) {
+    MOZ_ASSERT(!mIsRoot, "Root scroll frame shouldn't be resizable");
+
+    RefPtr<NodeInfo> nodeInfo;
+    nodeInfo = nodeInfoManager->GetNodeInfo(
+        nsGkAtoms::resizer, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
+    NS_ENSURE_TRUE(nodeInfo, NS_ERROR_OUT_OF_MEMORY);
+
+    NS_TrustedNewXULElement(getter_AddRefs(mResizerContent), nodeInfo.forget());
+
+    nsAutoString dir;
+    switch (StyleDisplay()->mResize) {
+      case StyleResize::Horizontal:
+        if (IsScrollbarOnRight()) {
+          dir.AssignLiteral("right");
+        } else {
+          dir.AssignLiteral("left");
+        }
+        break;
+      case StyleResize::Vertical:
+        dir.AssignLiteral("bottom");
+        if (!IsScrollbarOnRight()) {
+          mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::flip, u""_ns,
+                                   false);
+        }
+        break;
+      case StyleResize::Both:
+        if (IsScrollbarOnRight()) {
+          dir.AssignLiteral("bottomright");
+        } else {
+          dir.AssignLiteral("bottomleft");
+        }
+        break;
+      default:
+        NS_WARNING("only resizable types should have resizers");
+    }
+    mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::dir, dir, false);
+    aElements.AppendElement(mResizerContent);
+  }
+
+  if (neededAnonContent.contains(AnonymousContentType::HorizontalScrollbar) &&
+      neededAnonContent.contains(AnonymousContentType::VerticalScrollbar)) {
+    AnonymousContentKey key = AnonymousContentKey::Type_ScrollCorner;
+
+    RefPtr<NodeInfo> nodeInfo =
+        nodeInfoManager->GetNodeInfo(nsGkAtoms::scrollcorner, nullptr,
+                                     kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
+    NS_TrustedNewXULElement(getter_AddRefs(mScrollCornerContent),
+                            nodeInfo.forget());
+    if (mIsRoot) {
+      mScrollCornerContent->SetProperty(
+          nsGkAtoms::docLevelNativeAnonymousContent,
+          reinterpret_cast<void*>(true));
+      mScrollCornerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::root_,
+                                    u"true"_ns, false);
+
+      // Don't bother making style caching take [root="true"] styles into
+      // account.
+      key = AnonymousContentKey::None;
+    }
+    aElements.AppendElement(ContentInfo(mScrollCornerContent, key));
+  }
+
+  // Don't cache styles if we are a child of a <select> element, since we have
+  // some UA style sheet rules that depend on the <select>'s attributes.
+  if (GetContent()->IsHTMLElement(nsGkAtoms::select)) {
+    for (auto& info : aElements) {
+      info.mKey = AnonymousContentKey::None;
+    }
+  }
+
+  return NS_OK;
+}
+
+void nsHTMLScrollFrame::AppendAnonymousContentTo(
+    nsTArray<nsIContent*>& aElements, uint32_t aFilter) {
+  if (mHScrollbarContent) {
+    aElements.AppendElement(mHScrollbarContent);
+  }
+
+  if (mVScrollbarContent) {
+    aElements.AppendElement(mVScrollbarContent);
+  }
+
+  if (mScrollCornerContent) {
+    aElements.AppendElement(mScrollCornerContent);
+  }
+
+  if (mResizerContent) {
+    aElements.AppendElement(mResizerContent);
+  }
+}
+
+void nsHTMLScrollFrame::DidSetComputedStyle(ComputedStyle* aOldComputedStyle) {
+  nsContainerFrame::DidSetComputedStyle(aOldComputedStyle);
+  if (aOldComputedStyle && !mIsRoot &&
+      StyleDisplay()->mScrollSnapType !=
+          aOldComputedStyle->StyleDisplay()->mScrollSnapType) {
+    PostPendingResnap();
+  }
+}
+
+void nsHTMLScrollFrame::RemoveObservers() {
+  if (mAsyncScroll) {
+    mAsyncScroll->RemoveObserver();
+    mAsyncScroll = nullptr;
+  }
+  if (mAsyncSmoothMSDScroll) {
+    mAsyncSmoothMSDScroll->RemoveObserver();
+    mAsyncSmoothMSDScroll = nullptr;
+  }
+}
+
+/**
+ * Called when we want to update the scrollbar position, either because
+ * scrolling happened or the user moved the scrollbar position and we need to
+ * undo that (e.g., when the user clicks to scroll and we're using smooth
+ * scrolling, so we need to put the thumb back to its initial position for the
+ * start of the smooth sequence).
+ */
+void nsHTMLScrollFrame::UpdateScrollbarPosition() {
+  AutoWeakFrame weakFrame(this);
+  mFrameIsUpdatingScrollbar = true;
+
+  nsPoint pt = GetScrollPosition();
+  nsRect scrollRange = GetVisualScrollRange();
+
+  if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+    pt = GetVisualViewportOffset();
+    scrollRange = GetScrollRangeForUserInputEvents();
+  }
+
+  if (mVScrollbarBox) {
+    SetCoordAttribute(mVScrollbarBox->GetContent()->AsElement(),
+                      nsGkAtoms::curpos, pt.y - scrollRange.y);
+    if (!weakFrame.IsAlive()) {
+      return;
+    }
+  }
+  if (mHScrollbarBox) {
+    SetCoordAttribute(mHScrollbarBox->GetContent()->AsElement(),
+                      nsGkAtoms::curpos, pt.x - scrollRange.x);
+    if (!weakFrame.IsAlive()) {
+      return;
+    }
+  }
+
+  mFrameIsUpdatingScrollbar = false;
+}
+
+void nsHTMLScrollFrame::CurPosAttributeChangedInternal(nsIContent* aContent,
+                                                       bool aDoScroll) {
+  NS_ASSERTION(aContent, "aContent must not be null");
+  NS_ASSERTION((mHScrollbarBox && mHScrollbarBox->GetContent() == aContent) ||
+                   (mVScrollbarBox && mVScrollbarBox->GetContent() == aContent),
+               "unexpected child");
+  MOZ_ASSERT(aContent->IsElement());
+
+  // Attribute changes on the scrollbars happen in one of three ways:
+  // 1) The scrollbar changed the attribute in response to some user event
+  // 2) We changed the attribute in response to a ScrollPositionDidChange
+  // callback from the scrolling view
+  // 3) We changed the attribute to adjust the scrollbars for the start
+  // of a smooth scroll operation
+  //
+  // In cases 2 and 3 we do not need to scroll because we're just
+  // updating our scrollbar.
+  if (mFrameIsUpdatingScrollbar) {
+    return;
+  }
+
+  nsRect scrollRange = GetVisualScrollRange();
+
+  nsPoint current = GetScrollPosition() - scrollRange.TopLeft();
+
+  if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+    scrollRange = GetScrollRangeForUserInputEvents();
+    current = GetVisualViewportOffset() - scrollRange.TopLeft();
+  }
+
+  nsPoint dest;
+  nsRect allowedRange;
+  dest.x = GetCoordAttribute(mHScrollbarBox, nsGkAtoms::curpos, current.x,
+                             &allowedRange.x, &allowedRange.width);
+  dest.y = GetCoordAttribute(mVScrollbarBox, nsGkAtoms::curpos, current.y,
+                             &allowedRange.y, &allowedRange.height);
+  current += scrollRange.TopLeft();
+  dest += scrollRange.TopLeft();
+  allowedRange += scrollRange.TopLeft();
+
+  // Don't try to scroll if we're already at an acceptable place.
+  // Don't call Contains here since Contains returns false when the point is
+  // on the bottom or right edge of the rectangle.
+  if (allowedRange.ClampPoint(current) == current) {
+    return;
+  }
+
+  if (mScrollbarActivity &&
+      (mHasHorizontalScrollbar || mHasVerticalScrollbar)) {
+    RefPtr<ScrollbarActivity> scrollbarActivity(mScrollbarActivity);
+    scrollbarActivity->ActivityOccurred();
+  }
+
+  const bool isSmooth = aContent->AsElement()->HasAttr(nsGkAtoms::smooth);
+  if (isSmooth) {
+    // Make sure an attribute-setting callback occurs even if the view
+    // didn't actually move yet.  We need to make sure other listeners
+    // see that the scroll position is not (yet) what they thought it
+    // was.
+    AutoWeakFrame weakFrame(this);
+    UpdateScrollbarPosition();
+    if (!weakFrame.IsAlive()) {
+      return;
+    }
+  }
+
+  if (aDoScroll) {
+    ScrollToWithOrigin(dest, &allowedRange,
+                       ScrollOperationParams{
+                           isSmooth ? ScrollMode::Smooth : ScrollMode::Instant,
+                           ScrollOrigin::Scrollbars});
+  }
+  // 'this' might be destroyed here
+}
+
+/* ============= Scroll events ========== */
+
+nsHTMLScrollFrame::ScrollEvent::ScrollEvent(nsHTMLScrollFrame* aHelper,
+                                            bool aDelayed)
+    : Runnable("nsHTMLScrollFrame::ScrollEvent"), mHelper(aHelper) {
+  mHelper->PresContext()->RefreshDriver()->PostScrollEvent(this, aDelayed);
+}
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP
+nsHTMLScrollFrame::ScrollEvent::Run() {
+  if (mHelper) {
+    mHelper->FireScrollEvent();
+  }
+  return NS_OK;
+}
+
+nsHTMLScrollFrame::ScrollEndEvent::ScrollEndEvent(nsHTMLScrollFrame* aHelper)
+    : Runnable("nsHTMLScrollFrame::ScrollEndEvent"), mHelper(aHelper) {
+  mHelper->PresContext()->RefreshDriver()->PostScrollEvent(this);
+}
+
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP
+nsHTMLScrollFrame::ScrollEndEvent::Run() {
+  if (mHelper) {
+    mHelper->FireScrollEndEvent();
+  }
+  return NS_OK;
+}
+
+void nsHTMLScrollFrame::FireScrollEvent() {
+  RefPtr<nsIContent> content = GetContent();
+  RefPtr<nsPresContext> presContext = PresContext();
+  AUTO_PROFILER_TRACING_MARKER_DOCSHELL("Paint", "FireScrollEvent", GRAPHICS,
+                                        presContext->GetDocShell());
+
+  MOZ_ASSERT(mScrollEvent);
+  mScrollEvent->Revoke();
+  mScrollEvent = nullptr;
+
+  // If event handling is suppressed, keep posting the scroll event to the
+  // refresh driver until it is unsuppressed. The event is marked as delayed so
+  // that the refresh driver does not continue ticking.
+  if (content->GetComposedDoc() &&
+      content->GetComposedDoc()->EventHandlingSuppressed()) {
+    content->GetComposedDoc()->SetHasDelayedRefreshEvent();
+    PostScrollEvent(/* aDelayed = */ true);
+    return;
+  }
+
+  bool oldProcessing = mProcessingScrollEvent;
+  AutoWeakFrame weakFrame(this);
+  auto RestoreProcessingScrollEvent = mozilla::MakeScopeExit([&] {
+    if (weakFrame.IsAlive()) {  // Otherwise `this` will be dead too.
+      mProcessingScrollEvent = oldProcessing;
+    }
+  });
+
+  mProcessingScrollEvent = true;
+
+  WidgetGUIEvent event(true, eScroll, nullptr);
+  nsEventStatus status = nsEventStatus_eIgnore;
+  // Fire viewport scroll events at the document (where they
+  // will bubble to the window)
+  mozilla::layers::ScrollLinkedEffectDetector detector(
+      content->GetComposedDoc(),
+      presContext->RefreshDriver()->MostRecentRefresh());
+  if (mIsRoot) {
+    if (RefPtr<Document> doc = content->GetUncomposedDoc()) {
+      EventDispatcher::Dispatch(doc, presContext, &event, nullptr, &status);
+    }
+  } else {
+    // scroll events fired at elements don't bubble (although scroll events
+    // fired at documents do, to the window)
+    event.mFlags.mBubbles = false;
+    EventDispatcher::Dispatch(content, presContext, &event, nullptr, &status);
+  }
+}
+
+void nsHTMLScrollFrame::PostScrollEvent(bool aDelayed) {
+  if (mScrollEvent) {
+    return;
+  }
+
+  // The ScrollEvent constructor registers itself with the refresh driver.
