Adding upstream version 124.0.1.upstream/124.0.1

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

1 files changed, 6823 insertions, 0 deletions

diff --git a/layout/painting/nsDisplayList.h b/layout/painting/nsDisplayList.h
new file mode 100644
index 0000000000..cf4eb1dd16
--- /dev/null
+++ b/layout/painting/nsDisplayList.h
@@ -0,0 +1,6823 @@
+/* -*- 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/.
+ */
+
+/*
+ * structures that represent things to be painted (ordered in z-order),
+ * used during painting and hit testing
+ */
+
+#ifndef NSDISPLAYLIST_H_
+#define NSDISPLAYLIST_H_
+
+#include "DisplayItemClipChain.h"
+#include "DisplayListClipState.h"
+#include "FrameMetrics.h"
+#include "HitTestInfo.h"
+#include "ImgDrawResult.h"
+#include "RetainedDisplayListHelpers.h"
+#include "Units.h"
+#include "gfxContext.h"
+#include "mozilla/ArenaAllocator.h"
+#include "mozilla/Array.h"
+#include "mozilla/ArrayIterator.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/EffectCompositor.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MotionPathUtils.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TemplateLib.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/dom/EffectsInfo.h"
+#include "mozilla/gfx/UserData.h"
+#include "mozilla/layers/BSPTree.h"
+#include "mozilla/layers/ScrollableLayerGuid.h"
+#include "mozilla/layers/ScrollbarData.h"
+#include "nsAutoLayoutPhase.h"
+#include "nsCOMPtr.h"
+#include "nsCSSRenderingBorders.h"
+#include "nsContainerFrame.h"
+#include "nsDisplayItemTypes.h"
+#include "nsDisplayListInvalidation.h"
+#include "nsPoint.h"
+#include "nsPresArena.h"
+#include "nsRect.h"
+#include "nsRegion.h"
+#include "nsClassHashtable.h"
+#include "nsTHashSet.h"
+#include "nsTHashMap.h"
+
+#include <algorithm>
+#include <unordered_set>
+
+// XXX Includes that could be avoided by moving function implementations to the
+// cpp file.
+#include "gfxPlatform.h"
+
+class gfxContext;
+class nsIContent;
+class nsIScrollableFrame;
+class nsSubDocumentFrame;
+class nsCaret;
+struct WrFiltersHolder;
+
+namespace nsStyleTransformMatrix {
+class TransformReferenceBox;
+}
+
+namespace mozilla {
+
+enum class nsDisplayOwnLayerFlags;
+class nsDisplayCompositorHitTestInfo;
+class nsDisplayScrollInfoLayer;
+class PresShell;
+class StickyScrollContainer;
+
+namespace layers {
+struct FrameMetrics;
+class RenderRootStateManager;
+class Layer;
+class ImageContainer;
+class StackingContextHelper;
+class WebRenderScrollData;
+class WebRenderLayerScrollData;
+class WebRenderLayerManager;
+}  // namespace layers
+
+namespace wr {
+class DisplayListBuilder;
+}  // namespace wr
+
+namespace dom {
+class RemoteBrowser;
+class Selection;
+}  // namespace dom
+
+enum class DisplayListArenaObjectId {
+#define DISPLAY_LIST_ARENA_OBJECT(name_) name_,
+#include "nsDisplayListArenaTypes.h"
+#undef DISPLAY_LIST_ARENA_OBJECT
+  COUNT
+};
+
+extern LazyLogModule sContentDisplayListLog;
+extern LazyLogModule sParentDisplayListLog;
+
+LazyLogModule& GetLoggerByProcess();
+
+#define DL_LOG(lvl, ...) MOZ_LOG(GetLoggerByProcess(), lvl, (__VA_ARGS__))
+#define DL_LOGI(...) DL_LOG(LogLevel::Info, __VA_ARGS__)
+#define DL_LOG_TEST(lvl) MOZ_LOG_TEST(GetLoggerByProcess(), lvl)
+
+#ifdef DEBUG
+#  define DL_LOGD(...) DL_LOG(LogLevel::Debug, __VA_ARGS__)
+#  define DL_LOGV(...) DL_LOG(LogLevel::Verbose, __VA_ARGS__)
+#else
+// Disable Debug and Verbose logs for release builds.
+#  define DL_LOGD(...)
+#  define DL_LOGV(...)
+#endif
+
+/*
+ * An nsIFrame can have many different visual parts. For example an image frame
+ * can have a background, border, and outline, the image itself, and a
+ * translucent selection overlay. In general these parts can be drawn at
+ * discontiguous z-levels; see CSS2.1 appendix E:
+ * http://www.w3.org/TR/CSS21/zindex.html
+ *
+ * We construct a display list for a frame tree that contains one item
+ * for each visual part. The display list is itself a tree since some items
+ * are containers for other items; however, its structure does not match
+ * the structure of its source frame tree. The display list items are sorted
+ * by z-order. A display list can be used to paint the frames, to determine
+ * which frame is the target of a mouse event, and to determine what areas
+ * need to be repainted when scrolling. The display lists built for each task
+ * may be different for efficiency; in particular some frames need special
+ * display list items only for event handling, and do not create these items
+ * when the display list will be used for painting (the common case). For
+ * example, when painting we avoid creating nsDisplayBackground items for
+ * frames that don't display a visible background, but for event handling
+ * we need those backgrounds because they are not transparent to events.
+ *
+ * We could avoid constructing an explicit display list by traversing the
+ * frame tree multiple times in clever ways. However, reifying the display list
+ * reduces code complexity and reduces the number of times each frame must be
+ * traversed to one, which seems to be good for performance. It also means
+ * we can share code for painting, event handling and scroll analysis.
+ *
+ * Display lists are short-lived; content and frame trees cannot change
+ * between a display list being created and destroyed. Display lists should
+ * not be created during reflow because the frame tree may be in an
+ * inconsistent state (e.g., a frame's stored overflow-area may not include
+ * the bounds of all its children). However, it should be fine to create
+ * a display list while a reflow is pending, before it starts.
+ *
+ * A display list covers the "extended" frame tree; the display list for
+ * a frame tree containing FRAME/IFRAME elements can include frames from
+ * the subdocuments.
+ *
+ * Display item's coordinates are relative to their nearest reference frame
+ * ancestor. Both the display root and any frame with a transform act as a
+ * reference frame for their frame subtrees.
+ */
+
+/**
+ * An active scrolled root (ASR) is similar to an animated geometry root (AGR).
+ * The differences are:
+ *  - ASRs are only created for async-scrollable scroll frames. This is a
+ *    (hopefully) temporary restriction. In the future we will want to create
+ *    ASRs for all the things that are currently creating AGRs, and then
+ *    replace AGRs with ASRs and rename them from "active scrolled root" to
+ *    "animated geometry root".
+ *  - ASR objects are created during display list construction by the nsIFrames
+ *    that induce ASRs. This is done using AutoCurrentActiveScrolledRootSetter.
+ *    The current ASR is returned by
+ *    nsDisplayListBuilder::CurrentActiveScrolledRoot().
+ *  - There is no way to go from an nsIFrame pointer to the ASR of that frame.
+ *    If you need to look up an ASR after display list construction, you need
+ *    to store it while the AutoCurrentActiveScrolledRootSetter that creates it
+ *    is on the stack.
+ */
+struct ActiveScrolledRoot {
+  static already_AddRefed<ActiveScrolledRoot> CreateASRForFrame(
+      const ActiveScrolledRoot* aParent, nsIScrollableFrame* aScrollableFrame,
+      bool aIsRetained);
+
+  static const ActiveScrolledRoot* PickAncestor(
+      const ActiveScrolledRoot* aOne, const ActiveScrolledRoot* aTwo) {
+    MOZ_ASSERT(IsAncestor(aOne, aTwo) || IsAncestor(aTwo, aOne));
+    return Depth(aOne) <= Depth(aTwo) ? aOne : aTwo;
+  }
+
+  static const ActiveScrolledRoot* PickDescendant(
+      const ActiveScrolledRoot* aOne, const ActiveScrolledRoot* aTwo) {
+    MOZ_ASSERT(IsAncestor(aOne, aTwo) || IsAncestor(aTwo, aOne));
+    return Depth(aOne) >= Depth(aTwo) ? aOne : aTwo;
+  }
+
+  static bool IsAncestor(const ActiveScrolledRoot* aAncestor,
+                         const ActiveScrolledRoot* aDescendant);
+  static bool IsProperAncestor(const ActiveScrolledRoot* aAncestor,
+                               const ActiveScrolledRoot* aDescendant);
+
+  static nsCString ToString(const ActiveScrolledRoot* aActiveScrolledRoot);
+
+  // Call this when inserting an ancestor.
+  void IncrementDepth() { mDepth++; }
+
+  /**
+   * Find the view ID (or generate a new one) for the content element
+   * corresponding to the ASR.
+   */
+  layers::ScrollableLayerGuid::ViewID GetViewId() const {
+    if (!mViewId.isSome()) {
+      mViewId = Some(ComputeViewId());
+    }
+    return *mViewId;
+  }
+
+  RefPtr<const ActiveScrolledRoot> mParent;
+  nsIScrollableFrame* mScrollableFrame;
+
+  NS_INLINE_DECL_REFCOUNTING(ActiveScrolledRoot)
+
+ private:
+  ActiveScrolledRoot()
+      : mScrollableFrame(nullptr), mDepth(0), mRetained(false) {}
+
+  ~ActiveScrolledRoot();
+
+  static void DetachASR(ActiveScrolledRoot* aASR) {
+    aASR->mParent = nullptr;
+    aASR->mScrollableFrame = nullptr;
+    NS_RELEASE(aASR);
+  }
+  NS_DECLARE_FRAME_PROPERTY_WITH_DTOR(ActiveScrolledRootCache,
+                                      ActiveScrolledRoot, DetachASR)
+
+  static uint32_t Depth(const ActiveScrolledRoot* aActiveScrolledRoot) {
+    return aActiveScrolledRoot ? aActiveScrolledRoot->mDepth : 0;
+  }
+
+  layers::ScrollableLayerGuid::ViewID ComputeViewId() const;
+
+  // This field is lazily populated in GetViewId(). We don't want to do the
+  // work of populating if webrender is disabled, because it is often not
+  // needed.
+  mutable Maybe<layers::ScrollableLayerGuid::ViewID> mViewId;
+
+  uint32_t mDepth;
+  bool mRetained;
+};
+
+enum class nsDisplayListBuilderMode : uint8_t {
+  Painting,
+  PaintForPrinting,
+  EventDelivery,
+  FrameVisibility,
+  GenerateGlyph,
+};
+
+using ListArenaAllocator = ArenaAllocator<4096, 8>;
+
+class nsDisplayItem;
+class nsPaintedDisplayItem;
+class nsDisplayList;
+class nsDisplayWrapList;
+class nsDisplayTableBackgroundSet;
+class nsDisplayTableItem;
+
+class RetainedDisplayList;
+
+/**
+ * This manages a display list and is passed as a parameter to
+ * nsIFrame::BuildDisplayList.
+ * It contains the parameters that don't change from frame to frame and manages
+ * the display list memory using an arena. It also establishes the reference
+ * coordinate system for all display list items. Some of the parameters are
+ * available from the prescontext/presshell, but we copy them into the builder
+ * for faster/more convenient access.
+ */
+class nsDisplayListBuilder {
+  /**
+   * This manages status of a 3d context to collect visible rects of
+   * descendants and passing a dirty rect.
+   *
+   * Since some transforms maybe singular, passing visible rects or
+   * the dirty rect level by level from parent to children may get a
+   * wrong result, being different from the result of appling with
+   * effective transform directly.
+   *
+   * nsIFrame::BuildDisplayListForStackingContext() uses
+   * AutoPreserves3DContext to install an instance on the builder.
+   *
+   * \see AutoAccumulateTransform, AutoAccumulateRect,
+   *      AutoPreserves3DContext, Accumulate, GetCurrentTransform,
+   *      StartRoot.
+   */
+  class Preserves3DContext {
+   public:
+    Preserves3DContext()
+        : mAccumulatedRectLevels(0), mAllowAsyncAnimation(true) {}
+
+    Preserves3DContext(const Preserves3DContext& aOther)
+        : mAccumulatedRectLevels(0),
+          mVisibleRect(aOther.mVisibleRect),
+          mAllowAsyncAnimation(aOther.mAllowAsyncAnimation) {}
+
+    // Accmulate transforms of ancestors on the preserves-3d chain.
+    gfx::Matrix4x4 mAccumulatedTransform;
+    // Accmulate visible rect of descendants in the preserves-3d context.
+    nsRect mAccumulatedRect;
+    // How far this frame is from the root of the current 3d context.
+    int mAccumulatedRectLevels;
+    nsRect mVisibleRect;
+    // Allow async animation for this 3D context.
+    bool mAllowAsyncAnimation;
+  };
+
+ public:
+  using ViewID = layers::ScrollableLayerGuid::ViewID;
+
+  /**
+   * @param aReferenceFrame the frame at the root of the subtree; its origin
+   * is the origin of the reference coordinate system for this display list
+   * @param aMode encodes what the builder is being used for.
+   * @param aBuildCaret whether or not we should include the caret in any
+   * display lists that we make.
+   */
+  nsDisplayListBuilder(nsIFrame* aReferenceFrame,
+                       nsDisplayListBuilderMode aMode, bool aBuildCaret,
+                       bool aRetainingDisplayList = false);
+  ~nsDisplayListBuilder();
+
+  void BeginFrame();
+  void EndFrame();
+
+  void AddTemporaryItem(nsDisplayItem* aItem) {
+    mTemporaryItems.AppendElement(aItem);
+  }
+
+  WindowRenderer* GetWidgetWindowRenderer(nsView** aView = nullptr);
+  layers::WebRenderLayerManager* GetWidgetLayerManager(
+      nsView** aView = nullptr);
+
+  /**
+   * @return true if the display is being built in order to determine which
+   * frame is under the mouse position.
+   */
+  bool IsForEventDelivery() const {
+    return mMode == nsDisplayListBuilderMode::EventDelivery;
+  }
+
+  /**
+   * @return true if the display list is being built for painting. This
+   * includes both painting to a window or other buffer and painting to
+   * a print/pdf destination.
+   */
+  bool IsForPainting() const {
+    return mMode == nsDisplayListBuilderMode::Painting ||
+           mMode == nsDisplayListBuilderMode::PaintForPrinting;
+  }
+
+  /**
+   * @return true if the display list is being built specifically for printing.
+   */
+  bool IsForPrinting() const {
+    return mMode == nsDisplayListBuilderMode::PaintForPrinting;
+  }
+
+  /**
+   * @return true if the display list is being built for determining frame
+   * visibility.
+   */
+  bool IsForFrameVisibility() const {
+    return mMode == nsDisplayListBuilderMode::FrameVisibility;
+  }
+
+  /**
+   * @return true if the display list is being built for creating the glyph
+   * mask from text items.
+   */
+  bool IsForGenerateGlyphMask() const {
+    return mMode == nsDisplayListBuilderMode::GenerateGlyph;
+  }
+
+  bool BuildCompositorHitTestInfo() const {
+    return mBuildCompositorHitTestInfo;
+  }
+
+  /**
+   * @return true if "painting is suppressed" during page load and we
+   * should paint only the background of the document.
+   */
+  bool IsBackgroundOnly() {
+    NS_ASSERTION(mPresShellStates.Length() > 0,
+                 "don't call this if we're not in a presshell");
+    return CurrentPresShellState()->mIsBackgroundOnly;
+  }
+
+  /**
+   * @return the root of given frame's (sub)tree, whose origin
+   * establishes the coordinate system for the child display items.
+   */
+  const nsIFrame* FindReferenceFrameFor(const nsIFrame* aFrame,
+                                        nsPoint* aOffset = nullptr) const;
+
+  const Maybe<nsPoint>& AdditionalOffset() const { return mAdditionalOffset; }
+
+  /**
+   * @return the root of the display list's frame (sub)tree, whose origin
+   * establishes the coordinate system for the display list
+   */
+  nsIFrame* RootReferenceFrame() const { return mReferenceFrame; }
+
+  /**
+   * @return a point pt such that adding pt to a coordinate relative to aFrame
+   * makes it relative to ReferenceFrame(), i.e., returns
+   * aFrame->GetOffsetToCrossDoc(ReferenceFrame()). The returned point is in
+   * the appunits of aFrame.
+   */
+  const nsPoint ToReferenceFrame(const nsIFrame* aFrame) const {
+    nsPoint result;
+    FindReferenceFrameFor(aFrame, &result);
+    return result;
+  }
+  /**
+   * When building the display list, the scrollframe aFrame will be "ignored"
+   * for the purposes of clipping, and its scrollbars will be hidden. We use
+   * this to allow RenderOffscreen to render a whole document without beign
+   * clipped by the viewport or drawing the viewport scrollbars.
+   */
+  void SetIgnoreScrollFrame(nsIFrame* aFrame) { mIgnoreScrollFrame = aFrame; }
+  /**
+   * Get the scrollframe to ignore, if any.
+   */
+  nsIFrame* GetIgnoreScrollFrame() { return mIgnoreScrollFrame; }
+  void SetIsRelativeToLayoutViewport();
+  bool IsRelativeToLayoutViewport() const {
+    return mIsRelativeToLayoutViewport;
+  }
+  /**
+   * Get the ViewID of the nearest scrolling ancestor frame.
+   */
+  ViewID GetCurrentScrollParentId() const { return mCurrentScrollParentId; }
+  /**
+   * Get and set the flag that indicates if scroll parents should have layers
+   * forcibly created. This flag is set when a deeply nested scrollframe has
+   * a displayport, and for scroll handoff to work properly the ancestor
+   * scrollframes should also get their own scrollable layers.
+   */
+  void ForceLayerForScrollParent() { mForceLayerForScrollParent = true; }
+  /**
+   * Set the flag that indicates there is a non-minimal display port in the
+   * current subtree. This is used to determine display port expiry.
+   */
+  void SetContainsNonMinimalDisplayPort() {
+    mContainsNonMinimalDisplayPort = true;
+  }
+  /**
+   * Get the ViewID and the scrollbar flags corresponding to the scrollbar for
+   * which we are building display items at the moment.
+   */
+  ViewID GetCurrentScrollbarTarget() const { return mCurrentScrollbarTarget; }
+  Maybe<layers::ScrollDirection> GetCurrentScrollbarDirection() const {
+    return mCurrentScrollbarDirection;
+  }
+  /**
+   * Returns true if building a scrollbar, and the scrollbar will not be
+   * layerized.
+   */
+  bool IsBuildingNonLayerizedScrollbar() const {
+    return mIsBuildingScrollbar && !mCurrentScrollbarWillHaveLayer;
+  }
+  /**
+   * Calling this setter makes us include all out-of-flow descendant
+   * frames in the display list, wherever they may be positioned (even
+   * outside the dirty rects).
+   */
+  void SetIncludeAllOutOfFlows() { mIncludeAllOutOfFlows = true; }
+  bool GetIncludeAllOutOfFlows() const { return mIncludeAllOutOfFlows; }
+  /**
+   * Calling this setter makes us exclude all leaf frames that aren't
+   * selected.
+   */
+  void SetSelectedFramesOnly() { mSelectedFramesOnly = true; }
+  bool GetSelectedFramesOnly() { return mSelectedFramesOnly; }
+  /**
+   * @return Returns true if we should include the caret in any display lists
+   * that we make.
+   */
+  bool IsBuildingCaret() const { return mBuildCaret; }
+
+  bool IsRetainingDisplayList() const { return mRetainingDisplayList; }
+
+  bool IsPartialUpdate() const { return mPartialUpdate; }
+  void SetPartialUpdate(bool aPartial) { mPartialUpdate = aPartial; }
+
+  bool IsBuilding() const { return mIsBuilding; }
+  void SetIsBuilding(bool aIsBuilding) { mIsBuilding = aIsBuilding; }
+
+  bool InInvalidSubtree() const { return mInInvalidSubtree; }
+
+  /**
+   * Allows callers to selectively override the regular paint suppression
+   * checks, so that methods like GetFrameForPoint work when painting is
+   * suppressed.
+   */
+  void IgnorePaintSuppression() { mIgnoreSuppression = true; }
+  /**
+   * @return Returns if this builder will ignore paint suppression.
+   */
+  bool IsIgnoringPaintSuppression() { return mIgnoreSuppression; }
+  /**
+   * Call this if we're doing normal painting to the window.
+   */
+  void SetPaintingToWindow(bool aToWindow) { mIsPaintingToWindow = aToWindow; }
+  bool IsPaintingToWindow() const { return mIsPaintingToWindow; }
+  /**
+   * Call this if we're using high quality scaling for image decoding.
+   * It is also implied by IsPaintingToWindow.
+   */
+  void SetUseHighQualityScaling(bool aUseHighQualityScaling) {
+    mUseHighQualityScaling = aUseHighQualityScaling;
+  }
+  bool UseHighQualityScaling() const {
+    return mIsPaintingToWindow || mUseHighQualityScaling;
+  }
+  /**
+   * Call this if we're doing painting for WebRender
+   */
+  void SetPaintingForWebRender(bool aForWebRender) {
+    mIsPaintingForWebRender = true;
+  }
+  bool IsPaintingForWebRender() const { return mIsPaintingForWebRender; }
+  /**
+   * Call this to prevent descending into subdocuments.
+   */
+  void SetDescendIntoSubdocuments(bool aDescend) {
+    mDescendIntoSubdocuments = aDescend;
+  }
+
+  bool GetDescendIntoSubdocuments() { return mDescendIntoSubdocuments; }
+
+  /**
+   * Get dirty rect relative to current frame (the frame that we're calling
+   * BuildDisplayList on right now).
+   */
+  const nsRect& GetVisibleRect() { return mVisibleRect; }
+  const nsRect& GetDirtyRect() { return mDirtyRect; }
+
+  void SetVisibleRect(const nsRect& aVisibleRect) {
+    mVisibleRect = aVisibleRect;
+  }
+
+  void IntersectVisibleRect(const nsRect& aVisibleRect) {
+    mVisibleRect.IntersectRect(mVisibleRect, aVisibleRect);
+  }
+
+  void SetDirtyRect(const nsRect& aDirtyRect) { mDirtyRect = aDirtyRect; }
+
+  void IntersectDirtyRect(const nsRect& aDirtyRect) {
+    mDirtyRect.IntersectRect(mDirtyRect, aDirtyRect);
+  }
+
+  const nsIFrame* GetCurrentFrame() { return mCurrentFrame; }
+  const nsIFrame* GetCurrentReferenceFrame() { return mCurrentReferenceFrame; }
+
+  const nsPoint& GetCurrentFrameOffsetToReferenceFrame() const {
+    return mCurrentOffsetToReferenceFrame;
+  }
+
+  void Check() { mPool.Check(); }
+
+  /*
+   * Get the paint sequence number of the current paint.
+   */
+  static uint32_t GetPaintSequenceNumber() { return sPaintSequenceNumber; }
+
+  /*
+   * Increment the paint sequence number.
+   */
+  static void IncrementPaintSequenceNumber() { ++sPaintSequenceNumber; }
+
+  /**
+   * Returns true if merging and flattening of display lists should be
+   * performed while computing visibility.
+   */
+  bool AllowMergingAndFlattening() { return mAllowMergingAndFlattening; }
+  void SetAllowMergingAndFlattening(bool aAllow) {
+    mAllowMergingAndFlattening = aAllow;
+  }
+
+  void SetCompositorHitTestInfo(const gfx::CompositorHitTestInfo& aInfo) {
+    mCompositorHitTestInfo = aInfo;
+  }
+
+  const gfx::CompositorHitTestInfo& GetCompositorHitTestInfo() const {
+    return mCompositorHitTestInfo;
+  }
+
+  /**
+   * Builds a new nsDisplayCompositorHitTestInfo for the frame |aFrame| if
+   * needed, and adds it to the top of |aList|.
+   */
+  void BuildCompositorHitTestInfoIfNeeded(nsIFrame* aFrame,
+                                          nsDisplayList* aList);
+
+  bool IsInsidePointerEventsNoneDoc() {
+    return CurrentPresShellState()->mInsidePointerEventsNoneDoc;
+  }
+
+  bool IsTouchEventPrefEnabledDoc() {
+    return CurrentPresShellState()->mTouchEventPrefEnabledDoc;
+  }
+
+  bool GetAncestorHasApzAwareEventHandler() const {
+    return mAncestorHasApzAwareEventHandler;
+  }
+
+  void SetAncestorHasApzAwareEventHandler(bool aValue) {
+    mAncestorHasApzAwareEventHandler = aValue;
+  }
+
+  bool HaveScrollableDisplayPort() const { return mHaveScrollableDisplayPort; }
+  void SetHaveScrollableDisplayPort() { mHaveScrollableDisplayPort = true; }
+  void ClearHaveScrollableDisplayPort() { mHaveScrollableDisplayPort = false; }
+
+  bool SetIsCompositingCheap(bool aCompositingCheap) {
+    bool temp = mIsCompositingCheap;
+    mIsCompositingCheap = aCompositingCheap;
+    return temp;
+  }
+
+  bool IsCompositingCheap() const { return mIsCompositingCheap; }
+  /**
+   * Display the caret if needed.
+   */
+  bool DisplayCaret(nsIFrame* aFrame, nsDisplayList* aList) {
+    nsIFrame* frame = GetCaretFrame();
+    if (aFrame == frame && !IsBackgroundOnly()) {
+      frame->DisplayCaret(this, aList);
+      return true;
+    }
+    return false;
+  }
+  /**
+   * Get the frame that the caret is supposed to draw in.
+   * If the caret is currently invisible, this will be null.
+   */
+  nsIFrame* GetCaretFrame() { return mCaretFrame; }
+  /**
+   * Get the rectangle we're supposed to draw the caret into.
+   */
+  const nsRect& GetCaretRect() { return mCaretRect; }
+  /**
+   * Get the caret associated with the current presshell.
+   */
+  nsCaret* GetCaret();
+
+  /**
+   * Returns the root scroll frame for the current PresShell, if the PresShell
+   * is ignoring viewport scrolling.
+   */
+  nsIFrame* GetPresShellIgnoreScrollFrame() {
+    return CurrentPresShellState()->mPresShellIgnoreScrollFrame;
+  }
+
+  /**
+   * Notify the display list builder that we're entering a presshell.
+   * aReferenceFrame should be a frame in the new presshell.
+   * aPointerEventsNoneDoc should be set to true if the frame generating this
+   * document is pointer-events:none.
+   */
+  void EnterPresShell(const nsIFrame* aReferenceFrame,
+                      bool aPointerEventsNoneDoc = false);
+  /**
+   * For print-preview documents, we sometimes need to build display items for
+   * the same frames multiple times in the same presentation, with different
+   * clipping. Between each such batch of items, call
+   * ResetMarkedFramesForDisplayList to make sure that the results of
+   * MarkFramesForDisplayList do not carry over between batches.
+   */
+  void ResetMarkedFramesForDisplayList(const nsIFrame* aReferenceFrame);
+  /**
+   * Notify the display list builder that we're leaving a presshell.
+   */
+  void LeavePresShell(const nsIFrame* aReferenceFrame,
+                      nsDisplayList* aPaintedContents);
+
+  void IncrementPresShellPaintCount(PresShell* aPresShell);
+
+  /**
+   * Returns true if we're currently building a display list that's
+   * directly or indirectly under an nsDisplayTransform.
+   */
+  bool IsInTransform() const { return mInTransform; }
+
+  bool InEventsOnly() const { return mInEventsOnly; }
+  /**
+   * Indicate whether or not we're directly or indirectly under and
+   * nsDisplayTransform or SVG foreignObject.
+   */
+  void SetInTransform(bool aInTransform) { mInTransform = aInTransform; }
+
+  /**
+   * Returns true if we're currently building a display list that's
+   * under an nsDisplayFilters.
+   */
+  bool IsInFilter() const { return mInFilter; }
+
+  /**
+   * Return true if we're currently building a display list for a
+   * nested presshell.
+   */
+  bool IsInSubdocument() const { return mPresShellStates.Length() > 1; }
+
+  void SetDisablePartialUpdates(bool aDisable) {
+    mDisablePartialUpdates = aDisable;
+  }
+  bool DisablePartialUpdates() const { return mDisablePartialUpdates; }
+
+  void SetPartialBuildFailed(bool aFailed) { mPartialBuildFailed = aFailed; }
+  bool PartialBuildFailed() const { return mPartialBuildFailed; }
+
+  bool IsInActiveDocShell() const { return mIsInActiveDocShell; }
+  void SetInActiveDocShell(bool aActive) { mIsInActiveDocShell = aActive; }
+
+  /**
+   * Return true if we're currently building a display list for the presshell
+   * of a chrome document, or if we're building the display list for a popup.
+   */
+  bool IsInChromeDocumentOrPopup() const {
+    return mIsInChromePresContext || mIsBuildingForPopup;
+  }
+
+  /**
+   * @return true if images have been set to decode synchronously.
+   */
+  bool ShouldSyncDecodeImages() const { return mSyncDecodeImages; }
+
+  /**
+   * Indicates whether we should synchronously decode images. If true, we decode
+   * and draw whatever image data has been loaded. If false, we just draw
+   * whatever has already been decoded.
+   */
+  void SetSyncDecodeImages(bool aSyncDecodeImages) {
+    mSyncDecodeImages = aSyncDecodeImages;
+  }
+
+  nsDisplayTableBackgroundSet* SetTableBackgroundSet(
+      nsDisplayTableBackgroundSet* aTableSet) {
+    nsDisplayTableBackgroundSet* old = mTableBackgroundSet;
+    mTableBackgroundSet = aTableSet;
+    return old;
+  }
+  nsDisplayTableBackgroundSet* GetTableBackgroundSet() const {
+    return mTableBackgroundSet;
+  }
+
+  void FreeClipChains();
+
+  /*
+   * Frees the temporary display items created during merging.
+   */
+  void FreeTemporaryItems();
+
+  /**
+   * Helper method to generate background painting flags based on the
+   * information available in the display list builder.
+   */
+  uint32_t GetBackgroundPaintFlags();
+
+  /**
+   * Helper method to generate nsImageRenderer flags based on the information
+   * available in the display list builder.
+   */
+  uint32_t GetImageRendererFlags() const;
+
+  /**
+   * Helper method to generate image decoding flags based on the
+   * information available in the display list builder.
+   */
+  uint32_t GetImageDecodeFlags() const;
+
+  /**
+   * Subtracts aRegion from *aVisibleRegion. We avoid letting
+   * aVisibleRegion become overcomplex by simplifying it if necessary.
+   */
+  void SubtractFromVisibleRegion(nsRegion* aVisibleRegion,
+                                 const nsRegion& aRegion);
+
+  /**
+   * Mark the frames in aFrames to be displayed if they intersect aDirtyRect
+   * (which is relative to aDirtyFrame). If the frames have placeholders
+   * that might not be displayed, we mark the placeholders and their ancestors
+   * to ensure that display list construction descends into them
+   * anyway. nsDisplayListBuilder will take care of unmarking them when it is
+   * destroyed.
+   */
+  void MarkFramesForDisplayList(nsIFrame* aDirtyFrame,
+                                const nsFrameList& aFrames);
+  void MarkFrameForDisplay(nsIFrame* aFrame, const nsIFrame* aStopAtFrame);
+  void MarkFrameForDisplayIfVisible(nsIFrame* aFrame,
+                                    const nsIFrame* aStopAtFrame);
+  void AddFrameMarkedForDisplayIfVisible(nsIFrame* aFrame);
+
+  void ClearFixedBackgroundDisplayData();
+  /**
+   * Mark all child frames that Preserve3D() as needing display.
+   * Because these frames include transforms set on their parent, dirty rects
+   * for intermediate frames may be empty, yet child frames could still be
+   * visible.
+   */
+  void MarkPreserve3DFramesForDisplayList(nsIFrame* aDirtyFrame);
+
+  /**
+   * Returns true if we need to descend into this frame when building
+   * the display list, even though it doesn't intersect the dirty
+   * rect, because it may have out-of-flows that do so.
+   */
+  bool ShouldDescendIntoFrame(nsIFrame* aFrame, bool aVisible) const {
+    return aFrame->HasAnyStateBits(NS_FRAME_FORCE_DISPLAY_LIST_DESCEND_INTO) ||
+           (aVisible && aFrame->ForceDescendIntoIfVisible()) ||
+           GetIncludeAllOutOfFlows();
+  }
+
+  /**
+   * Returns the list of registered theme geometries.