+  mScrollEvent = new ScrollEvent(this, aDelayed);
+}
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP
+nsHTMLScrollFrame::AsyncScrollPortEvent::Run() {
+  return mHelper ? mHelper->FireScrollPortEvent() : NS_OK;
+}
+
+void nsHTMLScrollFrame::PostOverflowEvent() {
+  if (mAsyncScrollPortEvent.IsPending()) {
+    return;
+  }
+
+  OverflowState overflowState = GetOverflowState();
+
+  bool newVerticalOverflow = !!(overflowState & OverflowState::Vertical);
+  bool vertChanged = mVerticalOverflow != newVerticalOverflow;
+
+  bool newHorizontalOverflow = !!(overflowState & OverflowState::Horizontal);
+  bool horizChanged = mHorizontalOverflow != newHorizontalOverflow;
+
+  if (!vertChanged && !horizChanged) {
+    return;
+  }
+
+  nsRootPresContext* rpc = PresContext()->GetRootPresContext();
+  if (!rpc) {
+    return;
+  }
+
+  mAsyncScrollPortEvent = new AsyncScrollPortEvent(this);
+  rpc->AddWillPaintObserver(mAsyncScrollPortEvent.get());
+}
+
+nsIFrame* nsHTMLScrollFrame::GetFrameForStyle() const {
+  nsIFrame* styleFrame = nullptr;
+  if (mIsRoot) {
+    if (const Element* rootElement =
+            PresContext()->Document()->GetRootElement()) {
+      styleFrame = rootElement->GetPrimaryFrame();
+    }
+  } else {
+    styleFrame = const_cast<nsHTMLScrollFrame*>(this);
+  }
+
+  return styleFrame;
+}
+
+bool nsHTMLScrollFrame::NeedsScrollSnap() const {
+  nsIFrame* scrollSnapFrame = GetFrameForStyle();
+  if (!scrollSnapFrame) {
+    return false;
+  }
+  return scrollSnapFrame->StyleDisplay()->mScrollSnapType.strictness !=
+         StyleScrollSnapStrictness::None;
+}
+
+nsSize nsHTMLScrollFrame::GetSnapportSize() const {
+  nsRect snapport = GetScrollPortRect();
+  nsMargin scrollPadding = GetScrollPadding();
+  snapport.Deflate(scrollPadding);
+  return snapport.Size();
+}
+
+bool nsHTMLScrollFrame::IsScrollbarOnRight() const {
+  // The position of the scrollbar in top-level windows depends on the pref
+  // layout.scrollbar.side. For non-top-level elements, it depends only on the
+  // directionaliy of the element (equivalent to a value of "1" for the pref).
+  if (!mIsRoot) {
+    return IsPhysicalLTR();
+  }
+  switch (StaticPrefs::layout_scrollbar_side()) {
+    default:
+    case 0:  // UI directionality
+      return StaticPrefs::bidi_direction() == IBMBIDI_TEXTDIRECTION_LTR;
+    case 1:  // Document / content directionality
+      return IsPhysicalLTR();
+    case 2:  // Always right
+      return true;
+    case 3:  // Always left
+      return false;
+  }
+}
+
+bool nsHTMLScrollFrame::IsScrollingActive() const {
+  const nsStyleDisplay* disp = StyleDisplay();
+  if (disp->mWillChange.bits & StyleWillChangeBits::SCROLL) {
+    return true;
+  }
+
+  nsIContent* content = GetContent();
+  return mHasBeenScrolledRecently || IsAlwaysActive() ||
+         DisplayPortUtils::HasDisplayPort(content) ||
+         nsContentUtils::HasScrollgrab(content);
+}
+
+void nsHTMLScrollFrame::FinishReflowForScrollbar(Element* aElement,
+                                                 nscoord aMinXY, nscoord aMaxXY,
+                                                 nscoord aCurPosXY,
+                                                 nscoord aPageIncrement,
+                                                 nscoord aIncrement) {
+  // Scrollbars assume zero is the minimum position, so translate for them.
+  SetCoordAttribute(aElement, nsGkAtoms::curpos, aCurPosXY - aMinXY);
+  SetScrollbarEnabled(aElement, aMaxXY - aMinXY);
+  SetCoordAttribute(aElement, nsGkAtoms::maxpos, aMaxXY - aMinXY);
+  SetCoordAttribute(aElement, nsGkAtoms::pageincrement, aPageIncrement);
+  SetCoordAttribute(aElement, nsGkAtoms::increment, aIncrement);
+}
+
+class MOZ_RAII AutoMinimumScaleSizeChangeDetector final {
+ public:
+  explicit AutoMinimumScaleSizeChangeDetector(
+      nsHTMLScrollFrame* ansHTMLScrollFrame)
+      : mHelper(ansHTMLScrollFrame) {
+    MOZ_ASSERT(mHelper);
+    MOZ_ASSERT(mHelper->mIsRoot);
+
+    mPreviousMinimumScaleSize = ansHTMLScrollFrame->mMinimumScaleSize;
+    mPreviousIsUsingMinimumScaleSize =
+        ansHTMLScrollFrame->mIsUsingMinimumScaleSize;
+  }
+  ~AutoMinimumScaleSizeChangeDetector() {
+    if (mPreviousMinimumScaleSize != mHelper->mMinimumScaleSize ||
+        mPreviousIsUsingMinimumScaleSize != mHelper->mIsUsingMinimumScaleSize) {
+      mHelper->mMinimumScaleSizeChanged = true;
+    }
+  }
+
+ private:
+  nsHTMLScrollFrame* mHelper;
+
+  nsSize mPreviousMinimumScaleSize;
+  bool mPreviousIsUsingMinimumScaleSize;
+};
+
+nsSize nsHTMLScrollFrame::TrueOuterSize(nsDisplayListBuilder* aBuilder) const {
+  if (!PresShell()->UsesMobileViewportSizing()) {
+    return GetSize();
+  }
+
+  RefPtr<MobileViewportManager> manager =
+      PresShell()->GetMobileViewportManager();
+  MOZ_ASSERT(manager);
+
+  LayoutDeviceIntSize displaySize = manager->DisplaySize();
+
+  MOZ_ASSERT(aBuilder);
+  // In case of WebRender, we expand the outer size to include the dynamic
+  // toolbar area here.
+  // In case of non WebRender, we expand the size dynamically in
+  // MoveScrollbarForLayerMargin in AsyncCompositionManager.cpp.
+  WebRenderLayerManager* layerManager = aBuilder->GetWidgetLayerManager();
+  if (layerManager) {
+    displaySize.height += ViewAs<LayoutDevicePixel>(
+        PresContext()->GetDynamicToolbarMaxHeight(),
+        PixelCastJustification::LayoutDeviceIsScreenForBounds);
+  }
+
+  return LayoutDeviceSize::ToAppUnits(displaySize,
+                                      PresContext()->AppUnitsPerDevPixel());
+}
+
+void nsHTMLScrollFrame::UpdateMinimumScaleSize(
+    const nsRect& aScrollableOverflow, const nsSize& aICBSize) {
+  MOZ_ASSERT(mIsRoot);
+
+  AutoMinimumScaleSizeChangeDetector minimumScaleSizeChangeDetector(this);
+
+  mIsUsingMinimumScaleSize = false;
+
+  if (!PresShell()->UsesMobileViewportSizing()) {
+    return;
+  }
+
+  nsPresContext* pc = PresContext();
+  MOZ_ASSERT(pc->IsRootContentDocumentCrossProcess(),
+             "The pres context should be for the root content document");
+
+  RefPtr<MobileViewportManager> manager =
+      PresShell()->GetMobileViewportManager();
+  MOZ_ASSERT(manager);
+
+  ScreenIntSize displaySize = ViewAs<ScreenPixel>(
+      manager->DisplaySize(),
+      PixelCastJustification::LayoutDeviceIsScreenForBounds);
+  if (displaySize.width == 0 || displaySize.height == 0) {
+    return;
+  }
+  if (aScrollableOverflow.IsEmpty()) {
+    // Bail if the scrollable overflow rect is empty, as we're going to be
+    // dividing by it.
+    return;
+  }
+
+  Document* doc = pc->Document();
+  MOZ_ASSERT(doc, "The document should be valid");
+  if (doc->GetFullscreenElement()) {
+    // Don't use the minimum scale size in the case of fullscreen state.
+    // FIXME: 1508177: We will no longer need this.
+    return;
+  }
+
+  nsViewportInfo viewportInfo = doc->GetViewportInfo(displaySize);
+  if (!viewportInfo.IsZoomAllowed()) {
+    // Don't apply the minimum scale size if user-scalable=no is specified.
+    return;
+  }
+
+  // The intrinsic minimum scale is the scale that fits the entire content
+  // width into the visual viewport.
+  CSSToScreenScale intrinsicMinScale(
+      displaySize.width / CSSRect::FromAppUnits(aScrollableOverflow).XMost());
+
+  // The scale used to compute the minimum-scale size is the larger of the
+  // intrinsic minimum and the min-scale from the meta viewport tag.
+  CSSToScreenScale minScale =
+      std::max(intrinsicMinScale, viewportInfo.GetMinZoom());
+
+  // The minimum-scale size is the size of the visual viewport when zoomed
+  // to be the minimum scale.
+  mMinimumScaleSize = CSSSize::ToAppUnits(ScreenSize(displaySize) / minScale);
+
+  // Ensure the minimum-scale size is never smaller than the ICB size.
+  // That could happen if a page has a meta viewport tag with large explicitly
+  // specified viewport dimensions (making the ICB large) and also a large
+  // minimum scale (making the min-scale size small).
+  mMinimumScaleSize = Max(aICBSize, mMinimumScaleSize);
+
+  mIsUsingMinimumScaleSize = true;
+}
+
+bool nsHTMLScrollFrame::ReflowFinished() {
+  mPostedReflowCallback = false;
+
+  TryScheduleScrollAnimations();
+
+  if (mIsRoot) {
+    if (mMinimumScaleSizeChanged && PresShell()->UsesMobileViewportSizing() &&
+        !PresShell()->IsResolutionUpdatedByApz()) {
+      RefPtr<MobileViewportManager> manager =
+          PresShell()->GetMobileViewportManager();
+      MOZ_ASSERT(manager);
+
+      manager->ShrinkToDisplaySizeIfNeeded();
+      mMinimumScaleSizeChanged = false;
+    }
+
+    if (!UsesOverlayScrollbars()) {
+      // Layout scrollbars may have added or removed during reflow, so let's
+      // update the visual viewport accordingly. Note that this may be a no-op
+      // because we might have recomputed the visual viewport size during the
+      // reflow itself, just before laying out the fixed-pos items. But there
+      // might be cases where that code doesn't run, so this is a sort of
+      // backstop to ensure we do that recomputation.
+      if (RefPtr<MobileViewportManager> manager =
+              PresShell()->GetMobileViewportManager()) {
+        manager->UpdateVisualViewportSizeForPotentialScrollbarChange();
+      }
+    }
+
+#if defined(MOZ_WIDGET_ANDROID)
+    const bool hasVerticalOverflow =
+        GetOverflowState() & OverflowState::Vertical &&
+        GetScrollStyles().mVertical != StyleOverflow::Hidden;
+    if (!mFirstReflow && mHasVerticalOverflowForDynamicToolbar &&
+        !hasVerticalOverflow) {
+      PresShell()->MaybeNotifyShowDynamicToolbar();
+    }
+    mHasVerticalOverflowForDynamicToolbar = hasVerticalOverflow;
+#endif  // defined(MOZ_WIDGET_ANDROID)
+  }
+
+  bool doScroll = true;
+  if (IsSubtreeDirty()) {
+    // We will get another call after the next reflow and scrolling
+    // later is less janky.
+    doScroll = false;
+  }
+
+  if (mFirstReflow) {
+    nsPoint currentScrollPos = GetScrollPosition();
+    if (!mScrollUpdates.IsEmpty() &&
+        mScrollUpdates.LastElement().GetOrigin() == ScrollOrigin::None &&
+        currentScrollPos != nsPoint()) {
+      // With frame reconstructions, the reconstructed frame may have a nonzero
+      // scroll position by the end of the reflow, but without going through
+      // RestoreState. In particular this can happen with RTL XUL scrollframes,
+      // see https://bugzilla.mozilla.org/show_bug.cgi?id=1664638#c14.
+      // Upon construction, the nsHTMLScrollFrame constructor will have inserted
+      // a ScrollPositionUpdate into mScrollUpdates with origin None and a zero
+      // scroll position, but here we update that to hold the correct scroll
+      // position. Otherwise APZ may end up resetting the scroll position to
+      // zero incorrectly. If we ever hit this codepath, it must be on a reflow
+      // immediately following the scrollframe construction, so there should be
+      // exactly one ScrollPositionUpdate in mScrollUpdates.
+      MOZ_ASSERT(mScrollUpdates.Length() == 1);
+      MOZ_ASSERT(mScrollUpdates.LastElement().GetGeneration() ==
+                 mScrollGeneration);
+      MOZ_ASSERT(mScrollUpdates.LastElement().GetDestination() == CSSPoint());
+      SCROLLRESTORE_LOG("%p: updating initial SPU to pos %s\n", this,
+                        ToString(currentScrollPos).c_str());
+      mScrollUpdates.Clear();
+      AppendScrollUpdate(
+          ScrollPositionUpdate::NewScrollframe(currentScrollPos));
+    }
+
+    mFirstReflow = false;
+  }
+
+  nsAutoScriptBlocker scriptBlocker;
+
+  if (mReclampVVOffsetInReflowFinished) {
+    MOZ_ASSERT(mIsRoot && PresShell()->IsVisualViewportOffsetSet());
+    mReclampVVOffsetInReflowFinished = false;
+    AutoWeakFrame weakFrame(this);
+    PresShell()->SetVisualViewportOffset(PresShell()->GetVisualViewportOffset(),
+                                         GetScrollPosition());
+    NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
+  }
+
+  if (doScroll) {
+    ScrollToRestoredPosition();
+
+    // Clamp current scroll position to new bounds. Normally this won't
+    // do anything.
+    nsPoint currentScrollPos = GetScrollPosition();
+    ScrollToImpl(currentScrollPos, nsRect(currentScrollPos, nsSize(0, 0)),
+                 ScrollOrigin::Clamp);
+    if (ScrollAnimationState().isEmpty()) {
+      // We need to have mDestination track the current scroll position,
+      // in case it falls outside the new reflow area. mDestination is used
+      // by ScrollBy as its starting position.
+      mDestination = GetScrollPosition();
+    }
+  }
+
+  if (!mUpdateScrollbarAttributes) {
+    return false;
+  }
+  mUpdateScrollbarAttributes = false;
+
+  // Update scrollbar attributes.
+  if (mMayHaveDirtyFixedChildren) {
+    mMayHaveDirtyFixedChildren = false;
+    nsIFrame* parentFrame = GetParent();
+    for (nsIFrame* fixedChild =
+             parentFrame->GetChildList(FrameChildListID::Fixed).FirstChild();
+         fixedChild; fixedChild = fixedChild->GetNextSibling()) {
+      // force a reflow of the fixed child
+      PresShell()->FrameNeedsReflow(fixedChild, IntrinsicDirty::None,
+                                    NS_FRAME_HAS_DIRTY_CHILDREN);
+    }
+  }
+
+  // Suppress handling of the curpos attribute changes we make here.
+  NS_ASSERTION(!mFrameIsUpdatingScrollbar, "We shouldn't be reentering here");
+  mFrameIsUpdatingScrollbar = true;
+
+  // FIXME(emilio): Why this instead of mHScrollbarContent / mVScrollbarContent?
+  RefPtr<Element> vScroll =
+      mVScrollbarBox ? mVScrollbarBox->GetContent()->AsElement() : nullptr;
+  RefPtr<Element> hScroll =
+      mHScrollbarBox ? mHScrollbarBox->GetContent()->AsElement() : nullptr;
+
+  // Note, in some cases this may get deleted while finishing reflow
+  // for scrollbars. XXXmats is this still true now that we have a script
+  // blocker in this scope? (if not, remove the weak frame checks below).
+  if (vScroll || hScroll) {
+    nsSize visualViewportSize = GetVisualViewportSize();
+    nsRect scrollRange = GetVisualScrollRange();
+    nsPoint scrollPos = GetScrollPosition();
+    nsSize lineScrollAmount = GetLineScrollAmount();
+
+    if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+      scrollRange = GetScrollRangeForUserInputEvents();
+      scrollPos = GetVisualViewportOffset();
+    }
+
+    // If modifying the logic here, be sure to modify the corresponding
+    // compositor-side calculation in ScrollThumbUtils::ApplyTransformForAxis().
+    AutoWeakFrame weakFrame(this);
+    if (vScroll) {
+      const double kScrollMultiplier =
+          StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+      nscoord increment = lineScrollAmount.height * kScrollMultiplier;
+      // We normally use (visualViewportSize.height - increment) for height of
+      // page scrolling.  However, it is too small when increment is very large.
+      // (If increment is larger than visualViewportSize.height, direction of
+      // scrolling will be opposite). To avoid it, we use
+      // (float(visualViewportSize.height) * 0.8) as lower bound value of height
+      // of page scrolling. (bug 383267)
+      // XXX shouldn't we use GetPageScrollAmount here?
+      nscoord pageincrement = nscoord(visualViewportSize.height - increment);
+      nscoord pageincrementMin =
+          nscoord(float(visualViewportSize.height) * 0.8);
+      FinishReflowForScrollbar(
+          vScroll, scrollRange.y, scrollRange.YMost(), scrollPos.y,
+          std::max(pageincrement, pageincrementMin), increment);
+    }
+    if (hScroll) {
+      const double kScrollMultiplier =
+          StaticPrefs::toolkit_scrollbox_horizontalScrollDistance();
+      nscoord increment = lineScrollAmount.width * kScrollMultiplier;
+      FinishReflowForScrollbar(
+          hScroll, scrollRange.x, scrollRange.XMost(), scrollPos.x,
+          nscoord(float(visualViewportSize.width) * 0.8), increment);
+    }
+    NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
+  }
+
+  mFrameIsUpdatingScrollbar = false;
+  // We used to rely on the curpos attribute changes above to scroll the
+  // view.  However, for scrolling to the left of the viewport, we
+  // rescale the curpos attribute, which means that operations like
+  // resizing the window while it is scrolled all the way to the left
+  // hold the curpos attribute constant at 0 while still requiring
+  // scrolling.  So we suppress the effect of the changes above with
+  // mFrameIsUpdatingScrollbar and call CurPosAttributeChanged here.
+  // (It actually even works some of the time without this, thanks to
+  // nsSliderFrame::AttributeChanged's handling of maxpos, but not when
+  // we hide the scrollbar on a large size change, such as
+  // maximization.)
+  if (!mHScrollbarBox && !mVScrollbarBox) {
+    return false;
+  }
+  CurPosAttributeChangedInternal(
+      mVScrollbarBox ? mVScrollbarBox->GetContent()->AsElement()
+                     : mHScrollbarBox->GetContent()->AsElement(),
+      doScroll);
+  return doScroll;
+}
+
+void nsHTMLScrollFrame::ReflowCallbackCanceled() {
+  mPostedReflowCallback = false;
+}
+
+bool nsHTMLScrollFrame::ComputeCustomOverflow(OverflowAreas& aOverflowAreas) {
+  ScrollStyles ss = GetScrollStyles();
+
+  // Reflow when the change in overflow leads to one of our scrollbars
+  // changing or might require repositioning the scrolled content due to
+  // reduced extents.
+  nsRect scrolledRect = GetScrolledRect();
+  ScrollDirections overflowChange =
+      GetOverflowChange(scrolledRect, mPrevScrolledRect);
+  mPrevScrolledRect = scrolledRect;
+
+  bool needReflow = false;
+  nsPoint scrollPosition = GetScrollPosition();
+  if (overflowChange.contains(ScrollDirection::eHorizontal)) {
+    if (ss.mHorizontal != StyleOverflow::Hidden || scrollPosition.x) {
+      needReflow = true;
+    }
+  }
+  if (overflowChange.contains(ScrollDirection::eVertical)) {
+    if (ss.mVertical != StyleOverflow::Hidden || scrollPosition.y) {
+      needReflow = true;
+    }
+  }
+
+  if (needReflow) {
+    // If there are scrollbars, or we're not at the beginning of the pane,
+    // the scroll position may change. In this case, mark the frame as
+    // needing reflow. Don't use NS_FRAME_IS_DIRTY as dirty as that means
+    // we have to reflow the frame and all its descendants, and we don't
+    // have to do that here. Only this frame needs to be reflowed.
+    PresShell()->FrameNeedsReflow(this, IntrinsicDirty::None,
+                                  NS_FRAME_HAS_DIRTY_CHILDREN);
+    // Ensure that next time nsHTMLScrollFrame::Reflow runs, we don't skip
+    // updating the scrollbars. (Because the overflow area of the scrolled
+    // frame has probably just been updated, Reflow won't see it change.)
+    mSkippedScrollbarLayout = true;
+    return false;  // reflowing will update overflow
+  }
+  PostOverflowEvent();
+  return nsContainerFrame::ComputeCustomOverflow(aOverflowAreas);
+}
+
+void nsHTMLScrollFrame::UpdateSticky() {
+  StickyScrollContainer* ssc =
+      StickyScrollContainer::GetStickyScrollContainerForScrollFrame(this);
+  if (ssc) {
+    ssc->UpdatePositions(GetScrollPosition(), this);
+  }
+}
+
+void nsHTMLScrollFrame::UpdatePrevScrolledRect() {
+  // The layout scroll range is determinated by the scrolled rect and the scroll
+  // port, so if the scrolled rect is updated, we may have to schedule the
+  // associated scroll-driven animations' restyles.
+  nsRect currScrolledRect = GetScrolledRect();
+  if (!currScrolledRect.IsEqualEdges(mPrevScrolledRect)) {
+    mMayScheduleScrollAnimations = true;
+  }
+  mPrevScrolledRect = currScrolledRect;
+}
+
+void nsHTMLScrollFrame::AdjustScrollbarRectForResizer(
+    nsIFrame* aFrame, nsPresContext* aPresContext, nsRect& aRect,
+    bool aHasResizer, ScrollDirection aDirection) {
+  if ((aDirection == ScrollDirection::eVertical ? aRect.width : aRect.height) ==
+      0) {
+    return;
+  }
+
+  // if a content resizer is present, use its size. Otherwise, check if the
+  // widget has a resizer.
+  nsRect resizerRect;
+  if (aHasResizer) {
+    resizerRect = mResizerBox->GetRect();
+  } else {
+    nsPoint offset;
+    nsIWidget* widget = aFrame->GetNearestWidget(offset);
+    LayoutDeviceIntRect widgetRect;
+    if (!widget || !widget->ShowsResizeIndicator(&widgetRect)) {
+      return;
+    }
+
+    resizerRect =
+        nsRect(aPresContext->DevPixelsToAppUnits(widgetRect.x) - offset.x,
+               aPresContext->DevPixelsToAppUnits(widgetRect.y) - offset.y,
+               aPresContext->DevPixelsToAppUnits(widgetRect.width),
+               aPresContext->DevPixelsToAppUnits(widgetRect.height));
+  }
+
+  if (resizerRect.Contains(aRect.BottomRight() - nsPoint(1, 1))) {
+    switch (aDirection) {
+      case ScrollDirection::eVertical:
+        aRect.height = std::max(0, resizerRect.y - aRect.y);
+        break;
+      case ScrollDirection::eHorizontal:
+        aRect.width = std::max(0, resizerRect.x - aRect.x);
+        break;
+    }
+  } else if (resizerRect.Contains(aRect.BottomLeft() + nsPoint(1, -1))) {
+    switch (aDirection) {
+      case ScrollDirection::eVertical:
+        aRect.height = std::max(0, resizerRect.y - aRect.y);
+        break;
+      case ScrollDirection::eHorizontal: {
+        nscoord xmost = aRect.XMost();
+        aRect.x = std::max(aRect.x, resizerRect.XMost());
+        aRect.width = xmost - aRect.x;
+        break;
+      }
+    }
+  }
+}
+
+static void AdjustOverlappingScrollbars(nsRect& aVRect, nsRect& aHRect) {
+  if (aVRect.IsEmpty() || aHRect.IsEmpty()) return;
+
+  const nsRect oldVRect = aVRect;
+  const nsRect oldHRect = aHRect;
+  if (oldVRect.Contains(oldHRect.BottomRight() - nsPoint(1, 1))) {
+    aHRect.width = std::max(0, oldVRect.x - oldHRect.x);
+  } else if (oldVRect.Contains(oldHRect.BottomLeft() - nsPoint(0, 1))) {
+    nscoord overlap = std::min(oldHRect.width, oldVRect.XMost() - oldHRect.x);
+    aHRect.x += overlap;
+    aHRect.width -= overlap;
+  }
+  if (oldHRect.Contains(oldVRect.BottomRight() - nsPoint(1, 1))) {
+    aVRect.height = std::max(0, oldHRect.y - oldVRect.y);
+  }
+}
+
+void nsHTMLScrollFrame::LayoutScrollbarPartAtRect(
+    const ScrollReflowInput& aState, ReflowInput& aKidReflowInput,
+    const nsRect& aRect) {
+  nsPresContext* pc = PresContext();
+  nsIFrame* kid = aKidReflowInput.mFrame;
+  const auto wm = kid->GetWritingMode();
+  ReflowOutput desiredSize(wm);
+  MOZ_ASSERT(!wm.IsVertical(),
+             "Scrollbar parts should have writing-mode: initial");
+  MOZ_ASSERT(!wm.IsInlineReversed(),
+             "Scrollbar parts should have writing-mode: initial");
+  // XXX Maybe get a meaningful container size or something. Shouldn't matter
+  // given our asserts above.