+   */
+  nsTArray<nsIWidget::ThemeGeometry> GetThemeGeometries() const {
+    nsTArray<nsIWidget::ThemeGeometry> geometries;
+
+    for (const auto& data : mThemeGeometries.Values()) {
+      geometries.AppendElements(*data);
+    }
+
+    return geometries;
+  }
+
+  /**
+   * Notifies the builder that a particular themed widget exists
+   * at the given rectangle within the currently built display list.
+   * For certain appearance values (currently only StyleAppearance::Toolbar and
+   * StyleAppearance::WindowTitlebar) this gets called during every display list
+   * construction, for every themed widget of the right type within the
+   * display list, except for themed widgets which are transformed or have
+   * effects applied to them (e.g. CSS opacity or filters).
+   *
+   * @param aWidgetType the -moz-appearance value for the themed widget
+   * @param aItem the item associated with the theme geometry
+   * @param aRect the device-pixel rect relative to the widget's displayRoot
+   * for the themed widget
+   */
+  void RegisterThemeGeometry(uint8_t aWidgetType, nsDisplayItem* aItem,
+                             const LayoutDeviceIntRect& aRect) {
+    if (!mIsPaintingToWindow) {
+      return;
+    }
+
+    nsTArray<nsIWidget::ThemeGeometry>* geometries =
+        mThemeGeometries.GetOrInsertNew(aItem);
+    geometries->AppendElement(nsIWidget::ThemeGeometry(aWidgetType, aRect));
+  }
+
+  /**
+   * Removes theme geometries associated with the given display item |aItem|.
+   */
+  void UnregisterThemeGeometry(nsDisplayItem* aItem) {
+    mThemeGeometries.Remove(aItem);
+  }
+
+  /**
+   * Adjusts mWindowDraggingRegion to take into account aFrame. If aFrame's
+   * -moz-window-dragging value is |drag|, its border box is added to the
+   * collected dragging region; if the value is |no-drag|, the border box is
+   * subtracted from the region; if the value is |default|, that frame does
+   * not influence the window dragging region.
+   */
+  void AdjustWindowDraggingRegion(nsIFrame* aFrame);
+
+  LayoutDeviceIntRegion GetWindowDraggingRegion() const;
+
+  void RemoveModifiedWindowRegions();
+  void ClearRetainedWindowRegions();
+
+  const nsTHashMap<nsPtrHashKey<dom::RemoteBrowser>, dom::EffectsInfo>&
+  GetEffectUpdates() const {
+    return mEffectsUpdates;
+  }
+
+  void AddEffectUpdate(dom::RemoteBrowser* aBrowser,
+                       const dom::EffectsInfo& aUpdate);
+
+  /**
+   * Allocate memory in our arena. It will only be freed when this display list
+   * builder is destroyed. This memory holds nsDisplayItems and
+   * DisplayItemClipChain objects.
+   *
+   * Destructors are called as soon as the item is no longer used.
+   */
+  void* Allocate(size_t aSize, DisplayListArenaObjectId aId) {
+    return mPool.Allocate(aId, aSize);
+  }
+  void* Allocate(size_t aSize, DisplayItemType aType) {
+#define DECLARE_DISPLAY_ITEM_TYPE(name_, ...)                \
+  static_assert(size_t(DisplayItemType::TYPE_##name_) ==     \
+                    size_t(DisplayListArenaObjectId::name_), \
+                "");
+#include "nsDisplayItemTypesList.h"
+    static_assert(size_t(DisplayItemType::TYPE_MAX) ==
+                      size_t(DisplayListArenaObjectId::CLIPCHAIN),
+                  "");
+    static_assert(size_t(DisplayItemType::TYPE_MAX) + 1 ==
+                      size_t(DisplayListArenaObjectId::LISTNODE),
+                  "");
+#undef DECLARE_DISPLAY_ITEM_TYPE
+    return Allocate(aSize, DisplayListArenaObjectId(size_t(aType)));
+  }
+
+  void Destroy(DisplayListArenaObjectId aId, void* aPtr) {
+    return mPool.Free(aId, aPtr);
+  }
+  void Destroy(DisplayItemType aType, void* aPtr) {
+    return Destroy(DisplayListArenaObjectId(size_t(aType)), aPtr);
+  }
+
+  /**
+   * Allocate a new ActiveScrolledRoot in the arena. Will be cleaned up
+   * automatically when the arena goes away.
+   */
+  ActiveScrolledRoot* AllocateActiveScrolledRoot(
+      const ActiveScrolledRoot* aParent, nsIScrollableFrame* aScrollableFrame);
+
+  /**
+   * Allocate a new DisplayItemClipChain object in the arena. Will be cleaned
+   * up automatically when the arena goes away.
+   */
+  const DisplayItemClipChain* AllocateDisplayItemClipChain(
+      const DisplayItemClip& aClip, const ActiveScrolledRoot* aASR,
+      const DisplayItemClipChain* aParent);
+
+  /**
+   * Intersect two clip chains, allocating the new clip chain items in this
+   * builder's arena. The result is parented to aAncestor, and no intersections
+   * happen past aAncestor's ASR.
+   * That means aAncestor has to be living in this builder's arena already.
+   * aLeafClip1 and aLeafClip2 only need to outlive the call to this function,
+   * their values are copied into the newly-allocated intersected clip chain
+   * and this function does not hold on to any pointers to them.
+   */
+  const DisplayItemClipChain* CreateClipChainIntersection(
+      const DisplayItemClipChain* aAncestor,
+      const DisplayItemClipChain* aLeafClip1,
+      const DisplayItemClipChain* aLeafClip2);
+
+  /**
+   * Same as above, except aAncestor is computed as the nearest common
+   * ancestor of the two provided clips.
+   */
+  const DisplayItemClipChain* CreateClipChainIntersection(
+      const DisplayItemClipChain* aLeafClip1,
+      const DisplayItemClipChain* aLeafClip2);
+
+  /**
+   * Clone the supplied clip chain's chain items into this builder's arena.
+   */
+  const DisplayItemClipChain* CopyWholeChain(
+      const DisplayItemClipChain* aClipChain);
+
+  const ActiveScrolledRoot* GetFilterASR() const { return mFilterASR; }
+
+  /**
+   * Merges the display items in |aMergedItems| and returns a new temporary
+   * display item.
+   * The display items in |aMergedItems| have to be mergeable with each other.
+   */
+  nsDisplayWrapList* MergeItems(nsTArray<nsDisplayItem*>& aItems);
+
+  /**
+   * A helper class used to temporarily set nsDisplayListBuilder properties for
+   * building display items.
+   * aVisibleRect and aDirtyRect are relative to aForChild.
+   */
+  class AutoBuildingDisplayList {
+   public:
+    AutoBuildingDisplayList(nsDisplayListBuilder* aBuilder, nsIFrame* aForChild,
+                            const nsRect& aVisibleRect,
+                            const nsRect& aDirtyRect)
+        : AutoBuildingDisplayList(aBuilder, aForChild, aVisibleRect, aDirtyRect,
+                                  aForChild->IsTransformed()) {}
+
+    AutoBuildingDisplayList(nsDisplayListBuilder* aBuilder, nsIFrame* aForChild,
+                            const nsRect& aVisibleRect,
+                            const nsRect& aDirtyRect,
+                            const bool aIsTransformed);
+
+    void SetReferenceFrameAndCurrentOffset(const nsIFrame* aFrame,
+                                           const nsPoint& aOffset) {
+      mBuilder->mCurrentReferenceFrame = aFrame;
+      mBuilder->mCurrentOffsetToReferenceFrame = aOffset;
+    }
+
+    void SetAdditionalOffset(const nsPoint& aOffset) {
+      MOZ_ASSERT(!mBuilder->mAdditionalOffset);
+      mBuilder->mAdditionalOffset = Some(aOffset);
+
+      mBuilder->mCurrentOffsetToReferenceFrame += aOffset;
+    }
+
+    void RestoreBuildingInvisibleItemsValue() {
+      mBuilder->mBuildingInvisibleItems = mPrevBuildingInvisibleItems;
+    }
+
+    ~AutoBuildingDisplayList() {
+      mBuilder->mCurrentFrame = mPrevFrame;
+      mBuilder->mCurrentReferenceFrame = mPrevReferenceFrame;
+      mBuilder->mCurrentOffsetToReferenceFrame = mPrevOffset;
+      mBuilder->mVisibleRect = mPrevVisibleRect;
+      mBuilder->mDirtyRect = mPrevDirtyRect;
+      mBuilder->mAncestorHasApzAwareEventHandler =
+          mPrevAncestorHasApzAwareEventHandler;
+      mBuilder->mBuildingInvisibleItems = mPrevBuildingInvisibleItems;
+      mBuilder->mInInvalidSubtree = mPrevInInvalidSubtree;
+      mBuilder->mAdditionalOffset = mPrevAdditionalOffset;
+      mBuilder->mCompositorHitTestInfo = mPrevCompositorHitTestInfo;
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    const nsIFrame* mPrevFrame;
+    const nsIFrame* mPrevReferenceFrame;
+    nsPoint mPrevOffset;
+    Maybe<nsPoint> mPrevAdditionalOffset;
+    nsRect mPrevVisibleRect;
+    nsRect mPrevDirtyRect;
+    gfx::CompositorHitTestInfo mPrevCompositorHitTestInfo;
+    bool mPrevAncestorHasApzAwareEventHandler;
+    bool mPrevBuildingInvisibleItems;
+    bool mPrevInInvalidSubtree;
+  };
+
+  /**
+   * A helper class to temporarily set the value of mInTransform.
+   */
+  class AutoInTransformSetter {
+   public:
+    AutoInTransformSetter(nsDisplayListBuilder* aBuilder, bool aInTransform)
+        : mBuilder(aBuilder), mOldValue(aBuilder->mInTransform) {
+      aBuilder->mInTransform = aInTransform;
+    }
+
+    ~AutoInTransformSetter() { mBuilder->mInTransform = mOldValue; }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    bool mOldValue;
+  };
+
+  class AutoInEventsOnly {
+   public:
+    AutoInEventsOnly(nsDisplayListBuilder* aBuilder, bool aInEventsOnly)
+        : mBuilder(aBuilder), mOldValue(aBuilder->mInEventsOnly) {
+      aBuilder->mInEventsOnly |= aInEventsOnly;
+    }
+
+    ~AutoInEventsOnly() { mBuilder->mInEventsOnly = mOldValue; }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    bool mOldValue;
+  };
+
+  /**
+   * A helper class to temporarily set the value of mFilterASR and
+   * mInFilter.
+   */
+  class AutoEnterFilter {
+   public:
+    AutoEnterFilter(nsDisplayListBuilder* aBuilder, bool aUsingFilter)
+        : mBuilder(aBuilder),
+          mOldValue(aBuilder->mFilterASR),
+          mOldInFilter(aBuilder->mInFilter) {
+      if (!aBuilder->mFilterASR && aUsingFilter) {
+        aBuilder->mFilterASR = aBuilder->CurrentActiveScrolledRoot();
+        aBuilder->mInFilter = true;
+      }
+    }
+
+    ~AutoEnterFilter() {
+      mBuilder->mFilterASR = mOldValue;
+      mBuilder->mInFilter = mOldInFilter;
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    const ActiveScrolledRoot* mOldValue;
+    bool mOldInFilter;
+  };
+
+  /**
+   * Used to update the current active scrolled root on the display list
+   * builder, and to create new active scrolled roots.
+   */
+  class AutoCurrentActiveScrolledRootSetter {
+   public:
+    explicit AutoCurrentActiveScrolledRootSetter(nsDisplayListBuilder* aBuilder)
+        : mBuilder(aBuilder),
+          mSavedActiveScrolledRoot(aBuilder->mCurrentActiveScrolledRoot),
+          mContentClipASR(aBuilder->ClipState().GetContentClipASR()),
+          mDescendantsStartIndex(aBuilder->mActiveScrolledRoots.Length()),
+          mUsed(false),
+          mOldScrollParentId(aBuilder->mCurrentScrollParentId),
+          mOldForceLayer(aBuilder->mForceLayerForScrollParent),
+          mOldContainsNonMinimalDisplayPort(
+              mBuilder->mContainsNonMinimalDisplayPort),
+          mCanBeScrollParent(false) {}
+
+    void SetCurrentScrollParentId(ViewID aScrollId) {
+      // Update the old scroll parent id.
+      mOldScrollParentId = mBuilder->mCurrentScrollParentId;
+      // If this AutoCurrentActiveScrolledRootSetter has the same aScrollId as
+      // the previous one on the stack, then that means the scrollframe that
+      // created this isn't actually scrollable and cannot participate in
+      // scroll handoff. We set mCanBeScrollParent to false to indicate this.
+      mCanBeScrollParent = (mOldScrollParentId != aScrollId);
+      mBuilder->mCurrentScrollParentId = aScrollId;
+      mBuilder->mForceLayerForScrollParent = false;
+      mBuilder->mContainsNonMinimalDisplayPort = false;
+    }
+
+    bool ShouldForceLayerForScrollParent() const {
+      // Only scrollframes participating in scroll handoff can be forced to
+      // layerize
+      return mCanBeScrollParent && mBuilder->mForceLayerForScrollParent;
+    }
+
+    bool GetContainsNonMinimalDisplayPort() const {
+      // Only for scrollframes participating in scroll handoff can we return
+      // true.
+      return mCanBeScrollParent && mBuilder->mContainsNonMinimalDisplayPort;
+    }
+
+    ~AutoCurrentActiveScrolledRootSetter() {
+      mBuilder->mCurrentActiveScrolledRoot = mSavedActiveScrolledRoot;
+      mBuilder->mCurrentScrollParentId = mOldScrollParentId;
+      if (mCanBeScrollParent) {
+        // If this flag is set, caller code is responsible for having dealt
+        // with the current value of mBuilder->mForceLayerForScrollParent, so
+        // we can just restore the old value.
+        mBuilder->mForceLayerForScrollParent = mOldForceLayer;
+      } else {
+        // Otherwise we need to keep propagating the force-layerization flag
+        // upwards to the next ancestor scrollframe that does participate in
+        // scroll handoff.
+        mBuilder->mForceLayerForScrollParent |= mOldForceLayer;
+      }
+      mBuilder->mContainsNonMinimalDisplayPort |=
+          mOldContainsNonMinimalDisplayPort;
+    }
+
+    void SetCurrentActiveScrolledRoot(
+        const ActiveScrolledRoot* aActiveScrolledRoot);
+
+    void EnterScrollFrame(nsIScrollableFrame* aScrollableFrame) {
+      MOZ_ASSERT(!mUsed);
+      ActiveScrolledRoot* asr = mBuilder->AllocateActiveScrolledRoot(
+          mBuilder->mCurrentActiveScrolledRoot, aScrollableFrame);
+      mBuilder->mCurrentActiveScrolledRoot = asr;
+      mUsed = true;
+    }
+
+    void InsertScrollFrame(nsIScrollableFrame* aScrollableFrame);
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    /**
+     * The builder's mCurrentActiveScrolledRoot at construction time which
+     * needs to be restored at destruction time.
+     */
+    const ActiveScrolledRoot* mSavedActiveScrolledRoot;
+    /**
+     * If there's a content clip on the builder at construction time, then
+     * mContentClipASR is that content clip's ASR, otherwise null. The
+     * assumption is that the content clip doesn't get relaxed while this
+     * object is on the stack.
+     */
+    const ActiveScrolledRoot* mContentClipASR;
+    /**
+     * InsertScrollFrame needs to mutate existing ASRs (those that were
+     * created while this object was on the stack), and mDescendantsStartIndex
+     * makes it easier to skip ASRs that were created in the past.
+     */
+    size_t mDescendantsStartIndex;
+    /**
+     * Flag to make sure that only one of SetCurrentActiveScrolledRoot /
+     * EnterScrollFrame / InsertScrollFrame is called per instance of this
+     * class.
+     */
+    bool mUsed;
+    ViewID mOldScrollParentId;
+    bool mOldForceLayer;
+    bool mOldContainsNonMinimalDisplayPort;
+    bool mCanBeScrollParent;
+  };
+
+  /**
+   * Keeps track of the innermost ASR that can be used as the ASR for a
+   * container item that wraps all items that were created while this
+   * object was on the stack.