+  const nsSize containerSize;
+  aKidReflowInput.SetComputedISize(aRect.Width());
+  aKidReflowInput.SetComputedBSize(aRect.Height());
+
+  const LogicalPoint pos(wm, aRect.TopLeft(), containerSize);
+  const auto flags = ReflowChildFlags::Default;
+  nsReflowStatus status;
+  ReflowOutput kidDesiredSize(wm);
+  ReflowChild(kid, pc, kidDesiredSize, aKidReflowInput, wm, pos, containerSize,
+              flags, status);
+  FinishReflowChild(kid, pc, kidDesiredSize, &aKidReflowInput, wm, pos,
+                    containerSize, flags);
+}
+
+void nsHTMLScrollFrame::LayoutScrollbars(ScrollReflowInput& aState,
+                                         const nsRect& aInsideBorderArea,
+                                         const nsRect& aOldScrollPort) {
+  NS_ASSERTION(!mSuppressScrollbarUpdate, "This should have been suppressed");
+
+  const bool scrollbarOnLeft = !IsScrollbarOnRight();
+  const bool overlayScrollbars = UsesOverlayScrollbars();
+  const bool overlayScrollBarsOnRoot = overlayScrollbars && mIsRoot;
+  const bool showVScrollbar = mVScrollbarBox && mHasVerticalScrollbar;
+  const bool showHScrollbar = mHScrollbarBox && mHasHorizontalScrollbar;
+
+  nsSize compositionSize = mScrollPort.Size();
+  if (overlayScrollBarsOnRoot) {
+    compositionSize = nsLayoutUtils::CalculateCompositionSizeForFrame(
+        this, false, &compositionSize);
+  }
+
+  nsPresContext* presContext = mScrolledFrame->PresContext();
+  nsRect vRect;
+  if (showVScrollbar) {
+    vRect.height =
+        overlayScrollBarsOnRoot ? compositionSize.height : mScrollPort.height;
+    vRect.y = mScrollPort.y;
+    if (scrollbarOnLeft) {
+      vRect.width = mScrollPort.x - aInsideBorderArea.x;
+      vRect.x = aInsideBorderArea.x;
+    } else {
+      vRect.width = aInsideBorderArea.XMost() - mScrollPort.XMost();
+      vRect.x = mScrollPort.x + compositionSize.width;
+    }
+    if (overlayScrollbars || mOnlyNeedVScrollbarToScrollVVInsideLV) {
+      const nscoord width = aState.VScrollbarPrefWidth();
+      // There is no space reserved for the layout scrollbar, it is currently
+      // not visible because it is positioned just outside the scrollport. But
+      // we know that it needs to be made visible so we shift it back in.
+      vRect.width += width;
+      if (!scrollbarOnLeft) {
+        vRect.x -= width;
+      }
+    }
+  }
+
+  nsRect hRect;
+  if (showHScrollbar) {
+    hRect.width =
+        overlayScrollBarsOnRoot ? compositionSize.width : mScrollPort.width;
+    hRect.x = mScrollPort.x;
+    hRect.height = aInsideBorderArea.YMost() - mScrollPort.YMost();
+    hRect.y = mScrollPort.y + compositionSize.height;
+
+    if (overlayScrollbars || mOnlyNeedHScrollbarToScrollVVInsideLV) {
+      const nscoord height = aState.HScrollbarPrefHeight();
+      hRect.height += height;
+      // There is no space reserved for the layout scrollbar, it is currently
+      // not visible because it is positioned just outside the scrollport. But
+      // we know that it needs to be made visible so we shift it back in.
+      hRect.y -= height;
+    }
+  }
+
+  const bool hasVisualOnlyScrollbarsOnBothDirections =
+      !overlayScrollbars && showHScrollbar &&
+      mOnlyNeedHScrollbarToScrollVVInsideLV && showVScrollbar &&
+      mOnlyNeedVScrollbarToScrollVVInsideLV;
+  nsPresContext* pc = PresContext();
+
+  // place the scrollcorner
+  if (mScrollCornerBox) {
+    nsRect r(0, 0, 0, 0);
+    if (scrollbarOnLeft) {
+      // scrollbar (if any) on left
+      r.width = showVScrollbar ? mScrollPort.x - aInsideBorderArea.x : 0;
+      r.x = aInsideBorderArea.x;
+    } else {
+      // scrollbar (if any) on right
+      r.width =
+          showVScrollbar ? aInsideBorderArea.XMost() - mScrollPort.XMost() : 0;
+      r.x = aInsideBorderArea.XMost() - r.width;
+    }
+    NS_ASSERTION(r.width >= 0, "Scroll area should be inside client rect");
+
+    if (showHScrollbar) {
+      // scrollbar (if any) on bottom
+      // Note we don't support the horizontal scrollbar at the top side.
+      r.height = aInsideBorderArea.YMost() - mScrollPort.YMost();
+      NS_ASSERTION(r.height >= 0, "Scroll area should be inside client rect");
+    }
+    r.y = aInsideBorderArea.YMost() - r.height;
+
+    // If we have layout scrollbars and both scrollbars are present and both are
+    // only needed to scroll the VV inside the LV then we need a scrollcorner
+    // but the above calculation will result in an empty rect, so adjust it.
+    if (r.IsEmpty() && hasVisualOnlyScrollbarsOnBothDirections) {
+      r.width = vRect.width;
+      r.height = hRect.height;
+      r.x = scrollbarOnLeft ? mScrollPort.x : mScrollPort.XMost() - r.width;
+      r.y = mScrollPort.YMost() - r.height;
+    }
+
+    ReflowInput scrollCornerRI(
+        pc, aState.mReflowInput, mScrollCornerBox,
+        LogicalSize(mScrollCornerBox->GetWritingMode(), r.Size()));
+    LayoutScrollbarPartAtRect(aState, scrollCornerRI, r);
+  }
+
+  if (mResizerBox) {
+    // If a resizer is present, get its size.
+    //
+    // TODO(emilio): Should this really account for scrollbar-width?
+    auto scrollbarWidth = nsLayoutUtils::StyleForScrollbar(this)
+                              ->StyleUIReset()
+                              ->ScrollbarWidth();
+    const nscoord scrollbarSize =
+        GetNonOverlayScrollbarSize(pc, scrollbarWidth);
+    ReflowInput resizerRI(pc, aState.mReflowInput, mResizerBox,
+                          LogicalSize(mResizerBox->GetWritingMode()));
+    nsSize resizerMinSize = {resizerRI.ComputedMinWidth(),
+                             resizerRI.ComputedMinHeight()};
+
+    nsRect r;
+    r.width = std::max(std::max(r.width, scrollbarSize), resizerMinSize.width);
+    r.x = scrollbarOnLeft ? aInsideBorderArea.x
+                          : aInsideBorderArea.XMost() - r.width;
+    r.height =
+        std::max(std::max(r.height, scrollbarSize), resizerMinSize.height);
+    r.y = aInsideBorderArea.YMost() - r.height;
+
+    LayoutScrollbarPartAtRect(aState, resizerRI, r);
+  }
+
+  // Note that AdjustScrollbarRectForResizer has to be called after the
+  // resizer has been laid out immediately above this because it gets the rect
+  // of the resizer frame.
+  if (mVScrollbarBox) {
+    AdjustScrollbarRectForResizer(this, presContext, vRect, mResizerBox,
+                                  ScrollDirection::eVertical);
+  }
+  if (mHScrollbarBox) {
+    AdjustScrollbarRectForResizer(this, presContext, hRect, mResizerBox,
+                                  ScrollDirection::eHorizontal);
+  }
+
+  // Layout scrollbars can overlap at this point if they are both present and
+  // both only needed to scroll the VV inside the LV.
+  if (!LookAndFeel::GetInt(LookAndFeel::IntID::AllowOverlayScrollbarsOverlap) ||
+      hasVisualOnlyScrollbarsOnBothDirections) {
+    AdjustOverlappingScrollbars(vRect, hRect);
+  }
+  if (mVScrollbarBox) {
+    ReflowInput vScrollbarRI(
+        pc, aState.mReflowInput, mVScrollbarBox,
+        LogicalSize(mVScrollbarBox->GetWritingMode(), vRect.Size()));
+    LayoutScrollbarPartAtRect(aState, vScrollbarRI, vRect);
+  }
+  if (mHScrollbarBox) {
+    ReflowInput hScrollbarRI(
+        pc, aState.mReflowInput, mHScrollbarBox,
+        LogicalSize(mHScrollbarBox->GetWritingMode(), hRect.Size()));
+    LayoutScrollbarPartAtRect(aState, hScrollbarRI, hRect);
+  }
+
+  // may need to update fixed position children of the viewport,
+  // if the client area changed size because of an incremental
+  // reflow of a descendant.  (If the outer frame is dirty, the fixed
+  // children will be re-laid out anyway)
+  if (aOldScrollPort.Size() != mScrollPort.Size() &&
+      !HasAnyStateBits(NS_FRAME_IS_DIRTY) && mIsRoot) {
+    mMayHaveDirtyFixedChildren = true;
+  }
+
+  // post reflow callback to modify scrollbar attributes
+  mUpdateScrollbarAttributes = true;
+  if (!mPostedReflowCallback) {
+    PresShell()->PostReflowCallback(this);
+    mPostedReflowCallback = true;
+  }
+}
+
+#if DEBUG
+static bool ShellIsAlive(nsWeakPtr& aWeakPtr) {
+  RefPtr<PresShell> presShell = do_QueryReferent(aWeakPtr);
+  return !!presShell;
+}
+#endif
+
+void nsHTMLScrollFrame::SetScrollbarEnabled(Element* aElement,
+                                            nscoord aMaxPos) {
+  DebugOnly<nsWeakPtr> weakShell(do_GetWeakReference(PresShell()));
+  if (aMaxPos) {
+    aElement->UnsetAttr(kNameSpaceID_None, nsGkAtoms::disabled, true);
+  } else {
+    aElement->SetAttr(kNameSpaceID_None, nsGkAtoms::disabled, u"true"_ns, true);
+  }
+  MOZ_ASSERT(ShellIsAlive(weakShell), "pres shell was destroyed by scrolling");
+}
+
+void nsHTMLScrollFrame::SetCoordAttribute(Element* aElement, nsAtom* aAtom,
+                                          nscoord aSize) {
+  DebugOnly<nsWeakPtr> weakShell(do_GetWeakReference(PresShell()));
+  // convert to pixels
+  int32_t pixelSize = nsPresContext::AppUnitsToIntCSSPixels(aSize);
+
+  // only set the attribute if it changed.
+
+  nsAutoString newValue;
+  newValue.AppendInt(pixelSize);
+
+  if (aElement->AttrValueIs(kNameSpaceID_None, aAtom, newValue, eCaseMatters)) {
+    return;
+  }
+
+  AutoWeakFrame weakFrame(this);
+  RefPtr<Element> kungFuDeathGrip = aElement;
+  aElement->SetAttr(kNameSpaceID_None, aAtom, newValue, true);
+  MOZ_ASSERT(ShellIsAlive(weakShell), "pres shell was destroyed by scrolling");
+  if (!weakFrame.IsAlive()) {
+    return;
+  }
+
+  if (mScrollbarActivity &&
+      (mHasHorizontalScrollbar || mHasVerticalScrollbar)) {
+    RefPtr<ScrollbarActivity> scrollbarActivity(mScrollbarActivity);
+    scrollbarActivity->ActivityOccurred();
+  }
+}
+
+static void ReduceRadii(nscoord aXBorder, nscoord aYBorder, nscoord& aXRadius,
+                        nscoord& aYRadius) {
+  // In order to ensure that the inside edge of the border has no
+  // curvature, we need at least one of its radii to be zero.
+  if (aXRadius <= aXBorder || aYRadius <= aYBorder) return;
+
+  // For any corner where we reduce the radii, preserve the corner's shape.
+  double ratio =
+      std::max(double(aXBorder) / aXRadius, double(aYBorder) / aYRadius);
+  aXRadius *= ratio;
+  aYRadius *= ratio;
+}
+
+/**
+ * Implement an override for nsIFrame::GetBorderRadii to ensure that
+ * the clipping region for the border radius does not clip the scrollbars.
+ *
+ * In other words, we require that the border radius be reduced until the
+ * inner border radius at the inner edge of the border is 0 wherever we
+ * have scrollbars.
+ */
+bool nsHTMLScrollFrame::GetBorderRadii(const nsSize& aFrameSize,
+                                       const nsSize& aBorderArea,
+                                       Sides aSkipSides,
+                                       nscoord aRadii[8]) const {
+  if (!nsContainerFrame::GetBorderRadii(aFrameSize, aBorderArea, aSkipSides,
+                                        aRadii)) {
+    return false;
+  }
+
+  // Since we can use GetActualScrollbarSizes (rather than
+  // GetDesiredScrollbarSizes) since this doesn't affect reflow, we
+  // probably should.
+  nsMargin sb = GetActualScrollbarSizes();
+  nsMargin border = GetUsedBorder();
+
+  if (sb.left > 0 || sb.top > 0) {
+    ReduceRadii(border.left, border.top, aRadii[eCornerTopLeftX],
+                aRadii[eCornerTopLeftY]);
+  }
+
+  if (sb.top > 0 || sb.right > 0) {
+    ReduceRadii(border.right, border.top, aRadii[eCornerTopRightX],
+                aRadii[eCornerTopRightY]);
+  }
+
+  if (sb.right > 0 || sb.bottom > 0) {
+    ReduceRadii(border.right, border.bottom, aRadii[eCornerBottomRightX],
+                aRadii[eCornerBottomRightY]);
+  }
+
+  if (sb.bottom > 0 || sb.left > 0) {
+    ReduceRadii(border.left, border.bottom, aRadii[eCornerBottomLeftX],
+                aRadii[eCornerBottomLeftY]);
+  }
+
+  return true;
+}
+
+static nscoord SnapCoord(nscoord aCoord, double aRes,
+                         nscoord aAppUnitsPerPixel) {
+  if (StaticPrefs::layout_scroll_disable_pixel_alignment()) {
+    return aCoord;
+  }
+  double snappedToLayerPixels = NS_round((aRes * aCoord) / aAppUnitsPerPixel);
+  return NSToCoordRoundWithClamp(snappedToLayerPixels * aAppUnitsPerPixel /
+                                 aRes);
+}
+
+nsRect nsHTMLScrollFrame::GetScrolledRect() const {
+  nsRect result = GetUnsnappedScrolledRectInternal(
+      mScrolledFrame->ScrollableOverflowRect(), mScrollPort.Size());
+
+#if 0
+  // This happens often enough.
+  if (result.width < mScrollPort.width || result.height < mScrollPort.height) {
+    NS_WARNING("Scrolled rect smaller than scrollport?");
+  }
+#endif
+
+  // Expand / contract the result by up to half a layer pixel so that scrolling
+  // to the right / bottom edge does not change the layer pixel alignment of
+  // the scrolled contents.
+
+  if (result.x == 0 && result.y == 0 && result.width == mScrollPort.width &&
+      result.height == mScrollPort.height) {
+    // The edges that we would snap are already aligned with the scroll port,
+    // so we can skip all the work below.
+    return result;
+  }
+
+  // For that, we first convert the scroll port and the scrolled rect to rects
+  // relative to the reference frame, since that's the space where painting does
+  // snapping.
+  nsSize visualViewportSize = GetVisualViewportSize();
+  const nsIFrame* referenceFrame =
+      mReferenceFrameDuringPainting ? mReferenceFrameDuringPainting
+                                    : nsLayoutUtils::GetReferenceFrame(
+                                          const_cast<nsHTMLScrollFrame*>(this));
+  nsPoint toReferenceFrame = GetOffsetToCrossDoc(referenceFrame);
+  nsRect scrollPort(mScrollPort.TopLeft() + toReferenceFrame,
+                    visualViewportSize);
+  nsRect scrolledRect = result + scrollPort.TopLeft();
+
+  if (scrollPort.Overflows() || scrolledRect.Overflows()) {
+    return result;
+  }
+
+  // Now, snap the bottom right corner of both of these rects.
+  // We snap to layer pixels, so we need to respect the layer's scale.
+  nscoord appUnitsPerDevPixel =
+      mScrolledFrame->PresContext()->AppUnitsPerDevPixel();
+  MatrixScales scale = GetPaintedLayerScaleForFrame(
+      mScrolledFrame, /* aIncludeCSSTransform = */ false);
+  if (scale.xScale == 0 || scale.yScale == 0) {
+    scale = MatrixScales();
+  }
+
+  // Compute bounds for the scroll position, and computed the snapped scrolled
+  // rect from the scroll position bounds.
+  nscoord snappedScrolledAreaBottom =
+      SnapCoord(scrolledRect.YMost(), scale.yScale, appUnitsPerDevPixel);
+  nscoord snappedScrollPortBottom =
+      SnapCoord(scrollPort.YMost(), scale.yScale, appUnitsPerDevPixel);
+  nscoord maximumScrollOffsetY =
+      snappedScrolledAreaBottom - snappedScrollPortBottom;
+  result.SetBottomEdge(scrollPort.height + maximumScrollOffsetY);
+
+  if (GetScrolledFrameDir() == StyleDirection::Ltr) {
+    nscoord snappedScrolledAreaRight =
+        SnapCoord(scrolledRect.XMost(), scale.xScale, appUnitsPerDevPixel);
+    nscoord snappedScrollPortRight =
+        SnapCoord(scrollPort.XMost(), scale.xScale, appUnitsPerDevPixel);
+    nscoord maximumScrollOffsetX =
+        snappedScrolledAreaRight - snappedScrollPortRight;
+    result.SetRightEdge(scrollPort.width + maximumScrollOffsetX);
+  } else {
+    // In RTL, the scrolled area's right edge is at scrollPort.XMost(),
+    // and the scrolled area's x position is zero or negative. We want
+    // the right edge to stay flush with the scroll port, so we snap the
+    // left edge.
+    nscoord snappedScrolledAreaLeft =
+        SnapCoord(scrolledRect.x, scale.xScale, appUnitsPerDevPixel);
+    nscoord snappedScrollPortLeft =
+        SnapCoord(scrollPort.x, scale.xScale, appUnitsPerDevPixel);
+    nscoord minimumScrollOffsetX =
+        snappedScrolledAreaLeft - snappedScrollPortLeft;
+    result.SetLeftEdge(minimumScrollOffsetX);
+  }
+
+  return result;
+}
+
+StyleDirection nsHTMLScrollFrame::GetScrolledFrameDir() const {
+  // If the scrolled frame has unicode-bidi: plaintext, the paragraph
+  // direction set by the text content overrides the direction of the frame
+  if (mScrolledFrame->StyleTextReset()->mUnicodeBidi ==
+      StyleUnicodeBidi::Plaintext) {
+    if (nsIFrame* child = mScrolledFrame->PrincipalChildList().FirstChild()) {
+      return nsBidiPresUtils::ParagraphDirection(child) ==
+                     mozilla::intl::BidiDirection::LTR
+                 ? StyleDirection::Ltr
+                 : StyleDirection::Rtl;
+    }
+  }
+  return IsBidiLTR() ? StyleDirection::Ltr : StyleDirection::Rtl;
+}
+
+nsRect nsHTMLScrollFrame::GetUnsnappedScrolledRectInternal(
+    const nsRect& aScrolledOverflowArea, const nsSize& aScrollPortSize) const {
+  return nsLayoutUtils::GetScrolledRect(mScrolledFrame, aScrolledOverflowArea,
+                                        aScrollPortSize, GetScrolledFrameDir());
+}
+
+nsMargin nsHTMLScrollFrame::GetActualScrollbarSizes(
+    nsIScrollableFrame::ScrollbarSizesOptions
+        aOptions /* = nsIScrollableFrame::ScrollbarSizesOptions::NONE */)
+    const {
+  nsRect r = GetPaddingRectRelativeToSelf();
+
+  nsMargin m(mScrollPort.y - r.y, r.XMost() - mScrollPort.XMost(),
+             r.YMost() - mScrollPort.YMost(), mScrollPort.x - r.x);
+
+  if (aOptions == nsIScrollableFrame::ScrollbarSizesOptions::
+                      INCLUDE_VISUAL_VIEWPORT_SCROLLBARS &&
+      !UsesOverlayScrollbars()) {
+    // If we are using layout scrollbars and they only exist to scroll the
+    // visual viewport then they do not take up any layout space (so the
+    // scrollport is the same as the padding rect) but they do cover everything
+    // below them so some callers may want to include this special type of
+    // scrollbars in the returned value.
+    if (mHScrollbarBox && mHasHorizontalScrollbar &&
+        mOnlyNeedHScrollbarToScrollVVInsideLV) {
+      m.bottom += mHScrollbarBox->GetRect().height;
+    }
+    if (mVScrollbarBox && mHasVerticalScrollbar &&
+        mOnlyNeedVScrollbarToScrollVVInsideLV) {
+      if (IsScrollbarOnRight()) {
+        m.right += mVScrollbarBox->GetRect().width;
+      } else {
+        m.left += mVScrollbarBox->GetRect().width;
+      }
+    }
+  }
+
+  return m;
+}
+
+void nsHTMLScrollFrame::SetScrollbarVisibility(nsIFrame* aScrollbar,
+                                               bool aVisible) {
+  nsScrollbarFrame* scrollbar = do_QueryFrame(aScrollbar);
+  if (scrollbar) {
+    // See if we have a mediator.
+    nsIScrollbarMediator* mediator = scrollbar->GetScrollbarMediator();
+    if (mediator) {
+      // Inform the mediator of the visibility change.
+      mediator->VisibilityChanged(aVisible);
+    }
+  }
+}
+
+nscoord nsHTMLScrollFrame::GetCoordAttribute(nsIFrame* aBox, nsAtom* aAtom,
+                                             nscoord aDefaultValue,
+                                             nscoord* aRangeStart,
+                                             nscoord* aRangeLength) {
+  if (aBox) {
+    nsIContent* content = aBox->GetContent();
+
+    nsAutoString value;
+    if (content->IsElement()) {
+      content->AsElement()->GetAttr(aAtom, value);
+    }
+    if (!value.IsEmpty()) {
+      nsresult error;
+      // convert it to appunits
+      nscoord result =
+          nsPresContext::CSSPixelsToAppUnits(value.ToInteger(&error));
+      nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f);
+      // Any nscoord value that would round to the attribute value when
+      // converted to CSS pixels is allowed.
+      *aRangeStart = result - halfPixel;
+      *aRangeLength = halfPixel * 2 - 1;
+      return result;
+    }
+  }
+
+  // Only this exact default value is allowed.
+  *aRangeStart = aDefaultValue;
+  *aRangeLength = 0;
+  return aDefaultValue;
+}
+
+bool nsHTMLScrollFrame::IsLastScrollUpdateAnimating() const {
+  if (!mScrollUpdates.IsEmpty()) {
+    switch (mScrollUpdates.LastElement().GetMode()) {
+      case ScrollMode::Smooth:
+      case ScrollMode::SmoothMsd:
+        return true;
+      case ScrollMode::Instant:
+      case ScrollMode::Normal:
+        break;
+    }
+  }
+  return false;
+}
+
+bool nsHTMLScrollFrame::IsLastScrollUpdateTriggeredByScriptAnimating() const {
+  if (!mScrollUpdates.IsEmpty()) {
+    const ScrollPositionUpdate& lastUpdate = mScrollUpdates.LastElement();
+    if (lastUpdate.WasTriggeredByScript() &&
+        (mScrollUpdates.LastElement().GetMode() == ScrollMode::Smooth ||
+         mScrollUpdates.LastElement().GetMode() == ScrollMode::SmoothMsd)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+using AnimationState = nsIScrollableFrame::AnimationState;
+EnumSet<AnimationState> nsHTMLScrollFrame::ScrollAnimationState() const {
+  EnumSet<AnimationState> retval;
+  if (IsApzAnimationInProgress()) {
+    retval += AnimationState::APZInProgress;
+    if (mCurrentAPZScrollAnimationType ==
+        APZScrollAnimationType::TriggeredByScript) {
+      retval += AnimationState::TriggeredByScript;
+    }
+  }
+
+  if (mApzAnimationRequested) {
+    retval += AnimationState::APZRequested;
+    if (mApzAnimationTriggeredByScriptRequested) {
+      retval += AnimationState::TriggeredByScript;
+    }
+  }
+
+  if (IsLastScrollUpdateAnimating()) {
+    retval += AnimationState::APZPending;
+    if (IsLastScrollUpdateTriggeredByScriptAnimating()) {
+      retval += AnimationState::TriggeredByScript;
+    }
+  }
+  if (mAsyncScroll) {
+    retval += AnimationState::MainThread;
+    if (mAsyncScroll->WasTriggeredByScript()) {
+      retval += AnimationState::TriggeredByScript;
+    }
+  }
+
+  if (mAsyncSmoothMSDScroll) {
+    retval += AnimationState::MainThread;
+    if (mAsyncSmoothMSDScroll->WasTriggeredByScript()) {
+      retval += AnimationState::TriggeredByScript;
+    }
+  }
+  return retval;
+}
+
+void nsHTMLScrollFrame::ResetScrollInfoIfNeeded(
+    const MainThreadScrollGeneration& aGeneration,
+    const APZScrollGeneration& aGenerationOnApz,
+    APZScrollAnimationType aAPZScrollAnimationType,
+    InScrollingGesture aInScrollingGesture) {
+  if (aGeneration == mScrollGeneration) {
+    mLastScrollOrigin = ScrollOrigin::None;
+    mApzAnimationRequested = false;
+    mApzAnimationTriggeredByScriptRequested = false;
+  }
+
+  mScrollGenerationOnApz = aGenerationOnApz;
+  // We can reset this regardless of scroll generation, as this is only set
+  // here, as a response to APZ requesting a repaint.
+  mCurrentAPZScrollAnimationType = aAPZScrollAnimationType;
+
+  mInScrollingGesture = aInScrollingGesture;
+}
+
+UniquePtr<PresState> nsHTMLScrollFrame::SaveState() {
+  nsIScrollbarMediator* mediator = do_QueryFrame(GetScrolledFrame());
+  if (mediator) {
+    // child handles its own scroll state, so don't bother saving state here
+    return nullptr;
+  }
+
+  // Don't store a scroll state if we never have been scrolled or restored
+  // a previous scroll state, and we're not in the middle of a smooth scroll.
+  auto scrollAnimationState = ScrollAnimationState();
+  bool isScrollAnimating =
+      scrollAnimationState.contains(AnimationState::MainThread) ||
+      scrollAnimationState.contains(AnimationState::APZPending) ||
+      scrollAnimationState.contains(AnimationState::APZRequested);
+  if (!mHasBeenScrolled && !mDidHistoryRestore && !isScrollAnimating) {
+    return nullptr;
+  }
+
+  UniquePtr<PresState> state = NewPresState();
+  bool allowScrollOriginDowngrade =
+      !nsLayoutUtils::CanScrollOriginClobberApz(mLastScrollOrigin) ||
+      mAllowScrollOriginDowngrade;
+  // Save mRestorePos instead of our actual current scroll position, if it's
+  // valid and we haven't moved since the last update of mLastPos (same check
+  // that ScrollToRestoredPosition uses). This ensures if a reframe occurs
+  // while we're in the process of loading content to scroll to a restored
+  // position, we'll keep trying after the reframe. Similarly, if we're in the
+  // middle of a smooth scroll, store the destination so that when we restore
+  // we'll jump straight to the end of the scroll animation, rather than
+  // effectively dropping it. Note that the mRestorePos will override the
+  // smooth scroll destination if both are present.