+   * The rule is: all child items of the container item need to have
+   * clipped bounds with respect to the container ASR.
+   */
+  class AutoContainerASRTracker {
+   public:
+    explicit AutoContainerASRTracker(nsDisplayListBuilder* aBuilder);
+
+    const ActiveScrolledRoot* GetContainerASR() {
+      return mBuilder->mCurrentContainerASR;
+    }
+
+    ~AutoContainerASRTracker() {
+      mBuilder->mCurrentContainerASR = ActiveScrolledRoot::PickAncestor(
+          mBuilder->mCurrentContainerASR, mSavedContainerASR);
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    const ActiveScrolledRoot* mSavedContainerASR;
+  };
+
+  /**
+   * A helper class to temporarily set the value of mCurrentScrollbarTarget
+   * and mCurrentScrollbarFlags.
+   */
+  class AutoCurrentScrollbarInfoSetter {
+   public:
+    AutoCurrentScrollbarInfoSetter(
+        nsDisplayListBuilder* aBuilder, ViewID aScrollTargetID,
+        const Maybe<layers::ScrollDirection>& aScrollbarDirection,
+        bool aWillHaveLayer)
+        : mBuilder(aBuilder) {
+      aBuilder->mIsBuildingScrollbar = true;
+      aBuilder->mCurrentScrollbarTarget = aScrollTargetID;
+      aBuilder->mCurrentScrollbarDirection = aScrollbarDirection;
+      aBuilder->mCurrentScrollbarWillHaveLayer = aWillHaveLayer;
+    }
+
+    ~AutoCurrentScrollbarInfoSetter() {
+      // No need to restore old values because scrollbars cannot be nested.
+      mBuilder->mIsBuildingScrollbar = false;
+      mBuilder->mCurrentScrollbarTarget =
+          layers::ScrollableLayerGuid::NULL_SCROLL_ID;
+      mBuilder->mCurrentScrollbarDirection.reset();
+      mBuilder->mCurrentScrollbarWillHaveLayer = false;
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+  };
+
+  /**
+   * A helper class to temporarily set mBuildingExtraPagesForPageNum.
+   */
+  class MOZ_RAII AutoPageNumberSetter {
+   public:
+    AutoPageNumberSetter(nsDisplayListBuilder* aBuilder, const uint8_t aPageNum)
+        : mBuilder(aBuilder),
+          mOldPageNum(aBuilder->GetBuildingExtraPagesForPageNum()) {
+      mBuilder->SetBuildingExtraPagesForPageNum(aPageNum);
+    }
+    ~AutoPageNumberSetter() {
+      mBuilder->SetBuildingExtraPagesForPageNum(mOldPageNum);
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    uint8_t mOldPageNum;
+  };
+
+  /**
+   * A helper class to track current effective transform for items.
+   *
+   * For frames that is Combines3DTransformWithAncestors(), we need to
+   * apply all transforms of ancestors on the same preserves3D chain
+   * on the bounds of current frame to the coordination of the 3D
+   * context root.  The 3D context root computes it's bounds from
+   * these transformed bounds.
+   */
+  class AutoAccumulateTransform {
+   public:
+    explicit AutoAccumulateTransform(nsDisplayListBuilder* aBuilder)
+        : mBuilder(aBuilder),
+          mSavedTransform(aBuilder->mPreserves3DCtx.mAccumulatedTransform) {}
+
+    ~AutoAccumulateTransform() {
+      mBuilder->mPreserves3DCtx.mAccumulatedTransform = mSavedTransform;
+    }
+
+    void Accumulate(const gfx::Matrix4x4& aTransform) {
+      mBuilder->mPreserves3DCtx.mAccumulatedTransform =
+          aTransform * mBuilder->mPreserves3DCtx.mAccumulatedTransform;
+    }
+
+    const gfx::Matrix4x4& GetCurrentTransform() {
+      return mBuilder->mPreserves3DCtx.mAccumulatedTransform;
+    }
+
+    void StartRoot() {
+      mBuilder->mPreserves3DCtx.mAccumulatedTransform = gfx::Matrix4x4();
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    gfx::Matrix4x4 mSavedTransform;
+  };
+
+  /**
+   * A helper class to collect bounds rects of descendants.
+   *
+   * For a 3D context root, it's bounds is computed from the bounds of
+   * descendants.  If we transform bounds frame by frame applying
+   * transforms, the bounds may turn to empty for any singular
+   * transform on the path, but it is not empty for the accumulated
+   * transform.
+   */
+  class AutoAccumulateRect {
+   public:
+    explicit AutoAccumulateRect(nsDisplayListBuilder* aBuilder)
+        : mBuilder(aBuilder),
+          mSavedRect(aBuilder->mPreserves3DCtx.mAccumulatedRect) {
+      aBuilder->mPreserves3DCtx.mAccumulatedRect = nsRect();
+      aBuilder->mPreserves3DCtx.mAccumulatedRectLevels++;
+    }
+
+    ~AutoAccumulateRect() {
+      mBuilder->mPreserves3DCtx.mAccumulatedRect = mSavedRect;
+      mBuilder->mPreserves3DCtx.mAccumulatedRectLevels--;
+    }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    nsRect mSavedRect;
+  };
+
+  void AccumulateRect(const nsRect& aRect) {
+    mPreserves3DCtx.mAccumulatedRect.UnionRect(mPreserves3DCtx.mAccumulatedRect,
+                                               aRect);
+  }
+
+  const nsRect& GetAccumulatedRect() {
+    return mPreserves3DCtx.mAccumulatedRect;
+  }
+
+  /**
+   * The level is increased by one for items establishing 3D rendering
+   * context and starting a new accumulation.
+   */
+  int GetAccumulatedRectLevels() {
+    return mPreserves3DCtx.mAccumulatedRectLevels;
+  }
+
+  struct OutOfFlowDisplayData {
+    OutOfFlowDisplayData(
+        const DisplayItemClipChain* aContainingBlockClipChain,
+        const DisplayItemClipChain* aCombinedClipChain,
+        const ActiveScrolledRoot* aContainingBlockActiveScrolledRoot,
+        const ViewID& aScrollParentId, const nsRect& aVisibleRect,
+        const nsRect& aDirtyRect)
+        : mContainingBlockClipChain(aContainingBlockClipChain),
+          mCombinedClipChain(aCombinedClipChain),
+          mContainingBlockActiveScrolledRoot(
+              aContainingBlockActiveScrolledRoot),
+          mVisibleRect(aVisibleRect),
+          mDirtyRect(aDirtyRect),
+          mScrollParentId(aScrollParentId) {}
+    const DisplayItemClipChain* mContainingBlockClipChain;
+    const DisplayItemClipChain*
+        mCombinedClipChain;  // only necessary for the special case of top layer
+    const ActiveScrolledRoot* mContainingBlockActiveScrolledRoot;
+
+    // If this OutOfFlowDisplayData is associated with the ViewportFrame
+    // of a document that has a resolution (creating separate visual and
+    // layout viewports with their own coordinate spaces), these rects
+    // are in layout coordinates. Similarly, GetVisibleRectForFrame() in
+    // such a case returns a quantity in layout coordinates.
+    nsRect mVisibleRect;
+    nsRect mDirtyRect;
+    ViewID mScrollParentId;
+
+    static nsRect ComputeVisibleRectForFrame(nsDisplayListBuilder* aBuilder,
+                                             nsIFrame* aFrame,
+                                             const nsRect& aVisibleRect,
+                                             const nsRect& aDirtyRect,
+                                             nsRect* aOutDirtyRect);
+
+    nsRect GetVisibleRectForFrame(nsDisplayListBuilder* aBuilder,
+                                  nsIFrame* aFrame, nsRect* aDirtyRect) {
+      return ComputeVisibleRectForFrame(aBuilder, aFrame, mVisibleRect,
+                                        mDirtyRect, aDirtyRect);
+    }
+  };
+
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(OutOfFlowDisplayDataProperty,
+                                      OutOfFlowDisplayData)
+
+  struct DisplayListBuildingData {
+    nsIFrame* mModifiedAGR = nullptr;
+    nsRect mDirtyRect;
+  };
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(DisplayListBuildingRect,
+                                      DisplayListBuildingData)
+
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(DisplayListBuildingDisplayPortRect,
+                                      nsRect)
+
+  static OutOfFlowDisplayData* GetOutOfFlowData(nsIFrame* aFrame) {
+    if (!aFrame->GetParent()) {
+      return nullptr;
+    }
+    return aFrame->GetParent()->GetProperty(OutOfFlowDisplayDataProperty());
+  }
+
+  nsPresContext* CurrentPresContext();
+
+  OutOfFlowDisplayData* GetCurrentFixedBackgroundDisplayData() {
+    auto& displayData = CurrentPresShellState()->mFixedBackgroundDisplayData;
+    return displayData ? displayData.ptr() : nullptr;
+  }
+
+  /**
+   * Accumulates opaque stuff into the window opaque region.
+   */
+  void AddWindowOpaqueRegion(nsIFrame* aFrame, const nsRect& aBounds) {
+    if (IsRetainingDisplayList()) {
+      mRetainedWindowOpaqueRegion.Add(aFrame, aBounds);
+      return;
+    }
+    mWindowOpaqueRegion.Or(mWindowOpaqueRegion, aBounds);
+  }
+  /**
+   * Returns the window opaque region built so far. This may be incomplete
+   * since the opaque region is built during layer construction.
+   */
+  const nsRegion GetWindowOpaqueRegion() {
+    return IsRetainingDisplayList() ? mRetainedWindowOpaqueRegion.ToRegion()
+                                    : mWindowOpaqueRegion;
+  }
+
+  /**
+   * mContainsBlendMode is true if we processed a display item that
+   * has a blend mode attached. We do this so we can insert a
+   * nsDisplayBlendContainer in the parent stacking context.
+   */
+  void SetContainsBlendMode(bool aContainsBlendMode) {
+    mContainsBlendMode = aContainsBlendMode;
+  }
+  bool ContainsBlendMode() const { return mContainsBlendMode; }
+
+  DisplayListClipState& ClipState() { return mClipState; }
+  const ActiveScrolledRoot* CurrentActiveScrolledRoot() {
+    return mCurrentActiveScrolledRoot;
+  }
+  const ActiveScrolledRoot* CurrentAncestorASRStackingContextContents() {
+    return mCurrentContainerASR;
+  }
+
+  /**
+   * Add the current frame to the will-change budget if possible and
+   * remeber the outcome. Subsequent calls to IsInWillChangeBudget
+   * will return the same value as return here.
+   */
+  bool AddToWillChangeBudget(nsIFrame* aFrame, const nsSize& aSize);
+
+  /**
+   * This will add the current frame to the will-change budget the first
+   * time it is seen. On subsequent calls this will return the same
+   * answer. This effectively implements a first-come, first-served
+   * allocation of the will-change budget.
+   */
+  bool IsInWillChangeBudget(nsIFrame* aFrame, const nsSize& aSize);
+
+  /**
+   * Clears the will-change budget status for the given |aFrame|.
+   * This will also remove the frame from will-change budgets.
+   */
+  void ClearWillChangeBudgetStatus(nsIFrame* aFrame);
+
+  /**
+   * Removes the given |aFrame| from will-change budgets.
+   */
+  void RemoveFromWillChangeBudgets(const nsIFrame* aFrame);
+
+  /**
+   * Clears the will-change budgets.
+   */
+  void ClearWillChangeBudgets();
+
+  void EnterSVGEffectsContents(nsIFrame* aEffectsFrame,
+                               nsDisplayList* aHoistedItemsStorage);
+  void ExitSVGEffectsContents();
+
+  bool ShouldBuildScrollInfoItemsForHoisting() const;
+
+  void AppendNewScrollInfoItemForHoisting(
+      nsDisplayScrollInfoLayer* aScrollInfoItem);
+
+  /**
+   * A helper class to install/restore nsDisplayListBuilder::mPreserves3DCtx.
+   *
+   * mPreserves3DCtx is used by class AutoAccumulateTransform &
+   * AutoAccumulateRect to passing data between frames in the 3D
+   * context.  If a frame create a new 3D context, it should restore
+   * the value of mPreserves3DCtx before returning back to the parent.
+   * This class do it for the users.
+   */
+  class AutoPreserves3DContext {
+   public:
+    explicit AutoPreserves3DContext(nsDisplayListBuilder* aBuilder)
+        : mBuilder(aBuilder), mSavedCtx(aBuilder->mPreserves3DCtx) {}
+
+    ~AutoPreserves3DContext() { mBuilder->mPreserves3DCtx = mSavedCtx; }
+
+   private:
+    nsDisplayListBuilder* mBuilder;
+    Preserves3DContext mSavedCtx;
+  };
+
+  const nsRect GetPreserves3DRect() const {
+    return mPreserves3DCtx.mVisibleRect;
+  }
+
+  void SavePreserves3DRect() { mPreserves3DCtx.mVisibleRect = mVisibleRect; }
+
+  void SavePreserves3DAllowAsyncAnimation(bool aValue) {
+    mPreserves3DCtx.mAllowAsyncAnimation = aValue;
+  }
+
+  bool GetPreserves3DAllowAsyncAnimation() const {
+    return mPreserves3DCtx.mAllowAsyncAnimation;
+  }
+
+  bool IsBuildingInvisibleItems() const { return mBuildingInvisibleItems; }
+
+  void SetBuildingInvisibleItems(bool aBuildingInvisibleItems) {
+    mBuildingInvisibleItems = aBuildingInvisibleItems;
+  }
+
+  void SetBuildingExtraPagesForPageNum(uint8_t aPageNum) {
+    mBuildingExtraPagesForPageNum = aPageNum;
+  }
+  uint8_t GetBuildingExtraPagesForPageNum() const {
+    return mBuildingExtraPagesForPageNum;
+  }
+
+  bool HitTestIsForVisibility() const { return mVisibleThreshold.isSome(); }
+
+  float VisibilityThreshold() const {
+    MOZ_DIAGNOSTIC_ASSERT(HitTestIsForVisibility());
+    return mVisibleThreshold.valueOr(1.0f);
+  }
+
+  void SetHitTestIsForVisibility(float aVisibleThreshold) {
+    mVisibleThreshold = Some(aVisibleThreshold);
+  }
+
+  bool ShouldBuildAsyncZoomContainer() const {
+    return mBuildAsyncZoomContainer;
+  }
+  void UpdateShouldBuildAsyncZoomContainer();
+
+  void UpdateShouldBuildBackdropRootContainer();
+
+  bool ShouldRebuildDisplayListDueToPrefChange();
+
+  /**
+   * Represents a region composed of frame/rect pairs.
+   * WeakFrames are used to track whether a rect still belongs to the region.
+   * Modified frames and rects are removed and re-added to the region if needed.
+   */
+  struct WeakFrameRegion {
+    /**
+     * A wrapper to store WeakFrame and the pointer to the underlying frame.
+     * This is needed because WeakFrame does not store the frame pointer after
+     * the frame has been deleted.
+     */
+    struct WeakFrameWrapper {
+      explicit WeakFrameWrapper(nsIFrame* aFrame)
+          : mWeakFrame(new WeakFrame(aFrame)), mFrame(aFrame) {}
+
+      UniquePtr<WeakFrame> mWeakFrame;
+      void* mFrame;
+    };
+
+    nsTHashSet<void*> mFrameSet;
+    nsTArray<WeakFrameWrapper> mFrames;
+    nsTArray<pixman_box32_t> mRects;
+
+    template <typename RectType>
+    void Add(nsIFrame* aFrame, const RectType& aRect) {
+      if (mFrameSet.Contains(aFrame)) {
+        return;
+      }
+
+      mFrameSet.Insert(aFrame);
+      mFrames.AppendElement(WeakFrameWrapper(aFrame));
+      mRects.AppendElement(nsRegion::RectToBox(aRect));
+    }
+
+    void Clear() {
+      mFrameSet.Clear();
+      mFrames.Clear();
+      mRects.Clear();
+    }
+
+    void RemoveModifiedFramesAndRects();
+
+    size_t SizeOfExcludingThis(MallocSizeOf) const;
+
+    typedef gfx::ArrayView<pixman_box32_t> BoxArrayView;
+
+    nsRegion ToRegion() const { return nsRegion(BoxArrayView(mRects)); }
+
+    LayoutDeviceIntRegion ToLayoutDeviceIntRegion() const {
+      return LayoutDeviceIntRegion(BoxArrayView(mRects));
+    }
+  };
+
+  void AddScrollFrameToNotify(nsIScrollableFrame* aScrollFrame);
+  void NotifyAndClearScrollFrames();
+
+  // Helper class to find what link spec (if any) to associate with a frame,
+  // recording it in the builder, and generate the corresponding DisplayItem.
+  // This also takes care of generating a named destination for internal links
+  // if the element has an id or name attribute.
+  class Linkifier {
+   public:
+    Linkifier(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+              nsDisplayList* aList);
+
+    ~Linkifier() {
+      if (mBuilderToReset) {
+        mBuilderToReset->mLinkSpec.Truncate(0);
+      }
+    }
+
+    void MaybeAppendLink(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame);
+
+   private:
+    nsDisplayListBuilder* mBuilderToReset = nullptr;
+    nsDisplayList* mList;
+  };
+
+  /**
+   * Returns the nearest ancestor frame to aFrame that is considered to have
+   * (or will have) animated geometry. This can return aFrame.
+   */
+  nsIFrame* FindAnimatedGeometryRootFrameFor(nsIFrame* aFrame);
+
+  /**
+   * Returns true if this is a retained builder and reuse stacking contexts
+   * mode is enabled by pref.
+   */
+  bool IsReusingStackingContextItems() const {
+    return mIsReusingStackingContextItems;
+  }
+
+  /**
+   * Adds display item |aItem| to the reuseable display items set.
+   */
+  void AddReusableDisplayItem(nsDisplayItem* aItem);
+
+  /**
+   * Removes display item |aItem| from the reuseable display items set.
+   * This is needed because display items are sometimes deleted during
+   * display list building.
+   * Called by |nsDisplayItem::Destroy()| when the item has been reused.
+   */
+  void RemoveReusedDisplayItem(nsDisplayItem* aItem);
+
+  /**
+   * Clears the reuseable display items set.
+   */
+  void ClearReuseableDisplayItems();
+
+  /**
+   * Marks the given display item |aItem| as reused, and updates the necessary
+   * display list builder state.
+   */
+  void ReuseDisplayItem(nsDisplayItem* aItem);
+
+ private:
+  bool MarkOutOfFlowFrameForDisplay(nsIFrame* aDirtyFrame, nsIFrame* aFrame,
+                                    const nsRect& aVisibleRect,
+                                    const nsRect& aDirtyRect);
+
+  friend class nsDisplayBackgroundImage;
+  friend class RetainedDisplayListBuilder;
+
+  /**
+   * Returns whether a frame acts as an animated geometry root, optionally
+   * returning the next ancestor to check.
+   */
+  bool IsAnimatedGeometryRoot(nsIFrame* aFrame, nsIFrame** aParent = nullptr);
+
+  struct PresShellState {
+    PresShell* mPresShell;
+#ifdef DEBUG
+    Maybe<nsAutoLayoutPhase> mAutoLayoutPhase;
+#endif
+    Maybe<OutOfFlowDisplayData> mFixedBackgroundDisplayData;
+    uint32_t mFirstFrameMarkedForDisplay;
+    uint32_t mFirstFrameWithOOFData;
+    bool mIsBackgroundOnly;
+    // This is a per-document flag turning off event handling for all content
+    // in the document, and is set when we enter a subdocument for a pointer-
+    // events:none frame.
+    bool mInsidePointerEventsNoneDoc;
+    bool mTouchEventPrefEnabledDoc;
+    nsIFrame* mPresShellIgnoreScrollFrame;
+  };
+
+  PresShellState* CurrentPresShellState() {
+    NS_ASSERTION(mPresShellStates.Length() > 0,
+                 "Someone forgot to enter a presshell");
+    return &mPresShellStates[mPresShellStates.Length() - 1];
+  }
+
+  void AddSizeOfExcludingThis(nsWindowSizes&) const;
+
+  struct FrameWillChangeBudget {
+    FrameWillChangeBudget() : mPresContext(nullptr), mUsage(0) {}
+
+    FrameWillChangeBudget(const nsPresContext* aPresContext, uint32_t aUsage)
+        : mPresContext(aPresContext), mUsage(aUsage) {}
+
+    const nsPresContext* mPresContext;
+    uint32_t mUsage;
+  };
+
+  // will-change budget tracker
+  typedef uint32_t DocumentWillChangeBudget;
+
+  nsIFrame* const mReferenceFrame;
+  nsIFrame* mIgnoreScrollFrame;
+
+  const ActiveScrolledRoot* mCurrentActiveScrolledRoot;
+  const ActiveScrolledRoot* mCurrentContainerASR;
+  // mCurrentFrame is the frame that we're currently calling (or about to call)
+  // BuildDisplayList on.
+  const nsIFrame* mCurrentFrame;
+  // The reference frame for mCurrentFrame.
+  const nsIFrame* mCurrentReferenceFrame;
+
+  nsIFrame* mCaretFrame;
+  // A temporary list that we append scroll info items to while building
+  // display items for the contents of frames with SVG effects.
+  // Only non-null when ShouldBuildScrollInfoItemsForHoisting() is true.
+  // This is a pointer and not a real nsDisplayList value because the
+  // nsDisplayList class is defined below this class, so we can't use it here.
+  nsDisplayList* mScrollInfoItemsForHoisting;
+  nsTArray<RefPtr<ActiveScrolledRoot>> mActiveScrolledRoots;
+  DisplayItemClipChain* mFirstClipChainToDestroy;
+  nsTArray<nsDisplayItem*> mTemporaryItems;
+  nsDisplayTableBackgroundSet* mTableBackgroundSet;
+  ViewID mCurrentScrollParentId;
+  ViewID mCurrentScrollbarTarget;
+
+  nsTArray<nsIFrame*> mSVGEffectsFrames;
+  // When we are inside a filter, the current ASR at the time we entered the
+  // filter. Otherwise nullptr.
+  const ActiveScrolledRoot* mFilterASR;
+  nsCString mLinkSpec;  // Destination of link currently being emitted, if any.
+
+  // Optimized versions for non-retained display list.
+  LayoutDeviceIntRegion mWindowDraggingRegion;
+  LayoutDeviceIntRegion mWindowNoDraggingRegion;
+  nsRegion mWindowOpaqueRegion;
+
+  nsClassHashtable<nsPtrHashKey<nsDisplayItem>,
+                   nsTArray<nsIWidget::ThemeGeometry>>
+      mThemeGeometries;
+  DisplayListClipState mClipState;
+  nsTHashMap<nsPtrHashKey<const nsPresContext>, DocumentWillChangeBudget>
+      mDocumentWillChangeBudgets;
+
+  // Any frame listed in this set is already counted in the budget
+  // and thus is in-budget.
+  nsTHashMap<nsPtrHashKey<const nsIFrame>, FrameWillChangeBudget>
+      mFrameWillChangeBudgets;
+
+  nsTHashMap<nsPtrHashKey<dom::RemoteBrowser>, dom::EffectsInfo>
+      mEffectsUpdates;
+
+  nsTHashSet<nsCString> mDestinations;  // Destination names emitted.
+
+  // Stores reusable items collected during display list preprocessing.
+  nsTHashSet<nsDisplayItem*> mReuseableItems;
+
+  // Tracked regions used for retained display list.
+  WeakFrameRegion mRetainedWindowDraggingRegion;
+  WeakFrameRegion mRetainedWindowNoDraggingRegion;
+
+  // Window opaque region is calculated during layer building.
+  WeakFrameRegion mRetainedWindowOpaqueRegion;
+
+  std::unordered_set<const DisplayItemClipChain*, DisplayItemClipChainHasher,
+                     DisplayItemClipChainEqualer>
+      mClipDeduplicator;
+  std::unordered_set<nsIScrollableFrame*> mScrollFramesToNotify;
+
+  AutoTArray<nsIFrame*, 20> mFramesWithOOFData;
+  AutoTArray<nsIFrame*, 40> mFramesMarkedForDisplayIfVisible;
+  AutoTArray<PresShellState, 8> mPresShellStates;
+
+  using Arena = nsPresArena<32768, DisplayListArenaObjectId,
+                            size_t(DisplayListArenaObjectId::COUNT)>;
+  Arena mPool;
+
+  AutoTArray<nsIFrame*, 400> mFramesMarkedForDisplay;
+
+  gfx::CompositorHitTestInfo mCompositorHitTestInfo;
+
+  // The offset from mCurrentFrame to mCurrentReferenceFrame.
+  nsPoint mCurrentOffsetToReferenceFrame;
+
+  Maybe<float> mVisibleThreshold;
+
+  Maybe<nsPoint> mAdditionalOffset;
+
+  // Relative to mCurrentFrame.
+  nsRect mVisibleRect;
+  nsRect mDirtyRect;
+  nsRect mCaretRect;
+
+  Preserves3DContext mPreserves3DCtx;
+
+  uint8_t mBuildingExtraPagesForPageNum;
+
+  nsDisplayListBuilderMode mMode;
+  static uint32_t sPaintSequenceNumber;
+
+  bool mContainsBlendMode;
+  bool mIsBuildingScrollbar;
+  bool mCurrentScrollbarWillHaveLayer;
+  bool mBuildCaret;
+  bool mRetainingDisplayList;
+  bool mPartialUpdate;
+  bool mIgnoreSuppression;
+  bool mIncludeAllOutOfFlows;
+  bool mDescendIntoSubdocuments;
+  bool mSelectedFramesOnly;
+  bool mAllowMergingAndFlattening;
+  // True when we're building a display list that's directly or indirectly
+  // under an nsDisplayTransform
+  bool mInTransform;
+  bool mInEventsOnly;
+  bool mInFilter;
+  bool mInPageSequence;
+  bool mIsInChromePresContext;
+  bool mSyncDecodeImages;
+  bool mIsPaintingToWindow;
+  bool mUseHighQualityScaling;
+  bool mIsPaintingForWebRender;
+  bool mIsCompositingCheap;
+  bool mAncestorHasApzAwareEventHandler;
+  // True when the first async-scrollable scroll frame for which we build a
+  // display list has a display port. An async-scrollable scroll frame is one
+  // which WantsAsyncScroll().
+  bool mHaveScrollableDisplayPort;
+  bool mWindowDraggingAllowed;
+  bool mIsBuildingForPopup;
+  bool mForceLayerForScrollParent;
+  bool mContainsNonMinimalDisplayPort;
+  bool mAsyncPanZoomEnabled;
+  bool mBuildingInvisibleItems;
+  bool mIsBuilding;
+  bool mInInvalidSubtree;
+  bool mBuildCompositorHitTestInfo;
+  bool mDisablePartialUpdates;
+  bool mPartialBuildFailed;
+  bool mIsInActiveDocShell;
+  bool mBuildAsyncZoomContainer;
+  bool mIsRelativeToLayoutViewport;
+  bool mUseOverlayScrollbars;
+  bool mAlwaysLayerizeScrollbars;
+
+  bool mIsReusingStackingContextItems;
+
+  Maybe<layers::ScrollDirection> mCurrentScrollbarDirection;
+};
+
+// All types are defined in nsDisplayItemTypes.h
+#define NS_DISPLAY_DECL_NAME(n, e)                                           \
+  const char* Name() const override { return n; }                            \
+  constexpr static DisplayItemType ItemType() { return DisplayItemType::e; } \
+                                                                             \
+ private:                                                                    \
+  void* operator new(size_t aSize, nsDisplayListBuilder* aBuilder) {         \
+    return aBuilder->Allocate(aSize, DisplayItemType::e);                    \
+  }                                                                          \
+                                                                             \
+  template <typename T, typename F, typename... Args>                        \
+  friend T* mozilla::MakeDisplayItemWithIndex(                               \
+      nsDisplayListBuilder* aBuilder, F* aFrame, const uint16_t aIndex,      \
+      Args&&... aArgs);                                                      \
+                                                                             \
+ public:
+
+#define NS_DISPLAY_ALLOW_CLONING()                                          \
+  template <typename T>                                                     \
+  friend T* mozilla::MakeClone(nsDisplayListBuilder* aBuilder,              \
+                               const T* aItem);                             \
+                                                                            \
+  nsDisplayWrapList* Clone(nsDisplayListBuilder* aBuilder) const override { \
+    return MakeClone(aBuilder, this);                                       \
+  }
+
+template <typename T>
+MOZ_ALWAYS_INLINE T* MakeClone(nsDisplayListBuilder* aBuilder, const T* aItem) {
+  static_assert(std::is_base_of<nsDisplayWrapList, T>::value,
+                "Display item type should be derived from nsDisplayWrapList");
+  T* item = new (aBuilder) T(aBuilder, *aItem);
+  item->SetType(T::ItemType());
+  return item;
+}
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+void AssertUniqueItem(nsDisplayItem* aItem);
+#endif
+
+/**
+ * Returns true, if a display item of given |aType| needs to be built within
+ * opacity:0 container.
+ */
+bool ShouldBuildItemForEvents(const DisplayItemType aType);
+
+/**
+ * Initializes the hit test information of |aItem| if the item type supports it.
+ */
+void InitializeHitTestInfo(nsDisplayListBuilder* aBuilder,
+                           nsPaintedDisplayItem* aItem,
+                           const DisplayItemType aType);
+
+template <typename T, typename F, typename... Args>
+MOZ_ALWAYS_INLINE T* MakeDisplayItemWithIndex(nsDisplayListBuilder* aBuilder,
+                                              F* aFrame, const uint16_t aIndex,
+                                              Args&&... aArgs) {
+  static_assert(std::is_base_of<nsDisplayItem, T>::value,
+                "Display item type should be derived from nsDisplayItem");
+  static_assert(std::is_base_of<nsIFrame, F>::value,
+                "Frame type should be derived from nsIFrame");
+
+  const DisplayItemType type = T::ItemType();
+  if (aBuilder->InEventsOnly() && !ShouldBuildItemForEvents(type)) {
+    // This item is not needed for events.
+    return nullptr;
+  }
+
+  T* item = new (aBuilder) T(aBuilder, aFrame, std::forward<Args>(aArgs)...);
+
+  if (type != DisplayItemType::TYPE_GENERIC) {
+    item->SetType(type);
+  }
+
+  item->SetPerFrameIndex(aIndex);
+  item->SetExtraPageForPageNum(aBuilder->GetBuildingExtraPagesForPageNum());
+
+  nsPaintedDisplayItem* paintedItem = item->AsPaintedDisplayItem();
+  if (paintedItem) {
+    InitializeHitTestInfo(aBuilder, paintedItem, type);
+  }
+
+  if (aBuilder->InInvalidSubtree() ||
+      item->FrameForInvalidation()->IsFrameModified()) {
+    item->SetModifiedFrame(true);
+  }
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  if (aBuilder->IsRetainingDisplayList() && aBuilder->IsBuilding()) {
+    AssertUniqueItem(item);
+  }
+
+  // Verify that InInvalidSubtree matches invalidation frame's modified state.
+  if (aBuilder->InInvalidSubtree()) {
+    MOZ_DIAGNOSTIC_ASSERT(
+        AnyContentAncestorModified(item->FrameForInvalidation()));
+  }
+
+  DebugOnly<bool> isContainerType =
+      (GetDisplayItemFlagsForType(type) & TYPE_IS_CONTAINER);
+
+  MOZ_ASSERT(item->HasChildren() == isContainerType,
+             "Container items must have container display item flag set.");
+#endif
+
+  DL_LOGV("Created display item %p (%s) (frame: %p)", item, item->Name(),
+          aFrame);
+
+  return item;
+}
+
+template <typename T, typename F, typename... Args>
+MOZ_ALWAYS_INLINE T* MakeDisplayItem(nsDisplayListBuilder* aBuilder, F* aFrame,
+                                     Args&&... aArgs) {
+  return MakeDisplayItemWithIndex<T>(aBuilder, aFrame, 0,
+                                     std::forward<Args>(aArgs)...);
+}
+
+/*
+ * nsDisplayItemBase is a base-class for all display items. It is mainly
+ * responsible for handling the frame-display item 1:n relationship, as well as
+ * storing the state needed for display list merging.
+ *
+ * Display items are arena-allocated during display list construction.
+ *
+ * Display items can be containers --- i.e., they can perform hit testing
+ * and painting by recursively traversing a list of child items.