+  nsPoint pt = GetLogicalVisualViewportOffset();
+  if (isScrollAnimating) {
+    pt = mDestination;
+    allowScrollOriginDowngrade = false;
+  }
+  SCROLLRESTORE_LOG("%p: SaveState, pt=%s, mLastPos=%s, mRestorePos=%s\n", this,
+                    ToString(pt).c_str(), ToString(mLastPos).c_str(),
+                    ToString(mRestorePos).c_str());
+  if (mRestorePos.y != -1 && pt == mLastPos) {
+    pt = mRestorePos;
+  }
+  state->scrollState() = pt;
+  state->allowScrollOriginDowngrade() = allowScrollOriginDowngrade;
+  if (mIsRoot) {
+    // Only save resolution properties for root scroll frames
+    state->resolution() = PresShell()->GetResolution();
+  }
+  return state;
+}
+
+NS_IMETHODIMP nsHTMLScrollFrame::RestoreState(PresState* aState) {
+  mRestorePos = aState->scrollState();
+  MOZ_ASSERT(mLastScrollOrigin == ScrollOrigin::None);
+  mAllowScrollOriginDowngrade = aState->allowScrollOriginDowngrade();
+  // When restoring state, we promote mLastScrollOrigin to a stronger value
+  // from the default of eNone, to restore the behaviour that existed when
+  // the state was saved. If mLastScrollOrigin was a weaker value previously,
+  // then mAllowScrollOriginDowngrade will be true, and so the combination of
+  // mAllowScrollOriginDowngrade and the stronger mLastScrollOrigin will allow
+  // the same types of scrolls as before. It might be possible to also just
+  // save and restore the mAllowScrollOriginDowngrade and mLastScrollOrigin
+  // values directly without this sort of fiddling. Something to try in the
+  // future or if we tinker with this code more.
+  mLastScrollOrigin = ScrollOrigin::Other;
+  mDidHistoryRestore = true;
+  mLastPos = mScrolledFrame ? GetLogicalVisualViewportOffset() : nsPoint(0, 0);
+  SCROLLRESTORE_LOG("%p: RestoreState, set mRestorePos=%s mLastPos=%s\n", this,
+                    ToString(mRestorePos).c_str(), ToString(mLastPos).c_str());
+
+  // Resolution properties should only exist on root scroll frames.
+  MOZ_ASSERT(mIsRoot || aState->resolution() == 1.0);
+
+  if (mIsRoot) {
+    PresShell()->SetResolutionAndScaleTo(
+        aState->resolution(), ResolutionChangeOrigin::MainThreadRestore);
+  }
+  return NS_OK;
+}
+
+void nsHTMLScrollFrame::PostScrolledAreaEvent() {
+  if (mScrolledAreaEvent.IsPending()) {
+    return;
+  }
+  mScrolledAreaEvent = new ScrolledAreaEvent(this);
+  nsContentUtils::AddScriptRunner(mScrolledAreaEvent.get());
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// ScrolledArea change event dispatch
+
+// TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
+MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHODIMP
+nsHTMLScrollFrame::ScrolledAreaEvent::Run() {
+  if (mHelper) {
+    mHelper->FireScrolledAreaEvent();
+  }
+  return NS_OK;
+}
+
+void nsHTMLScrollFrame::FireScrolledAreaEvent() {
+  mScrolledAreaEvent.Forget();
+
+  InternalScrollAreaEvent event(true, eScrolledAreaChanged, nullptr);
+  RefPtr<nsPresContext> presContext = PresContext();
+  nsIContent* content = GetContent();
+
+  event.mArea = mScrolledFrame->ScrollableOverflowRectRelativeToParent();
+  if (RefPtr<Document> doc = content->GetUncomposedDoc()) {
+    EventDispatcher::Dispatch(doc, presContext, &event, nullptr);
+  }
+}
+
+ScrollDirections nsIScrollableFrame::GetAvailableScrollingDirections() const {
+  nscoord oneDevPixel =
+      GetScrolledFrame()->PresContext()->AppUnitsPerDevPixel();
+  ScrollDirections directions;
+  nsRect scrollRange = GetScrollRange();
+  if (scrollRange.width >= oneDevPixel) {
+    directions += ScrollDirection::eHorizontal;
+  }
+  if (scrollRange.height >= oneDevPixel) {
+    directions += ScrollDirection::eVertical;
+  }
+  return directions;
+}
+
+nsRect nsHTMLScrollFrame::GetScrollRangeForUserInputEvents() const {
+  // This function computes a scroll range based on a scrolled rect and scroll
+  // port defined as follows:
+  //   scrollable rect = overflow:hidden ? layout viewport : scrollable rect
+  //   scroll port = have visual viewport ? visual viewport : layout viewport
+  // The results in the same notion of scroll range that APZ uses (the combined
+  // effect of FrameMetrics::CalculateScrollRange() and
+  // nsLayoutUtils::CalculateScrollableRectForFrame).
+
+  ScrollStyles ss = GetScrollStyles();
+
+  nsPoint scrollPos = GetScrollPosition();
+
+  nsRect scrolledRect = GetScrolledRect();
+  if (StyleOverflow::Hidden == ss.mHorizontal) {
+    scrolledRect.width = mScrollPort.width;
+    scrolledRect.x = scrollPos.x;
+  }
+  if (StyleOverflow::Hidden == ss.mVertical) {
+    scrolledRect.height = mScrollPort.height;
+    scrolledRect.y = scrollPos.y;
+  }
+
+  nsSize scrollPort = GetVisualViewportSize();
+
+  nsRect scrollRange = scrolledRect;
+  scrollRange.width = std::max(scrolledRect.width - scrollPort.width, 0);
+  scrollRange.height = std::max(scrolledRect.height - scrollPort.height, 0);
+
+  return scrollRange;
+}
+
+ScrollDirections
+nsHTMLScrollFrame::GetAvailableScrollingDirectionsForUserInputEvents() const {
+  nsRect scrollRange = GetScrollRangeForUserInputEvents();
+
+  // We check if there is at least one half of a screen pixel of scroll range to
+  // roughly match what apz does when it checks if the change in scroll position
+  // in screen pixels round to zero or not.
+  // (https://searchfox.org/mozilla-central/rev/2f09184ec781a2667feec87499d4b81b32b6c48e/gfx/layers/apz/src/AsyncPanZoomController.cpp#3210)
+  // This isn't quite half a screen pixel, it doesn't take into account CSS
+  // transforms, but should be good enough.
+  float halfScreenPixel =
+      GetScrolledFrame()->PresContext()->AppUnitsPerDevPixel() /
+      (PresShell()->GetCumulativeResolution() * 2.f);
+  ScrollDirections directions;
+  if (scrollRange.width >= halfScreenPixel) {
+    directions += ScrollDirection::eHorizontal;
+  }
+  if (scrollRange.height >= halfScreenPixel) {
+    directions += ScrollDirection::eVertical;
+  }
+  return directions;
+}
+
+/**
+ * Append scroll positions for valid snap positions into |aSnapInfo| if
+ * applicable.
+ */
+static void AppendScrollPositionsForSnap(
+    const nsIFrame* aFrame, const nsIFrame* aScrolledFrame,
+    const nsRect& aScrolledRect, const nsMargin& aScrollPadding,
+    const nsRect& aScrollRange, WritingMode aWritingModeOnScroller,
+    ScrollSnapInfo& aSnapInfo, nsHTMLScrollFrame::SnapTargetSet* aSnapTargets) {
+  ScrollSnapTargetId targetId = ScrollSnapUtils::GetTargetIdFor(aFrame);
+
+  nsRect snapArea =
+      ScrollSnapUtils::GetSnapAreaFor(aFrame, aScrolledFrame, aScrolledRect);
+  // Use the writing-mode on the target element if the snap area is larger than
+  // the snapport.
+  // https://drafts.csswg.org/css-scroll-snap/#snap-scope
+  WritingMode writingMode = ScrollSnapUtils::NeedsToRespectTargetWritingMode(
+                                snapArea.Size(), aSnapInfo.mSnapportSize)
+                                ? aFrame->GetWritingMode()
+                                : aWritingModeOnScroller;
+
+  // These snap range shouldn't be involved with scroll-margin since we just
+  // need the visible range of the target element.
+  if (snapArea.width > aSnapInfo.mSnapportSize.width) {
+    aSnapInfo.mXRangeWiderThanSnapport.AppendElement(
+        ScrollSnapInfo::ScrollSnapRange(snapArea, ScrollDirection::eHorizontal,
+                                        targetId));
+  }
+  if (snapArea.height > aSnapInfo.mSnapportSize.height) {
+    aSnapInfo.mYRangeWiderThanSnapport.AppendElement(
+        ScrollSnapInfo::ScrollSnapRange(snapArea, ScrollDirection::eVertical,
+                                        targetId));
+  }
+
+  // Shift target rect position by the scroll padding to get the padded
+  // position thus we don't need to take account scroll-padding values in
+  // ScrollSnapUtils::GetSnapPointForDestination() when it gets called from
+  // the compositor thread.
+  snapArea.y -= aScrollPadding.top;
+  snapArea.x -= aScrollPadding.left;
+
+  LogicalRect logicalTargetRect(writingMode, snapArea, aSnapInfo.mSnapportSize);
+  LogicalSize logicalSnapportRect(writingMode, aSnapInfo.mSnapportSize);
+  LogicalRect logicalScrollRange(aWritingModeOnScroller, aScrollRange,
+                                 // The origin of this logical coordinate system
+                                 // what we need here is (0, 0), so we use an
+                                 // empty size.
+                                 nsSize());
+
+  Maybe<nscoord> blockDirectionPosition;
+  const nsStyleDisplay* styleDisplay = aFrame->StyleDisplay();
+  nscoord containerBSize = logicalSnapportRect.BSize(writingMode);
+  switch (styleDisplay->mScrollSnapAlign.block) {
+    case StyleScrollSnapAlignKeyword::None:
+      break;
+    case StyleScrollSnapAlignKeyword::Start:
+      blockDirectionPosition.emplace(
+          writingMode.IsVerticalRL() ? -logicalTargetRect.BStart(writingMode)
+                                     : logicalTargetRect.BStart(writingMode));
+      break;
+    case StyleScrollSnapAlignKeyword::End: {
+      nscoord candidate = std::clamp(
+          // What we need here is the scroll position instead of the snap
+          // position itself, so we need, for example, the top edge of the
+          // scroll port on horizontal-tb when the frame is positioned at
+          // the bottom edge of the scroll port. For this reason we subtract
+          // containerBSize from BEnd of the target and clamp it inside the
+          // scrollable range.
+          logicalTargetRect.BEnd(writingMode) - containerBSize,
+          logicalScrollRange.BStart(writingMode),
+          logicalScrollRange.BEnd(writingMode));
+      blockDirectionPosition.emplace(writingMode.IsVerticalRL() ? -candidate
+                                                                : candidate);
+      break;
+    }
+    case StyleScrollSnapAlignKeyword::Center: {
+      nscoord targetCenter = (logicalTargetRect.BStart(writingMode) +
+                              logicalTargetRect.BEnd(writingMode)) /
+                             2;
+      nscoord halfSnapportSize = containerBSize / 2;
+      // Get the center of the target to align with the center of the snapport
+      // depending on direction.
+      nscoord candidate = std::clamp(targetCenter - halfSnapportSize,
+                                     logicalScrollRange.BStart(writingMode),
+                                     logicalScrollRange.BEnd(writingMode));
+      blockDirectionPosition.emplace(writingMode.IsVerticalRL() ? -candidate
+                                                                : candidate);
+      break;
+    }
+  }
+
+  Maybe<nscoord> inlineDirectionPosition;
+  nscoord containerISize = logicalSnapportRect.ISize(writingMode);
+  switch (styleDisplay->mScrollSnapAlign.inline_) {
+    case StyleScrollSnapAlignKeyword::None:
+      break;
+    case StyleScrollSnapAlignKeyword::Start:
+      inlineDirectionPosition.emplace(
+          writingMode.IsInlineReversed()
+              ? -logicalTargetRect.IStart(writingMode)
+              : logicalTargetRect.IStart(writingMode));
+      break;
+    case StyleScrollSnapAlignKeyword::End: {
+      nscoord candidate = std::clamp(
+          // Same as above BEnd case, we subtract containerISize.
+          //
+          // Note that the logical scroll range is mapped to [0, x] range even
+          // if it's in RTL contents. So for example, if the physical range is
+          // [-200, 0], it's mapped to [0, 200], i.e. IStart() is 0, IEnd() is
+          // 200. So we can just use std::clamp with the same arguments in both
+          // RTL/LTR cases.
+          logicalTargetRect.IEnd(writingMode) - containerISize,
+          logicalScrollRange.IStart(writingMode),
+          logicalScrollRange.IEnd(writingMode));
+      inlineDirectionPosition.emplace(
+          writingMode.IsInlineReversed() ? -candidate : candidate);
+      break;
+    }
+    case StyleScrollSnapAlignKeyword::Center: {
+      nscoord targetCenter = (logicalTargetRect.IStart(writingMode) +
+                              logicalTargetRect.IEnd(writingMode)) /
+                             2;
+      nscoord halfSnapportSize = containerISize / 2;
+      // Get the center of the target to align with the center of the snapport
+      // depending on direction.
+      nscoord candidate = std::clamp(targetCenter - halfSnapportSize,
+                                     logicalScrollRange.IStart(writingMode),
+                                     logicalScrollRange.IEnd(writingMode));
+      inlineDirectionPosition.emplace(
+          writingMode.IsInlineReversed() ? -candidate : candidate);
+      break;
+    }
+  }
+
+  if (blockDirectionPosition || inlineDirectionPosition) {
+    aSnapInfo.mSnapTargets.AppendElement(
+        writingMode.IsVertical()
+            ? ScrollSnapInfo::SnapTarget(
+                  std::move(blockDirectionPosition),
+                  std::move(inlineDirectionPosition), std::move(snapArea),
+                  styleDisplay->mScrollSnapStop, targetId)
+            : ScrollSnapInfo::SnapTarget(
+                  std::move(inlineDirectionPosition),
+                  std::move(blockDirectionPosition), std::move(snapArea),
+                  styleDisplay->mScrollSnapStop, targetId));
+    if (aSnapTargets) {
+      aSnapTargets->EnsureInserted(aFrame->GetContent());
+    }
+  }
+}
+
+/**
+ * Collect the scroll positions corresponding to snap positions of frames in the
+ * subtree rooted at |aFrame|, relative to |aScrolledFrame|, into |aSnapInfo|.
+ */
+static void CollectScrollPositionsForSnap(
+    nsIFrame* aFrame, nsIFrame* aScrolledFrame, const nsRect& aScrolledRect,
+    const nsMargin& aScrollPadding, const nsRect& aScrollRange,
+    WritingMode aWritingModeOnScroller, ScrollSnapInfo& aSnapInfo,
+    nsHTMLScrollFrame::SnapTargetSet* aSnapTargets) {
+  // Snap positions only affect the nearest ancestor scroll container on the
+  // element's containing block chain.
+  nsIScrollableFrame* sf = do_QueryFrame(aFrame);
+  if (sf) {
+    return;
+  }
+
+  for (const auto& childList : aFrame->ChildLists()) {
+    for (nsIFrame* f : childList.mList) {
+      const nsStyleDisplay* styleDisplay = f->StyleDisplay();
+      if (styleDisplay->mScrollSnapAlign.inline_ !=
+              StyleScrollSnapAlignKeyword::None ||
+          styleDisplay->mScrollSnapAlign.block !=
+              StyleScrollSnapAlignKeyword::None) {
+        AppendScrollPositionsForSnap(
+            f, aScrolledFrame, aScrolledRect, aScrollPadding, aScrollRange,
+            aWritingModeOnScroller, aSnapInfo, aSnapTargets);
+      }
+      CollectScrollPositionsForSnap(
+          f, aScrolledFrame, aScrolledRect, aScrollPadding, aScrollRange,
+          aWritingModeOnScroller, aSnapInfo, aSnapTargets);
+    }
+  }
+}
+
+static nscoord ResolveScrollPaddingStyleValue(
+    const StyleRect<mozilla::NonNegativeLengthPercentageOrAuto>&
+        aScrollPaddingStyle,
+    Side aSide, const nsSize& aScrollPortSize) {
+  if (aScrollPaddingStyle.Get(aSide).IsAuto()) {
+    // https://drafts.csswg.org/css-scroll-snap-1/#valdef-scroll-padding-auto
+    return 0;
+  }
+
+  nscoord percentageBasis;
+  switch (aSide) {
+    case eSideTop:
+    case eSideBottom:
+      percentageBasis = aScrollPortSize.height;
+      break;
+    case eSideLeft:
+    case eSideRight:
+      percentageBasis = aScrollPortSize.width;
+      break;
+  }
+
+  return aScrollPaddingStyle.Get(aSide).AsLengthPercentage().Resolve(
+      percentageBasis);
+}
+
+static nsMargin ResolveScrollPaddingStyle(
+    const StyleRect<mozilla::NonNegativeLengthPercentageOrAuto>&
+        aScrollPaddingStyle,
+    const nsSize& aScrollPortSize) {
+  return nsMargin(ResolveScrollPaddingStyleValue(aScrollPaddingStyle, eSideTop,
+                                                 aScrollPortSize),
+                  ResolveScrollPaddingStyleValue(aScrollPaddingStyle,
+                                                 eSideRight, aScrollPortSize),
+                  ResolveScrollPaddingStyleValue(aScrollPaddingStyle,
+                                                 eSideBottom, aScrollPortSize),
+                  ResolveScrollPaddingStyleValue(aScrollPaddingStyle, eSideLeft,
+                                                 aScrollPortSize));
+}
+
+nsMargin nsHTMLScrollFrame::GetScrollPadding() const {
+  nsIFrame* styleFrame = GetFrameForStyle();
+  if (!styleFrame) {
+    return nsMargin();
+  }
+
+  // The spec says percentage values are relative to the scroll port size.
+  // https://drafts.csswg.org/css-scroll-snap-1/#scroll-padding
+  return ResolveScrollPaddingStyle(styleFrame->StylePadding()->mScrollPadding,
+                                   GetScrollPortRect().Size());
+}
+
+ScrollSnapInfo nsHTMLScrollFrame::ComputeScrollSnapInfo() {
+  ScrollSnapInfo result;
+
+  nsIFrame* scrollSnapFrame = GetFrameForStyle();
+  if (!scrollSnapFrame) {
+    return result;
+  }
+
+  const nsStyleDisplay* disp = scrollSnapFrame->StyleDisplay();
+  if (disp->mScrollSnapType.strictness == StyleScrollSnapStrictness::None) {
+    // We won't be snapping, short-circuit the computation.
+    return result;
+  }
+
+  WritingMode writingMode = GetWritingMode();
+  result.InitializeScrollSnapStrictness(writingMode, disp);
+
+  result.mSnapportSize = GetSnapportSize();
+  CollectScrollPositionsForSnap(
+      mScrolledFrame, mScrolledFrame, GetScrolledRect(), GetScrollPadding(),
+      GetLayoutScrollRange(), writingMode, result, &mSnapTargets);
+  return result;
+}
+
+ScrollSnapInfo nsHTMLScrollFrame::GetScrollSnapInfo() {
+  // TODO(botond): Should we cache it?
+  return ComputeScrollSnapInfo();
+}
+
+Maybe<SnapDestination> nsHTMLScrollFrame::GetSnapPointForDestination(
+    ScrollUnit aUnit, ScrollSnapFlags aFlags, const nsPoint& aStartPos,
+    const nsPoint& aDestination) {
+  // We can release the strong references for the previous snap target
+  // elements here since calling this ComputeScrollSnapInfo means we are going
+  // to evaluate new snap points, thus there's no chance to generating
+  // nsIContent instances in between this function call and the function call
+  // for the (re-)evaluation.
+  mSnapTargets.Clear();
+  return ScrollSnapUtils::GetSnapPointForDestination(
+      ComputeScrollSnapInfo(), aUnit, aFlags, GetLayoutScrollRange(), aStartPos,
+      aDestination);
+}
+
+Maybe<SnapDestination> nsHTMLScrollFrame::GetSnapPointForResnap() {
+  // Same as in GetSnapPointForDestination, We can release the strong references
+  // for the previous snap targets here.
+  mSnapTargets.Clear();
+  nsIContent* focusedContent =
+      GetContent()->GetComposedDoc()->GetUnretargetedFocusedContent();
+  return ScrollSnapUtils::GetSnapPointForResnap(
+      ComputeScrollSnapInfo(), GetLayoutScrollRange(), GetScrollPosition(),
+      mLastSnapTargetIds, focusedContent);
+}
+
+bool nsHTMLScrollFrame::NeedsResnap() {
+  nsIContent* focusedContent =
+      GetContent()->GetComposedDoc()->GetUnretargetedFocusedContent();
+  return ScrollSnapUtils::GetSnapPointForResnap(
+             ComputeScrollSnapInfo(), GetLayoutScrollRange(),
+             GetScrollPosition(), mLastSnapTargetIds, focusedContent)
+      .isSome();
+}
+
+void nsHTMLScrollFrame::SetLastSnapTargetIds(
+    UniquePtr<ScrollSnapTargetIds> aIds) {
+  if (!aIds) {
+    mLastSnapTargetIds = nullptr;
+    return;
+  }
+
+  // This SetLastSnapTargetIds gets called asynchronously so that there's a race
+  // condition something like;
+  // 1) an async scroll operation triggered snapping to a point on an element
+  // 2) during the async scroll operation, the element got removed from this
+  //    scroll container
+  // 3) re-snapping triggered
+  // 4) this SetLastSnapTargetIds got called
+  // In such case |aIds| are stale, we shouldn't use it.
+  for (const auto* idList : {&aIds->mIdsOnX, &aIds->mIdsOnY}) {
+    for (const auto id : *idList) {
+      if (!mSnapTargets.Contains(reinterpret_cast<nsIContent*>(id))) {
+        mLastSnapTargetIds = nullptr;
+        return;
+      }
+    }
+  }
+
+  mLastSnapTargetIds = std::move(aIds);
+}
+
+bool nsHTMLScrollFrame::IsLastSnappedTarget(const nsIFrame* aFrame) const {
+  ScrollSnapTargetId id = ScrollSnapUtils::GetTargetIdFor(aFrame);
+  MOZ_ASSERT(id != ScrollSnapTargetId::None,
+             "This function is supposed to be called for contents");
+
+  if (!mLastSnapTargetIds) {
+    return false;
+  }
+
+  return mLastSnapTargetIds->mIdsOnX.Contains(id) ||
+         mLastSnapTargetIds->mIdsOnY.Contains(id);
+}
+
+void nsHTMLScrollFrame::TryResnap() {
+  // If there's any async scroll is running or we are processing pan/touch
+  // gestures or scroll thumb dragging,  don't clobber the scroll.
+  if (!ScrollAnimationState().isEmpty() ||
+      mInScrollingGesture == InScrollingGesture::Yes) {
+    return;
+  }
+
+  if (auto snapDestination = GetSnapPointForResnap()) {
+    // We are going to re-snap so that we need to clobber scroll anchoring.
+    mAnchor.UserScrolled();
+
+    // Snap to the nearest snap position if exists.
+    ScrollToWithOrigin(
+        snapDestination->mPosition, nullptr /* range */,
+        ScrollOperationParams{
+            IsSmoothScroll(ScrollBehavior::Auto) ? ScrollMode::SmoothMsd
+                                                 : ScrollMode::Instant,
+            ScrollOrigin::Other, std::move(snapDestination->mTargetIds)});
+  }
+}
+
+void nsHTMLScrollFrame::PostPendingResnapIfNeeded(const nsIFrame* aFrame) {
+  if (!IsLastSnappedTarget(aFrame)) {
+    return;
+  }
+
+  PostPendingResnap();
+}
+
+void nsHTMLScrollFrame::PostPendingResnap() {
+  PresShell()->PostPendingScrollResnap(this);
+}
+
+nsIScrollableFrame::PhysicalScrollSnapAlign
+nsHTMLScrollFrame::GetScrollSnapAlignFor(const nsIFrame* aFrame) const {
+  StyleScrollSnapAlignKeyword alignForY = StyleScrollSnapAlignKeyword::None;
+  StyleScrollSnapAlignKeyword alignForX = StyleScrollSnapAlignKeyword::None;
+
+  nsIFrame* styleFrame = GetFrameForStyle();
+  if (!styleFrame) {
+    return {alignForX, alignForY};
+  }
+
+  if (styleFrame->StyleDisplay()->mScrollSnapType.strictness ==
+      StyleScrollSnapStrictness::None) {
+    return {alignForX, alignForY};
+  }
+
+  const nsStyleDisplay* styleDisplay = aFrame->StyleDisplay();
+  if (styleDisplay->mScrollSnapAlign.inline_ ==
+          StyleScrollSnapAlignKeyword::None &&
+      styleDisplay->mScrollSnapAlign.block ==
+          StyleScrollSnapAlignKeyword::None) {
+    return {alignForX, alignForY};
+  }
+
+  nsSize snapAreaSize =
+      ScrollSnapUtils::GetSnapAreaFor(aFrame, mScrolledFrame, GetScrolledRect())
+          .Size();
+  const WritingMode writingMode =
+      ScrollSnapUtils::NeedsToRespectTargetWritingMode(snapAreaSize,
+                                                       GetSnapportSize())
+          ? aFrame->GetWritingMode()
+          : styleFrame->GetWritingMode();
+
+  switch (styleFrame->StyleDisplay()->mScrollSnapType.axis) {
+    case StyleScrollSnapAxis::X:
+      alignForX = writingMode.IsVertical()
+                      ? styleDisplay->mScrollSnapAlign.block
+                      : styleDisplay->mScrollSnapAlign.inline_;
+      break;
+    case StyleScrollSnapAxis::Y:
+      alignForY = writingMode.IsVertical()
+                      ? styleDisplay->mScrollSnapAlign.inline_
+                      : styleDisplay->mScrollSnapAlign.block;
+      break;
+    case StyleScrollSnapAxis::Block:
+      if (writingMode.IsVertical()) {
+        alignForX = styleDisplay->mScrollSnapAlign.block;
+      } else {
+        alignForY = styleDisplay->mScrollSnapAlign.block;
+      }
+      break;
+    case StyleScrollSnapAxis::Inline:
+      if (writingMode.IsVertical()) {
+        alignForY = styleDisplay->mScrollSnapAlign.inline_;
+      } else {
+        alignForX = styleDisplay->mScrollSnapAlign.inline_;
+      }
+      break;
+    case StyleScrollSnapAxis::Both:
+      if (writingMode.IsVertical()) {
+        alignForX = styleDisplay->mScrollSnapAlign.block;
+        alignForY = styleDisplay->mScrollSnapAlign.inline_;
+      } else {
+        alignForX = styleDisplay->mScrollSnapAlign.inline_;
+        alignForY = styleDisplay->mScrollSnapAlign.block;
+      }
+      break;
+  }
+
+  return {alignForX, alignForY};
+}
+
+bool nsHTMLScrollFrame::UsesOverlayScrollbars() const {
+  return PresContext()->UseOverlayScrollbars();
+}
+
+bool nsHTMLScrollFrame::DragScroll(WidgetEvent* aEvent) {
+  // Dragging is allowed while within a 20 pixel border. Note that device pixels
+  // are used so that the same margin is used even when zoomed in or out.