+ *
+ * Display items belong to a list at all times (except temporarily as they
+ * move from one list to another).
+ */
+class nsDisplayItem {
+ public:
+  using LayerManager = layers::LayerManager;
+  using WebRenderLayerManager = layers::WebRenderLayerManager;
+  using StackingContextHelper = layers::StackingContextHelper;
+  using ViewID = layers::ScrollableLayerGuid::ViewID;
+
+  /**
+   * Downcasts this item to nsPaintedDisplayItem, if possible.
+   */
+  virtual nsPaintedDisplayItem* AsPaintedDisplayItem() { return nullptr; }
+  virtual const nsPaintedDisplayItem* AsPaintedDisplayItem() const {
+    return nullptr;
+  }
+
+  /**
+   * Downcasts this item to nsDisplayWrapList, if possible.
+   */
+  virtual nsDisplayWrapList* AsDisplayWrapList() { return nullptr; }
+  virtual const nsDisplayWrapList* AsDisplayWrapList() const { return nullptr; }
+
+  /**
+   * Create a clone of this item.
+   */
+  virtual nsDisplayWrapList* Clone(nsDisplayListBuilder* aBuilder) const {
+    return nullptr;
+  }
+
+  /**
+   * Checks if the given display item can be merged with this item.
+   * @return true if the merging is possible, otherwise false.
+   */
+  virtual bool CanMerge(const nsDisplayItem* aItem) const { return false; }
+
+  /**
+   * Frees the memory allocated for this display item.
+   * The given display list builder must have allocated this display item.
+   */
+  virtual void Destroy(nsDisplayListBuilder* aBuilder) {
+    const DisplayItemType type = GetType();
+    DL_LOGV("Destroying display item %p (%s)", this, Name());
+
+    if (IsReusedItem()) {
+      aBuilder->RemoveReusedDisplayItem(this);
+    }
+
+    this->~nsDisplayItem();
+    aBuilder->Destroy(type, this);
+  }
+
+  /**
+   * Returns the frame that this display item was created for.
+   * Never returns null.
+   */
+  inline nsIFrame* Frame() const {
+    MOZ_ASSERT(mFrame, "Trying to use display item after frame deletion!");
+    return mFrame;
+  }
+
+  /**
+   * Called when the display item is prepared for deletion. The display item
+   * should not be used after calling this function.
+   */
+  virtual void RemoveFrame(nsIFrame* aFrame) {
+    MOZ_ASSERT(aFrame);
+
+    if (mFrame && aFrame == mFrame) {
+      mFrame = nullptr;
+      SetDeletedFrame();
+    }
+  }
+
+  /**
+   * A display item can depend on multiple different frames for invalidation.
+   */
+  virtual nsIFrame* GetDependentFrame() { return nullptr; }
+
+  /**
+   * Returns the frame that provides the style data, and should
+   * be checked when deciding if this display item can be reused.
+   */
+  virtual nsIFrame* FrameForInvalidation() const { return Frame(); }
+
+  /**
+   * Display items can override this to communicate that they won't
+   * contribute any visual information (for example fully transparent).
+   */
+  virtual bool IsInvisible() const { return false; }
+
+  /**
+   * Returns the printable name of this display item.
+   */
+  virtual const char* Name() const = 0;
+
+  /**
+   * Some consecutive items should be rendered together as a unit, e.g.,
+   * outlines for the same element. For this, we need a way for items to
+   * identify their type. We use the type for other purposes too.
+   */
+  DisplayItemType GetType() const {
+    MOZ_ASSERT(mType != DisplayItemType::TYPE_ZERO,
+               "Display item should have a valid type!");
+    return mType;
+  }
+
+  /**
+   * Pairing this with the Frame() pointer gives a key that
+   * uniquely identifies this display item in the display item tree.
+   */
+  uint32_t GetPerFrameKey() const {
+    // The top 8 bits are the page index
+    // The middle 16 bits of the per frame key uniquely identify the display
+    // item when there are more than one item of the same type for a frame.
+    // The low 8 bits are the display item type.
+    return (static_cast<uint32_t>(mExtraPageForPageNum)
+            << (TYPE_BITS + (sizeof(mPerFrameIndex) * 8))) |
+           (static_cast<uint32_t>(mPerFrameIndex) << TYPE_BITS) |
+           static_cast<uint32_t>(mType);
+  }
+
+  /**
+   * Returns true if this item was reused during display list merging.
+   */
+  bool IsReused() const { return mItemFlags.contains(ItemFlag::ReusedItem); }
+
+  void SetReused(bool aReused) { SetItemFlag(ItemFlag::ReusedItem, aReused); }
+
+  /**
+   * Returns true if this item can be reused during display list merging.
+   */
+  bool CanBeReused() const {
+    return !mItemFlags.contains(ItemFlag::CantBeReused);
+  }
+
+  void SetCantBeReused() { mItemFlags += ItemFlag::CantBeReused; }
+
+  bool CanBeCached() const {
+    return !mItemFlags.contains(ItemFlag::CantBeCached);
+  }
+
+  void SetCantBeCached() { mItemFlags += ItemFlag::CantBeCached; }
+
+  bool IsOldItem() const { return !!mOldList; }
+
+  /**
+   * Returns true if the frame of this display item is in a modified subtree.
+   */
+  bool HasModifiedFrame() const {
+    return mItemFlags.contains(ItemFlag::ModifiedFrame);
+  }
+
+  void SetModifiedFrame(bool aModified) {
+    SetItemFlag(ItemFlag::ModifiedFrame, aModified);
+  }
+
+  bool HasDeletedFrame() const;
+
+  /**
+   * Set the nsDisplayList that this item belongs to, and what index it is
+   * within that list.
+   * Temporary state for merging used by RetainedDisplayListBuilder.
+   */
+  void SetOldListIndex(nsDisplayList* aList, OldListIndex aIndex,
+                       uint32_t aListKey, uint32_t aNestingDepth) {
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    mOldListKey = aListKey;
+    mOldNestingDepth = aNestingDepth;
+#endif
+    mOldList = reinterpret_cast<uintptr_t>(aList);
+    mOldListIndex = aIndex;
+  }
+
+  bool GetOldListIndex(nsDisplayList* aList, uint32_t aListKey,
+                       OldListIndex* aOutIndex) {
+    if (mOldList != reinterpret_cast<uintptr_t>(aList)) {
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      MOZ_CRASH_UNSAFE_PRINTF(
+          "Item found was in the wrong list! type %d "
+          "(outer type was %d at depth %d, now is %d)",
+          GetPerFrameKey(), mOldListKey, mOldNestingDepth, aListKey);
+#endif
+      return false;
+    }
+    *aOutIndex = mOldListIndex;
+    return true;
+  }
+
+  /**
+   * Returns the display list containing the children of this display item.
+   * The children may be in a different coordinate system than this item.
+   */
+  virtual RetainedDisplayList* GetChildren() const { return nullptr; }
+  bool HasChildren() const { return GetChildren(); }
+
+  /**
+   * Display items with children may return true here. This causes the
+   * display list iterator to descend into the child display list.
+   */
+  virtual bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) {
+    return false;
+  }
+
+  virtual bool CreatesStackingContextHelper() { return false; }
+
+  /**
+   * Returns true if this item can be moved asynchronously on the compositor,
+   * see RetainedDisplayListBuilder.cpp comments.
+   */
+  virtual bool CanMoveAsync() { return false; }
+
+ protected:
+  // This is never instantiated directly (it has pure virtual methods), so no
+  // need to count constructors and destructors.
+  nsDisplayItem(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame);
+  nsDisplayItem(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                const ActiveScrolledRoot* aActiveScrolledRoot);
+
+  /**
+   * The custom copy-constructor is implemented to prevent copying the saved
+   * state of the item.
+   * This is currently only used when creating temporary items for merging.
+   */
+  nsDisplayItem(nsDisplayListBuilder* aBuilder, const nsDisplayItem& aOther)
+      : mFrame(aOther.mFrame),
+        mItemFlags(aOther.mItemFlags),
+        mType(aOther.mType),
+        mExtraPageForPageNum(aOther.mExtraPageForPageNum),
+        mPerFrameIndex(aOther.mPerFrameIndex),
+        mBuildingRect(aOther.mBuildingRect),
+        mToReferenceFrame(aOther.mToReferenceFrame),
+        mActiveScrolledRoot(aOther.mActiveScrolledRoot),
+        mClipChain(aOther.mClipChain) {
+    MOZ_COUNT_CTOR(nsDisplayItem);
+    // TODO: It might be better to remove the flags that aren't copied.
+    if (aOther.ForceNotVisible()) {
+      mItemFlags += ItemFlag::ForceNotVisible;
+    }
+    if (mFrame->In3DContextAndBackfaceIsHidden()) {
+      mItemFlags += ItemFlag::BackfaceHidden;
+    }
+    if (aOther.Combines3DTransformWithAncestors()) {
+      mItemFlags += ItemFlag::Combines3DTransformWithAncestors;
+    }
+  }
+
+  virtual ~nsDisplayItem() {
+    MOZ_COUNT_DTOR(nsDisplayItem);
+    if (mFrame) {
+      mFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  void SetType(const DisplayItemType aType) { mType = aType; }
+
+  void SetPerFrameIndex(const uint16_t aIndex) { mPerFrameIndex = aIndex; }
+
+  // Display list building for printing can build duplicate
+  // container display items when they contain a mixture of
+  // OOF and normal content that is spread across multiple
+  // pages. We include the page number for the duplicates
+  // to make our GetPerFrameKey unique.
+  void SetExtraPageForPageNum(const uint8_t aPageNum) {
+    mExtraPageForPageNum = aPageNum;
+  }
+
+  void SetDeletedFrame();
+
+ public:
+  nsDisplayItem() = delete;
+  nsDisplayItem(const nsDisplayItem&) = delete;
+
+  /**
+   * Invalidate cached information that depends on this node's contents, after
+   * a mutation of those contents.
+   *
+   * Specifically, if you mutate an |nsDisplayItem| in a way that would change
+   * the WebRender display list items generated for it, you should call this
+   * method.
+   *
+   * If a |RestoreState| method exists to restore some piece of state, that's a
+   * good indication that modifications to said state should be accompanied by a
+   * call to this method. Opacity flattening's effects on
+   * |nsDisplayBackgroundColor| items are one example.
+   */
+  virtual void InvalidateItemCacheEntry() {}
+
+  struct HitTestState {
+    explicit HitTestState() = default;
+
+    ~HitTestState() {
+      NS_ASSERTION(mItemBuffer.Length() == 0,
+                   "mItemBuffer should have been cleared");
+    }
+
+    // Handling transform items for preserve 3D frames.
+    bool mInPreserves3D = false;
+    // When hit-testing for visibility, we may hit an fully opaque item in a
+    // nested display list. We want to stop at that point, without looking
+    // further on other items.
+    bool mHitOccludingItem = false;
+
+    float mCurrentOpacity = 1.0f;
+
+    AutoTArray<nsDisplayItem*, 100> mItemBuffer;
+  };
+
+  uint8_t GetFlags() const { return GetDisplayItemFlagsForType(GetType()); }
+
+  virtual bool IsContentful() const { return GetFlags() & TYPE_IS_CONTENTFUL; }
+
+  /**
+   * This is called after we've constructed a display list for event handling.
+   * When this is called, we've already ensured that aRect intersects the
+   * item's bounds and that clipping has been taking into account.
+   *
+   * @param aRect the point or rect being tested, relative to the reference
+   * frame. If the width and height are both 1 app unit, it indicates we're
+   * hit testing a point, not a rect.
+   * @param aState must point to a HitTestState. If you don't have one,
+   * just create one with the default constructor and pass it in.
+   * @param aOutFrames each item appends the frame(s) in this display item that
+   * the rect is considered over (if any) to aOutFrames.
+   */
+  virtual void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+                       HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) {}
+
+  virtual nsIFrame* StyleFrame() const { return mFrame; }
+
+  /**
+   * Compute the used z-index of our frame; returns zero for elements to which
+   * z-index does not apply, and for z-index:auto.
+   * @note This can be overridden, @see nsDisplayWrapList::SetOverrideZIndex.
+   */
+  virtual int32_t ZIndex() const;
+  /**
+   * The default bounds is the frame border rect.
+   * @param aSnap *aSnap is set to true if the returned rect will be
+   * snapped to nearest device pixel edges during actual drawing.
+   * It might be set to false and snap anyway, so code computing the set of
+   * pixels affected by this display item needs to round outwards to pixel
+   * boundaries when *aSnap is set to false.
+   * This does not take the item's clipping into account.
+   * @return a rectangle relative to aBuilder->ReferenceFrame() that
+   * contains the area drawn by this display item
+   */
+  virtual nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const {
+    *aSnap = false;
+    return nsRect(ToReferenceFrame(), Frame()->GetSize());
+  }
+
+  /**
+   * Returns the untransformed bounds of this display item.
+   */
+  virtual nsRect GetUntransformedBounds(nsDisplayListBuilder* aBuilder,
+                                        bool* aSnap) const {
+    return GetBounds(aBuilder, aSnap);
+  }
+
+  virtual nsRegion GetTightBounds(nsDisplayListBuilder* aBuilder,
+                                  bool* aSnap) const {
+    *aSnap = false;
+    return nsRegion();
+  }
+
+  /**
+   * Returns true if nothing will be rendered inside aRect, false if uncertain.
+   * aRect is assumed to be contained in this item's bounds.
+   */
+  virtual bool IsInvisibleInRect(const nsRect& aRect) const { return false; }
+
+  /**
+   * Returns the result of GetBounds intersected with the item's clip.
+   * The intersection is approximate since rounded corners are not taking into
+   * account.
+   */
+  nsRect GetClippedBounds(nsDisplayListBuilder* aBuilder) const;
+
+  nsRect GetBorderRect() const {
+    return nsRect(ToReferenceFrame(), Frame()->GetSize());
+  }
+
+  nsRect GetPaddingRect() const {
+    return Frame()->GetPaddingRectRelativeToSelf() + ToReferenceFrame();
+  }
+
+  nsRect GetContentRect() const {
+    return Frame()->GetContentRectRelativeToSelf() + ToReferenceFrame();
+  }
+
+  /**
+   * Checks if the frame(s) owning this display item have been marked as
+   * invalid, and needing repainting.
+   */
+  virtual bool IsInvalid(nsRect& aRect) const {
+    bool result = mFrame ? mFrame->IsInvalid(aRect) : false;
+    aRect += ToReferenceFrame();
+    return result;
+  }
+
+  /**
+   * Creates and initializes an nsDisplayItemGeometry object that retains the
+   * current areas covered by this display item. These need to retain enough
+   * information such that they can be compared against a future nsDisplayItem
+   * of the same type, and determine if repainting needs to happen.
+   *
+   * Subclasses wishing to store more information need to override both this
+   * and ComputeInvalidationRegion, as well as implementing an
+   * nsDisplayItemGeometry subclass.
+   *
+   * The default implementation tracks both the display item bounds, and the
+   * frame's border rect.
+   */
+  virtual nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) {
+    return new nsDisplayItemGenericGeometry(this, aBuilder);
+  }
+
+  /**
+   * Compares an nsDisplayItemGeometry object from a previous paint against the
+   * current item. Computes if the geometry of the item has changed, and the
+   * invalidation area required for correct repainting.
+   *
+   * The existing geometry will have been created from a display item with a
+   * matching GetPerFrameKey()/mFrame pair to the current item.
+   *
+   * The default implementation compares the display item bounds, and the
+   * frame's border rect, and invalidates the entire bounds if either rect
+   * changes.
+   *
+   * @param aGeometry The geometry of the matching display item from the
+   * previous paint.
+   * @param aInvalidRegion Output param, the region to invalidate, or
+   * unchanged if none.
+   */
+  virtual void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                         const nsDisplayItemGeometry* aGeometry,
+                                         nsRegion* aInvalidRegion) const {
+    const nsDisplayItemGenericGeometry* geometry =
+        static_cast<const nsDisplayItemGenericGeometry*>(aGeometry);
+    bool snap;
+    if (!geometry->mBounds.IsEqualInterior(GetBounds(aBuilder, &snap)) ||
+        !geometry->mBorderRect.IsEqualInterior(GetBorderRect())) {
+      aInvalidRegion->Or(GetBounds(aBuilder, &snap), geometry->mBounds);
+    }
+  }
+
+  /**
+   * An alternative default implementation of ComputeInvalidationRegion,
+   * that instead invalidates only the changed area between the two items.