+  nscoord margin = 20 * PresContext()->AppUnitsPerDevPixel();
+
+  // Don't drag scroll for small scrollareas.
+  if (mScrollPort.width < margin * 2 || mScrollPort.height < margin * 2) {
+    return false;
+  }
+
+  // If willScroll is computed as false, then the frame is already scrolled as
+  // far as it can go in both directions. Return false so that an ancestor
+  // scrollframe can scroll instead.
+  bool willScroll = false;
+  nsPoint pnt =
+      nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, RelativeTo{this});
+  nsPoint scrollPoint = GetScrollPosition();
+  nsRect rangeRect = GetLayoutScrollRange();
+
+  // Only drag scroll when a scrollbar is present.
+  nsPoint offset;
+  if (mHasHorizontalScrollbar) {
+    if (pnt.x >= mScrollPort.x && pnt.x <= mScrollPort.x + margin) {
+      offset.x = -margin;
+      if (scrollPoint.x > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.x >= mScrollPort.XMost() - margin &&
+               pnt.x <= mScrollPort.XMost()) {
+      offset.x = margin;
+      if (scrollPoint.x < rangeRect.width) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (mHasVerticalScrollbar) {
+    if (pnt.y >= mScrollPort.y && pnt.y <= mScrollPort.y + margin) {
+      offset.y = -margin;
+      if (scrollPoint.y > 0) {
+        willScroll = true;
+      }
+    } else if (pnt.y >= mScrollPort.YMost() - margin &&
+               pnt.y <= mScrollPort.YMost()) {
+      offset.y = margin;
+      if (scrollPoint.y < rangeRect.height) {
+        willScroll = true;
+      }
+    }
+  }
+
+  if (offset.x || offset.y) {
+    ScrollToWithOrigin(
+        GetScrollPosition() + offset, nullptr /* range */,
+        ScrollOperationParams{ScrollMode::Normal, ScrollOrigin::Other});
+  }
+
+  return willScroll;
+}
+
+static nsSliderFrame* GetSliderFrame(nsIFrame* aScrollbarFrame) {
+  if (!aScrollbarFrame) {
+    return nullptr;
+  }
+
+  for (const auto& childList : aScrollbarFrame->ChildLists()) {
+    for (nsIFrame* frame : childList.mList) {
+      if (nsSliderFrame* sliderFrame = do_QueryFrame(frame)) {
+        return sliderFrame;
+      }
+    }
+  }
+  return nullptr;
+}
+
+static void AsyncScrollbarDragInitiated(uint64_t aDragBlockId,
+                                        nsIFrame* aScrollbar) {
+  if (nsSliderFrame* sliderFrame = GetSliderFrame(aScrollbar)) {
+    sliderFrame->AsyncScrollbarDragInitiated(aDragBlockId);
+  }
+}
+
+void nsHTMLScrollFrame::AsyncScrollbarDragInitiated(
+    uint64_t aDragBlockId, ScrollDirection aDirection) {
+  switch (aDirection) {
+    case ScrollDirection::eVertical:
+      ::AsyncScrollbarDragInitiated(aDragBlockId, mVScrollbarBox);
+      break;
+    case ScrollDirection::eHorizontal:
+      ::AsyncScrollbarDragInitiated(aDragBlockId, mHScrollbarBox);
+      break;
+  }
+}
+
+static void AsyncScrollbarDragRejected(nsIFrame* aScrollbar) {
+  if (nsSliderFrame* sliderFrame = GetSliderFrame(aScrollbar)) {
+    sliderFrame->AsyncScrollbarDragRejected();
+  }
+}
+
+void nsHTMLScrollFrame::AsyncScrollbarDragRejected() {
+  // We don't get told which scrollbar requested the async drag,
+  // so we notify both.
+  ::AsyncScrollbarDragRejected(mHScrollbarBox);
+  ::AsyncScrollbarDragRejected(mVScrollbarBox);
+}
+
+void nsHTMLScrollFrame::ApzSmoothScrollTo(
+    const nsPoint& aDestination, ScrollMode aMode, ScrollOrigin aOrigin,
+    ScrollTriggeredByScript aTriggeredByScript,
+    UniquePtr<ScrollSnapTargetIds> aSnapTargetIds) {
+  if (mApzSmoothScrollDestination == Some(aDestination)) {
+    // If we already sent APZ a smooth-scroll request to this
+    // destination (i.e. it was the last request
+    // we sent), then don't send another one because it is redundant.
+    // This is to avoid a scenario where pages do repeated scrollBy
+    // calls, incrementing the generation counter, and blocking APZ from
+    // syncing the scroll offset back to the main thread.
+    // Note that if we get two smooth-scroll requests to the same
+    // destination with some other scroll in between,
+    // mApzSmoothScrollDestination will get reset to Nothing() and so
+    // we shouldn't have the problem where this check discards a
+    // legitimate smooth-scroll.
+    return;
+  }
+
+  // The animation will be handled in the compositor, pass the
+  // information needed to start the animation and skip the main-thread
+  // animation for this scroll.
+  MOZ_ASSERT(aOrigin != ScrollOrigin::None);
+  mApzSmoothScrollDestination = Some(aDestination);
+  AppendScrollUpdate(ScrollPositionUpdate::NewSmoothScroll(
+      aMode, aOrigin, aDestination, aTriggeredByScript,
+      std::move(aSnapTargetIds)));
+
+  nsIContent* content = GetContent();
+  if (!DisplayPortUtils::HasNonMinimalNonZeroDisplayPort(content)) {
+    // If this frame doesn't have a displayport then there won't be an
+    // APZC instance for it and so there won't be anything to process
+    // this smooth scroll request. We should set a displayport on this
+    // frame to force an APZC which can handle the request.
+    if (MOZ_LOG_TEST(sDisplayportLog, LogLevel::Debug)) {
+      mozilla::layers::ScrollableLayerGuid::ViewID viewID =
+          mozilla::layers::ScrollableLayerGuid::NULL_SCROLL_ID;
+      nsLayoutUtils::FindIDFor(content, &viewID);
+      MOZ_LOG(
+          sDisplayportLog, LogLevel::Debug,
+          ("ApzSmoothScrollTo setting displayport on scrollId=%" PRIu64 "\n",
+           viewID));
+    }
+
+    DisplayPortUtils::CalculateAndSetDisplayPortMargins(
+        GetScrollTargetFrame(), DisplayPortUtils::RepaintMode::Repaint);
+    nsIFrame* frame = do_QueryFrame(GetScrollTargetFrame());
+    DisplayPortUtils::SetZeroMarginDisplayPortOnAsyncScrollableAncestors(frame);
+  }
+
+  // Schedule a paint to ensure that the frame metrics get updated on
+  // the compositor thread.
+  SchedulePaint();
+}
+
+bool nsHTMLScrollFrame::CanApzScrollInTheseDirections(
+    ScrollDirections aDirections) {
+  ScrollStyles styles = GetScrollStyles();
+  if (aDirections.contains(ScrollDirection::eHorizontal) &&
+      styles.mHorizontal == StyleOverflow::Hidden)
+    return false;
+  if (aDirections.contains(ScrollDirection::eVertical) &&
+      styles.mVertical == StyleOverflow::Hidden)
+    return false;
+  return true;
+}
+
+bool nsHTMLScrollFrame::SmoothScrollVisual(
+    const nsPoint& aVisualViewportOffset,
+    FrameMetrics::ScrollOffsetUpdateType aUpdateType) {
+  bool canDoApzSmoothScroll =
+      nsLayoutUtils::AsyncPanZoomEnabled(this) && WantAsyncScroll();
+  if (!canDoApzSmoothScroll) {
+    return false;
+  }
+
+  // Clamp the destination to the visual scroll range.
+  // There is a potential issue here, where |mDestination| is usually
+  // clamped to the layout scroll range, and so e.g. a subsequent
+  // window.scrollBy() may have an undesired effect. However, as this function
+  // is only called internally, this should not be a problem in practice.
+  // If it turns out to be, the fix would be:
+  //   - add a new "destination" field that doesn't have to be clamped to
+  //     the layout scroll range
+  //   - clamp mDestination to the layout scroll range here
+  //   - make sure ComputeScrollMetadata() picks up the former as the
+  //     smooth scroll destination to send to APZ.
+  mDestination = GetVisualScrollRange().ClampPoint(aVisualViewportOffset);
+
+  UniquePtr<ScrollSnapTargetIds> snapTargetIds;
+  // Perform the scroll.
+  ApzSmoothScrollTo(mDestination, ScrollMode::SmoothMsd,
+                    aUpdateType == FrameMetrics::eRestore
+                        ? ScrollOrigin::Restore
+                        : ScrollOrigin::Other,
+                    ScrollTriggeredByScript::No, std::move(snapTargetIds));
+  return true;
+}
+
+bool nsHTMLScrollFrame::IsSmoothScroll(dom::ScrollBehavior aBehavior) const {
+  if (aBehavior == dom::ScrollBehavior::Instant) {
+    return false;
+  }
+
+  // The user smooth scrolling preference should be honored for any requested
+  // smooth scrolls. A requested smooth scroll when smooth scrolling is
+  // disabled should be equivalent to an instant scroll. This is not enforced
+  // for the <scrollbox> XUL element to allow for the browser chrome to
+  // override this behavior when toolkit.scrollbox.smoothScroll is enabled.
+  if (!GetContent()->IsXULElement(nsGkAtoms::scrollbox)) {
+    if (!nsLayoutUtils::IsSmoothScrollingEnabled()) {
+      return false;
+    }
+  } else {
+    if (!StaticPrefs::toolkit_scrollbox_smoothScroll()) {
+      return false;
+    }
+  }
+
+  if (aBehavior == dom::ScrollBehavior::Smooth) {
+    return true;
+  }
+
+  nsIFrame* styleFrame = GetFrameForStyle();
+  if (!styleFrame) {
+    return false;
+  }
+  return (aBehavior == dom::ScrollBehavior::Auto &&
+          styleFrame->StyleDisplay()->mScrollBehavior ==
+              StyleScrollBehavior::Smooth);
+}
+
+nsTArray<ScrollPositionUpdate> nsHTMLScrollFrame::GetScrollUpdates() const {
+  return mScrollUpdates.Clone();
+}
+
+void nsHTMLScrollFrame::AppendScrollUpdate(
+    const ScrollPositionUpdate& aUpdate) {
+  mScrollGeneration = aUpdate.GetGeneration();
+  mScrollUpdates.AppendElement(aUpdate);
+}
+
+void nsHTMLScrollFrame::ScheduleScrollAnimations() {
+  nsIContent* content = GetContent();
+  MOZ_ASSERT(content && content->IsElement(),
+             "The nsIScrollableFrame should have the element.");
+
+  const Element* elementOrPseudo = content->AsElement();
+  PseudoStyleType pseudo = elementOrPseudo->GetPseudoElementType();
+  if (pseudo != PseudoStyleType::NotPseudo &&
+      !AnimationUtils::IsSupportedPseudoForAnimations(pseudo)) {
+    // This is not an animatable pseudo element, and so we don't generate
+    // scroll-timeline for it.
+    return;
+  }
+
+  const auto [element, type] =
+      AnimationUtils::GetElementPseudoPair(elementOrPseudo);
+  const auto* scheduler = ProgressTimelineScheduler::Get(element, type);
+  if (!scheduler) {
+    // We don't have scroll timelines associated with this frame.
+    return;
+  }
+
+  scheduler->ScheduleAnimations();
+}