+   */
+  void ComputeInvalidationRegionDifference(
+      nsDisplayListBuilder* aBuilder,
+      const nsDisplayItemBoundsGeometry* aGeometry,
+      nsRegion* aInvalidRegion) const {
+    bool snap;
+    nsRect bounds = GetBounds(aBuilder, &snap);
+
+    if (!aGeometry->mBounds.IsEqualInterior(bounds)) {
+      nscoord radii[8];
+      if (aGeometry->mHasRoundedCorners || Frame()->GetBorderRadii(radii)) {
+        aInvalidRegion->Or(aGeometry->mBounds, bounds);
+      } else {
+        aInvalidRegion->Xor(aGeometry->mBounds, bounds);
+      }
+    }
+  }
+
+  /**
+   * This function is called when an item's list of children has been modified
+   * by RetainedDisplayListBuilder.
+   */
+  virtual void InvalidateCachedChildInfo(nsDisplayListBuilder* aBuilder) {}
+
+  virtual void AddSizeOfExcludingThis(nsWindowSizes&) const {}
+
+  /**
+   * @param aSnap set to true if the edges of the rectangles of the opaque
+   * region would be snapped to device pixels when drawing
+   * @return a region of the item that is opaque --- that is, every pixel
+   * that is visible is painted with an opaque
+   * color. This is useful for determining when one piece
+   * of content completely obscures another so that we can do occlusion
+   * culling.
+   * This does not take clipping into account.
+   * This must return a simple region (1 rect) for painting display lists.
+   * It is only allowed to be a complex region for hit testing.
+   */
+  virtual nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                                   bool* aSnap) const {
+    *aSnap = false;
+    return nsRegion();
+  }
+  /**
+   * @return Some(nscolor) if the item is guaranteed to paint every pixel in its
+   * bounds with the same (possibly translucent) color
+   */
+  virtual Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const {
+    return Nothing();
+  }
+
+  /**
+   * @return true if the contents of this item are rendered fixed relative
+   * to the nearest viewport.
+   */
+  virtual bool ShouldFixToViewport(nsDisplayListBuilder* aBuilder) const {
+    return false;
+  }
+
+  /**
+   * Returns true if all layers that can be active should be forced to be
+   * active. Requires setting the pref layers.force-active=true.
+   */
+  static bool ForceActiveLayers();
+
+#ifdef MOZ_DUMP_PAINTING
+  /**
+   * Mark this display item as being painted via
+   * FrameLayerBuilder::DrawPaintedLayer.
+   */
+  bool Painted() const { return mItemFlags.contains(ItemFlag::Painted); }
+
+  /**
+   * Check if this display item has been painted.
+   */
+  void SetPainted() { mItemFlags += ItemFlag::Painted; }
+#endif
+
+  /**
+   * Function to create the WebRenderCommands.
+   * We should check if the layer state is
+   * active first and have an early return if the layer state is
+   * not active.
+   *
+   * @return true if successfully creating webrender commands.
+   */
+  virtual bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) {
+    return false;
+  }
+
+  /**
+   * Updates the provided aLayerData with any APZ-relevant scroll data
+   * that is specific to this display item. This is stuff that would normally
+   * be put on the layer during BuildLayer, but this is only called in
+   * layers-free webrender mode, where we don't have layers.
+   *
+   * This function returns true if and only if it has APZ-relevant scroll data
+   * to provide. Note that the arguments passed in may be nullptr, in which case
+   * the function should still return true if and only if it has APZ-relevant
+   * scroll data, but obviously in this case it can't actually put the
+   * data onto aLayerData, because there isn't one.
+   *
+   * This function assumes that aData and aLayerData will either both be null,
+   * or will both be non-null. The caller is responsible for enforcing this.
+   */
+  virtual bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                                layers::WebRenderLayerScrollData* aLayerData) {
+    return false;
+  }
+
+  /**
+   * Returns true if this item needs to have its geometry updated, despite
+   * returning empty invalidation region.
+   */
+  virtual bool NeedsGeometryUpdates() const { return false; }
+
+  /**
+   * When this item is rendered using fallback rendering, whether it should use
+   * blob rendering (i.e. a recording DrawTarget), as opposed to a pixel-backed
+   * DrawTarget.
+   * Some items, such as those calling into the native themed widget machinery,
+   * are more efficiently painted without blob recording. Those should return
+   * false here.
+   */
+  virtual bool ShouldUseBlobRenderingForFallback() const { return true; }
+
+  /**
+   * If this has a child list where the children are in the same coordinate
+   * system as this item (i.e., they have the same reference frame),
+   * return the list.
+   */
+  virtual RetainedDisplayList* GetSameCoordinateSystemChildren() const {
+    return nullptr;
+  }
+
+  virtual void UpdateBounds(nsDisplayListBuilder* aBuilder) {}
+  /**
+   * Do UpdateBounds() for items with frames establishing or extending
+   * 3D rendering context.
+   *
+   * This function is called by UpdateBoundsFor3D() of
+   * nsDisplayTransform(), and it is called by
+   * BuildDisplayListForStackingContext() on transform items
+   * establishing 3D rendering context.
+   *
+   * The bounds of a transform item with the frame establishing 3D
+   * rendering context should be computed by calling
+   * DoUpdateBoundsPreserves3D() on all descendants that participate
+   * the same 3d rendering context.
+   */
+  virtual void DoUpdateBoundsPreserves3D(nsDisplayListBuilder* aBuilder) {}
+
+  /**
+   * Returns the building rectangle used by nsDisplayListBuilder when
+   * this item was constructed.
+   */
+  const nsRect& GetBuildingRect() const { return mBuildingRect; }
+
+  void SetBuildingRect(const nsRect& aBuildingRect) {
+    mBuildingRect = aBuildingRect;
+  }
+
+  /**
+   * Returns the building rect for the children, relative to their
+   * reference frame. Can be different from mBuildingRect for
+   * nsDisplayTransform, since the reference frame for the children is different
+   * from the reference frame for the item itself.
+   */
+  virtual const nsRect& GetBuildingRectForChildren() const {
+    return mBuildingRect;
+  }
+
+  virtual void WriteDebugInfo(std::stringstream& aStream) {}
+
+  /**
+   * Returns the result of aBuilder->ToReferenceFrame(GetUnderlyingFrame())
+   */
+  const nsPoint& ToReferenceFrame() const {
+    NS_ASSERTION(mFrame, "No frame?");
+    return mToReferenceFrame;
+  }
+
+  /**
+   * Returns the reference frame for display item children of this item.
+   */
+  virtual const nsIFrame* ReferenceFrameForChildren() const { return nullptr; }
+
+  /**
+   * Checks if this display item (or any children) contains content that might
+   * be rendered with component alpha (e.g. subpixel antialiasing). Returns the
+   * bounds of the area that needs component alpha, or an empty rect if nothing
+   * in the item does.
+   */
+  virtual nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const {
+    return nsRect();
+  }
+
+  /**
+   * Check if we can add async animations to the layer for this display item.
+   */
+  virtual bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) {
+    return false;
+  }
+
+  virtual bool SupportsOptimizingToImage() const { return false; }
+
+  virtual const DisplayItemClip& GetClip() const;
+  void IntersectClip(nsDisplayListBuilder* aBuilder,
+                     const DisplayItemClipChain* aOther, bool aStore);
+
+  virtual void SetActiveScrolledRoot(
+      const ActiveScrolledRoot* aActiveScrolledRoot) {
+    mActiveScrolledRoot = aActiveScrolledRoot;
+  }
+  const ActiveScrolledRoot* GetActiveScrolledRoot() const {
+    return mActiveScrolledRoot;
+  }
+
+  virtual void SetClipChain(const DisplayItemClipChain* aClipChain,
+                            bool aStore);
+  const DisplayItemClipChain* GetClipChain() const { return mClipChain; }
+
+  bool BackfaceIsHidden() const {
+    return mItemFlags.contains(ItemFlag::BackfaceHidden);
+  }
+
+  bool Combines3DTransformWithAncestors() const {
+    return mItemFlags.contains(ItemFlag::Combines3DTransformWithAncestors);
+  }
+
+  bool ForceNotVisible() const {
+    return mItemFlags.contains(ItemFlag::ForceNotVisible);
+  }
+
+  bool In3DContextAndBackfaceIsHidden() const {
+    return mItemFlags.contains(ItemFlag::BackfaceHidden) &&
+           mItemFlags.contains(ItemFlag::Combines3DTransformWithAncestors);
+  }
+
+  bool HasDifferentFrame(const nsDisplayItem* aOther) const {
+    return mFrame != aOther->mFrame;
+  }
+
+  bool HasHitTestInfo() const {
+    return mItemFlags.contains(ItemFlag::HasHitTestInfo);
+  }
+
+  bool HasSameTypeAndClip(const nsDisplayItem* aOther) const {
+    return GetPerFrameKey() == aOther->GetPerFrameKey() &&
+           GetClipChain() == aOther->GetClipChain();
+  }
+
+  bool HasSameContent(const nsDisplayItem* aOther) const {
+    return mFrame->GetContent() == aOther->Frame()->GetContent();
+  }
+
+  virtual void NotifyUsed(nsDisplayListBuilder* aBuilder) {}
+
+  virtual Maybe<nsRect> GetClipWithRespectToASR(
+      nsDisplayListBuilder* aBuilder, const ActiveScrolledRoot* aASR) const;
+
+  virtual const nsRect& GetUntransformedPaintRect() const {
+    return GetBuildingRect();
+  }
+
+  nsRect GetPaintRect(nsDisplayListBuilder* aBuilder, gfxContext* aCtx);
+
+  virtual const HitTestInfo& GetHitTestInfo() { return HitTestInfo::Empty(); }
+
+  enum class ReuseState : uint8_t {
+    None,
+    // Set during display list building.
+    Reusable,
+    // Set during display list preprocessing.
+    PreProcessed,
+    // Set during partial display list build.
+    Reused,
+  };
+
+  void SetReusable() {
+    MOZ_ASSERT(mReuseState == ReuseState::None ||
+               mReuseState == ReuseState::Reused);
+    mReuseState = ReuseState::Reusable;
+  }
+
+  bool IsReusable() const { return mReuseState == ReuseState::Reusable; }
+
+  void SetPreProcessed() {
+    MOZ_ASSERT(mReuseState == ReuseState::Reusable);
+    mReuseState = ReuseState::PreProcessed;
+  }
+
+  bool IsPreProcessed() const {
+    return mReuseState == ReuseState::PreProcessed;
+  }
+
+  void SetReusedItem() {
+    MOZ_ASSERT(mReuseState == ReuseState::PreProcessed);
+    mReuseState = ReuseState::Reused;
+  }
+
+  bool IsReusedItem() const { return mReuseState == ReuseState::Reused; }
+
+  void ResetReuseState() { mReuseState = ReuseState::None; }
+
+  ReuseState GetReuseState() const { return mReuseState; }
+
+  nsIFrame* mFrame;  // 8
+
+ private:
+  enum class ItemFlag : uint16_t {
+    CantBeReused,
+    CantBeCached,
+    DeletedFrame,
+    ModifiedFrame,
+    ReusedItem,
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    MergedItem,
+    PreProcessedItem,
+#endif
+    BackfaceHidden,
+    Combines3DTransformWithAncestors,
+    ForceNotVisible,
+    HasHitTestInfo,
+#ifdef MOZ_DUMP_PAINTING
+    // True if this frame has been painted.
+    Painted,
+#endif
+  };
+
+  EnumSet<ItemFlag, uint16_t> mItemFlags;              // 2
+  DisplayItemType mType = DisplayItemType::TYPE_ZERO;  // 1
+  uint8_t mExtraPageForPageNum = 0;                    // 1
+  uint16_t mPerFrameIndex = 0;                         // 2
+  ReuseState mReuseState = ReuseState::None;
+  OldListIndex mOldListIndex;  // 4
+  uintptr_t mOldList = 0;      // 8
+
+  // This is the rectangle that nsDisplayListBuilder was using as the visible
+  // rect to decide which items to construct.
+  nsRect mBuildingRect;
+
+ protected:
+  void SetItemFlag(ItemFlag aFlag, const bool aValue) {
+    if (aValue) {
+      mItemFlags += aFlag;
+    } else {
+      mItemFlags -= aFlag;
+    }
+  }
+
+  void SetHasHitTestInfo() { mItemFlags += ItemFlag::HasHitTestInfo; }
+
+  // Result of ToReferenceFrame(mFrame), if mFrame is non-null
+  nsPoint mToReferenceFrame;
+
+  RefPtr<const ActiveScrolledRoot> mActiveScrolledRoot;
+  RefPtr<const DisplayItemClipChain> mClipChain;
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+ public:
+  bool IsMergedItem() const {
+    return mItemFlags.contains(ItemFlag::MergedItem);
+  }
+
+  bool IsPreProcessedItem() const {
+    return mItemFlags.contains(ItemFlag::PreProcessedItem);
+  }
+
+  void SetMergedPreProcessed(bool aMerged, bool aPreProcessed) {
+    SetItemFlag(ItemFlag::MergedItem, aMerged);
+    SetItemFlag(ItemFlag::PreProcessedItem, aPreProcessed);
+  }
+
+  uint32_t mOldListKey = 0;
+  uint32_t mOldNestingDepth = 0;
+#endif
+};
+
+class nsPaintedDisplayItem : public nsDisplayItem {
+ public:
+  nsPaintedDisplayItem* AsPaintedDisplayItem() final { return this; }
+  const nsPaintedDisplayItem* AsPaintedDisplayItem() const final {
+    return this;
+  }
+
+  /**
+   * Returns true if this display item would return true from ApplyOpacity
+   * without actually applying the opacity. Otherwise returns false.
+   */
+  virtual bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                               nsDisplayListBuilder* aBuilder) const {
+    return false;
+  }
+
+  /**
+   * Returns true if this item supports PaintWithClip, where the clipping
+   * is used directly as the primitive geometry instead of needing an explicit
+   * clip.
+   */
+  virtual bool CanPaintWithClip(const DisplayItemClip& aClip) { return false; }
+
+  /**
+   * Same as |Paint()|, except provides a clip to use the geometry to draw with.
+   * Must not be called unless |CanPaintWithClip()| returned true.
+   */
+  virtual void PaintWithClip(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+                             const DisplayItemClip& aClip) {
+    MOZ_ASSERT_UNREACHABLE("PaintWithClip() is not implemented!");
+  }
+
+  /**
+   * Paint this item to some rendering context.
+   */
+  virtual void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) = 0;
+
+  /**
+   * External storage used by |DisplayItemCache| to avoid hashmap lookups.
+   * If an item is reused and has the cache index set, it means that
+   * |DisplayItemCache| has assigned a cache slot for the item.
+   */
+  Maybe<uint16_t>& CacheIndex() { return mCacheIndex; }
+
+  void InvalidateItemCacheEntry() override {
+    // |nsPaintedDisplayItem|s may have |DisplayItemCache| entries
+    // that no longer match after a mutation. The cache will notice
+    // on its own that the entry is no longer in use, and free it.
+    mCacheIndex = Nothing();
+  }
+
+  const HitTestInfo& GetHitTestInfo() final { return mHitTestInfo; }
+  void InitializeHitTestInfo(nsDisplayListBuilder* aBuilder) {
+    mHitTestInfo.Initialize(aBuilder, Frame());
+    SetHasHitTestInfo();
+  }
+
+ protected:
+  nsPaintedDisplayItem(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame,
+                             aBuilder->CurrentActiveScrolledRoot()) {}
+
+  nsPaintedDisplayItem(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       const ActiveScrolledRoot* aActiveScrolledRoot)
+      : nsDisplayItem(aBuilder, aFrame, aActiveScrolledRoot) {}
+
+  nsPaintedDisplayItem(nsDisplayListBuilder* aBuilder,
+                       const nsPaintedDisplayItem& aOther)
+      : nsDisplayItem(aBuilder, aOther), mHitTestInfo(aOther.mHitTestInfo) {}
+
+ protected:
+  HitTestInfo mHitTestInfo;
+  Maybe<uint16_t> mCacheIndex;
+};
+
+template <typename T>
+struct MOZ_HEAP_CLASS LinkedListNode {
+  explicit LinkedListNode(T aValue) : mNext(nullptr), mValue(aValue) {}
+  LinkedListNode* mNext;
+  T mValue;
+};
+
+template <typename T>
+struct LinkedListIterator {
+  using iterator_category = std::forward_iterator_tag;
+  using difference_type = std::ptrdiff_t;
+  using value_type = T;
+  using pointer = T*;
+  using reference = T&;
+  using Node = LinkedListNode<T>;
+
+  explicit LinkedListIterator(Node* aNode = nullptr) : mNode(aNode) {}
+
+  bool HasNext() const { return mNode != nullptr; }
+
+  LinkedListIterator<T>& operator++() {
+    MOZ_ASSERT(mNode);
+    mNode = mNode->mNext;
+    return *this;
+  }
+
+  bool operator==(const LinkedListIterator<T>& aOther) const {
+    return mNode == aOther.mNode;
+  }
+
+  bool operator!=(const LinkedListIterator<T>& aOther) const {
+    return mNode != aOther.mNode;
+  }
+
+  const T operator*() const {
+    MOZ_ASSERT(mNode);
+    return mNode->mValue;
+  }
+
+  T operator*() {
+    MOZ_ASSERT(mNode);
+    return mNode->mValue;
+  }
+
+  Node* mNode;
+};
+
+/**
+ * Manages a singly-linked list of display list items.
+ *
+ * Stepping upward through this list is very fast. Stepping downward is very
+ * slow so we don't support it. The methods that need to step downward
+ * (HitTest()) internally build a temporary array of all
+ * the items while they do the downward traversal, so overall they're still
+ * linear time. We have optimized for efficient AppendToTop() of both
+ * items and lists, with minimal codesize.
+ *
+ * Internal linked list nodes are allocated using arena allocator.
+ * */
+class nsDisplayList {
+ public:
+  using Node = LinkedListNode<nsDisplayItem*>;
+  using iterator = LinkedListIterator<nsDisplayItem*>;
+  using const_iterator = iterator;
+
+  iterator begin() { return iterator(mBottom); }
+  iterator end() { return iterator(nullptr); }
+  const_iterator begin() const { return iterator(mBottom); }
+  const_iterator end() const { return iterator(nullptr); }
+
+  explicit nsDisplayList(nsDisplayListBuilder* aBuilder) : mBuilder(aBuilder) {}
+
+  nsDisplayList() = delete;
+  nsDisplayList(const nsDisplayList&) = delete;
+  nsDisplayList& operator=(const nsDisplayList&) = delete;
+
+  virtual ~nsDisplayList() {
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    if (!mAllowNonEmptyDestruction) {
+      MOZ_RELEASE_ASSERT(IsEmpty(), "Nonempty list left over?");
+    }
+#endif
+
+    DeallocateNodes();
+  }
+
+  nsDisplayList(nsDisplayList&& aOther)
+      : mBottom(aOther.mBottom),
+        mTop(aOther.mTop),
+        mLength(aOther.mLength),
+        mBuilder(aOther.mBuilder) {
+    aOther.SetEmpty();
+  }
+
+  nsDisplayList& operator=(nsDisplayList&& aOther) {
+    MOZ_RELEASE_ASSERT(mBuilder == aOther.mBuilder);
+
+    if (this != &aOther) {
+      MOZ_RELEASE_ASSERT(IsEmpty());
+      mBottom = std::move(aOther.mBottom);
+      mTop = std::move(aOther.mTop);
+      mLength = std::move(aOther.mLength);
+      aOther.SetEmpty();
+    }
+    return *this;
+  }
+
+  /**
+   * Append an item to the top of the list.
+   **/
+  void AppendToTop(nsDisplayItem* aItem) {
+    if (!aItem) {
+      return;
+    }
+
+    auto* next = Allocate(aItem);
+    MOZ_ASSERT(next);
+
+    if (IsEmpty()) {
+      mBottom = next;
+      mTop = next;
+    } else {
+      mTop->mNext = next;
+      mTop = next;
+    }
+
+    mLength++;
+
+    MOZ_ASSERT(mBottom && mTop);
+    MOZ_ASSERT(mTop->mNext == nullptr);
+  }
+
+  template <typename T, typename F, typename... Args>
+  void AppendNewToTop(nsDisplayListBuilder* aBuilder, F* aFrame,
+                      Args&&... aArgs) {
+    AppendNewToTopWithIndex<T>(aBuilder, aFrame, 0,
+                               std::forward<Args>(aArgs)...);
+  }
+
+  template <typename T, typename F, typename... Args>
+  void AppendNewToTopWithIndex(nsDisplayListBuilder* aBuilder, F* aFrame,
+                               const uint16_t aIndex, Args&&... aArgs) {
+    nsDisplayItem* item = MakeDisplayItemWithIndex<T>(
+        aBuilder, aFrame, aIndex, std::forward<Args>(aArgs)...);
+    AppendToTop(item);
+  }
+
+  /**
+   * Removes all items from aList and appends them to the top of this list.
+   */
+  void AppendToTop(nsDisplayList* aList) {
+    MOZ_ASSERT(aList != this);
+    MOZ_RELEASE_ASSERT(mBuilder == aList->mBuilder);
+
+    if (aList->IsEmpty()) {
+      return;
+    }
+
+    if (IsEmpty()) {
+      std::swap(mBottom, aList->mBottom);
+      std::swap(mTop, aList->mTop);
+      std::swap(mLength, aList->mLength);
+    } else {
+      MOZ_ASSERT(mTop && mTop->mNext == nullptr);
+      mTop->mNext = aList->mBottom;
+      mTop = aList->mTop;
+      mLength += aList->mLength;
+
+      aList->SetEmpty();
+    }
+  }
+
+  /**
+   * Clears the display list.
+   */
+  void Clear() {
+    DeallocateNodes();
+    SetEmpty();
+  }
+
+  /**
+   * Creates a shallow copy of this display list to |aDestination|.
+   */
+  void CopyTo(nsDisplayList* aDestination) const {
+    for (auto* item : *this) {
+      aDestination->AppendToTop(item);
+    }
+  }
+
+  /**
+   * Calls the function |aFn| for each display item in the display list.
+   */
+  void ForEach(const std::function<void(nsDisplayItem*)>& aFn) {
+    for (auto* item : *this) {
+      aFn(item);
+    }
+  }
+  /**
+   * Remove all items from the list and call their destructors.
+   */
+  virtual void DeleteAll(nsDisplayListBuilder* aBuilder);
+
+  /**
+   * @return the item at the bottom of the list, or null if the list is empty
+   */
+  nsDisplayItem* GetBottom() const {
+    return mBottom ? mBottom->mValue : nullptr;
+  }
+
+  /**
+   * @return the item at the top of the list, or null if the list is empty
+   */
+  nsDisplayItem* GetTop() const { return mTop ? mTop->mValue : nullptr; }
+
+  bool IsEmpty() const { return mBottom == nullptr; }
+
+  /**
+   * @return the number of items in the list
+   */
+  size_t Length() const { return mLength; }
+
+  /**
+   * Stable sort the list by the z-order of Frame() on
+   * each item. 'auto' is counted as zero.
+   * It is assumed that the list is already in content document order.
+   */
+  void SortByZOrder();
+
+  /**
+   * Stable sort the list by the tree order of the content of
+   * Frame() on each item. z-index is ignored.
+   * @param aCommonAncestor a common ancestor of all the content elements
+   * associated with the display items, for speeding up tree order
+   * checks, or nullptr if not known; it's only a hint, if it is not an
+   * ancestor of some elements, then we lose performance but not correctness
+   */
+  void SortByContentOrder(nsIContent* aCommonAncestor);
+
+  /**
+   * Sort the display list using a stable sort.
+   * aComparator(Item item1, Item item2) should return true if item1 should go
+   * before item2.
+   * We sort the items into increasing order.
+   */
+  template <typename Item, typename Comparator>
+  void Sort(const Comparator& aComparator) {
+    if (Length() < 2) {
+      // Only sort lists with more than one item.
+      return;
+    }
+
+    // Some casual local browsing testing suggests that a local preallocated
+    // array of 20 items should be able to avoid a lot of dynamic allocations
+    // here.
+    AutoTArray<Item, 20> items;
+
+    for (nsDisplayItem* item : TakeItems()) {
+      items.AppendElement(Item(item));
+    }
+
+    std::stable_sort(items.begin(), items.end(), aComparator);
+
+    for (Item& item : items) {
+      AppendToTop(item);
+    }
+  }
+
+  nsDisplayList TakeItems() {
+    nsDisplayList list = std::move(*this);
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    list.mAllowNonEmptyDestruction = true;
+#endif
+    return list;
+  }
+
+  nsDisplayItem* RemoveBottom() {
+    if (!mBottom) {
+      return nullptr;
+    }
+
+    nsDisplayItem* bottom = mBottom->mValue;
+
+    auto next = mBottom->mNext;
+    Deallocate(mBottom);
+    mBottom = next;
+
+    if (!mBottom) {
+      // No bottom item means no items at all.
+      mTop = nullptr;
+    }
+
+    MOZ_ASSERT(mLength > 0);
+    mLength--;
+
+    return bottom;
+  }
+
+  /**
+   * Paint the list to the rendering context. We assume that (0,0) in aCtx
+   * corresponds to the origin of the reference frame. For best results,
+   * aCtx's current transform should make (0,0) pixel-aligned. The
+   * rectangle in aDirtyRect is painted, which *must* be contained in the
+   * dirty rect used to construct the display list.
+   *
+   * If aFlags contains PAINT_USE_WIDGET_LAYERS and
+   * ShouldUseWidgetLayerManager() is set, then we will paint using
+   * the reference frame's widget's layer manager (and ctx may be null),
+   * otherwise we will use a temporary BasicLayerManager and ctx must
+   * not be null.
+   *
+   * If PAINT_EXISTING_TRANSACTION is set, the reference frame's widget's
+   * layer manager has already had BeginTransaction() called on it and
+   * we should not call it again.
+   *
+   * This must only be called on the root display list of the display list
+   * tree.
+   *
+   * We return the layer manager used for painting --- mainly so that
+   * callers can dump its layer tree if necessary.
+   */
+  enum {
+    PAINT_DEFAULT = 0,
+    PAINT_USE_WIDGET_LAYERS = 0x01,
+    PAINT_EXISTING_TRANSACTION = 0x04,
+    PAINT_IDENTICAL_DISPLAY_LIST = 0x08
+  };
+  void PaintRoot(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+                 uint32_t aFlags, Maybe<double> aDisplayListBuildTime);
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+             int32_t aAppUnitsPerDevPixel);
+
+  /**
+   * Get the bounds. Takes the union of the bounds of all children.
+   * The result is not cached.
+   */
+  nsRect GetClippedBounds(nsDisplayListBuilder* aBuilder) const;
+
+  /**
+   * Get this list's bounds, respecting clips relative to aASR. The result is
+   * the union of each item's clipped bounds with respect to aASR. That means
+   * that if an item can move asynchronously with an ASR that is a descendant
+   * of aASR, then the clipped bounds with respect to aASR will be the clip of
+   * that item for aASR, because the item can move anywhere inside that clip.
+   * If there is an item in this list which is not bounded with respect to
+   * aASR (i.e. which does not have "finite bounds" with respect to aASR),
+   * then this method trigger an assertion failure.
+   * The optional aBuildingRect out argument can be set to non-null if the
+   * caller is also interested to know the building rect.  This can be used
+   * to get the visible rect efficiently without traversing the display list
+   * twice.
+   */
+  nsRect GetClippedBoundsWithRespectToASR(
+      nsDisplayListBuilder* aBuilder, const ActiveScrolledRoot* aASR,
+      nsRect* aBuildingRect = nullptr) const;
+
+  /**
+   * Returns the opaque region of this display list.
+   */
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder) {
+    nsRegion result;
+    bool snap;
+    for (nsDisplayItem* item : *this) {
+      result.OrWith(item->GetOpaqueRegion(aBuilder, &snap));
+    }
+    return result;
+  }
+
+  /**
+   * Returns the bounds of the area that needs component alpha.
+   */
+  nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const {
+    nsRect bounds;
+    for (nsDisplayItem* item : *this) {
+      bounds.UnionRect(bounds, item->GetComponentAlphaBounds(aBuilder));
+    }
+    return bounds;
+  }
+
+  /**
+   * Find the topmost display item that returns a non-null frame, and return
+   * the frame.
+   */
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               nsDisplayItem::HitTestState* aState,
+               nsTArray<nsIFrame*>* aOutFrames) const;
+  /**
+   * Compute the union of the visible rects of the items in the list. The
+   * result is not cached.
+   */
+  nsRect GetBuildingRect() const;
+
+ private:
+  inline Node* Allocate(nsDisplayItem* aItem) {
+    void* ptr =
+        mBuilder->Allocate(sizeof(Node), DisplayListArenaObjectId::LISTNODE);
+    return new (ptr) Node(aItem);
+  }
+
+  inline void Deallocate(Node* aNode) {
+    aNode->~Node();
+    mBuilder->Destroy(DisplayListArenaObjectId::LISTNODE, aNode);
+  }
+
+  void DeallocateNodes() {
+    Node* current = mBottom;
+    Node* next = nullptr;
+
+    while (current) {
+      next = current->mNext;
+      Deallocate(current);
+      current = next;
+    }
+  }
+
+  inline void SetEmpty() {
+    mBottom = nullptr;
+    mTop = nullptr;
+    mLength = 0;
+  }
+
+  Node* mBottom = nullptr;
+  Node* mTop = nullptr;
+  size_t mLength = 0;
+  nsDisplayListBuilder* mBuilder = nullptr;
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  // This checks that the invariant of display lists owning their items is held.
+  bool mAllowNonEmptyDestruction = false;
+#endif
+};
+
+/**
+ * This is passed as a parameter to nsIFrame::BuildDisplayList. That method
+ * will put any generated items onto the appropriate list given here. It's
+ * basically just a collection with one list for each separate stacking layer.
+ * The lists themselves are external to this object and thus can be shared
+ * with others. Some of the list pointers may even refer to the same list.
+ */
+class nsDisplayListSet {
+ public:
+  /**
+   * @return a list where one should place the border and/or background for
+   * this frame (everything from steps 1 and 2 of CSS 2.1 appendix E)
+   */
+  nsDisplayList* BorderBackground() const { return mLists[0]; }
+  /**
+   * @return a list where one should place the borders and/or backgrounds for
+   * block-level in-flow descendants (step 4 of CSS 2.1 appendix E)
+   */
+  nsDisplayList* BlockBorderBackgrounds() const { return mLists[1]; }
+  /**
+   * @return a list where one should place descendant floats (step 5 of
+   * CSS 2.1 appendix E)
+   */
+  nsDisplayList* Floats() const { return mLists[2]; }
+  /**
+   * @return a list where one should place the (pseudo) stacking contexts
+   * for descendants of this frame (everything from steps 3, 7 and 8
+   * of CSS 2.1 appendix E)
+   */
+  nsDisplayList* PositionedDescendants() const { return mLists[3]; }
+  /**
+   * @return a list where one should place the outlines
+   * for this frame and its descendants (step 9 of CSS 2.1 appendix E)
+   */
+  nsDisplayList* Outlines() const { return mLists[4]; }
+  /**
+   * @return a list where one should place all other content
+   */
+  nsDisplayList* Content() const { return mLists[5]; }
+
+  const std::array<nsDisplayList*, 6>& Lists() const { return mLists; }
+
+  /**
+   * Clears all the display lists in the set.
+   */
+  void Clear() {
+    for (auto* list : mLists) {
+      MOZ_ASSERT(list);
+      list->Clear();
+    }
+  }
+
+  /**
+   * Deletes all the display items in the set.
+   */
+  void DeleteAll(nsDisplayListBuilder* aBuilder) {
+    for (auto* list : mLists) {
+      list->DeleteAll(aBuilder);
+    }
+  }
+
+  nsDisplayListSet(nsDisplayList* aBorderBackground,
+                   nsDisplayList* aBlockBorderBackgrounds,
+                   nsDisplayList* aFloats, nsDisplayList* aContent,
+                   nsDisplayList* aPositionedDescendants,
+                   nsDisplayList* aOutlines)
+      : mLists{aBorderBackground, aBlockBorderBackgrounds, aFloats,
+               aContent,          aPositionedDescendants,  aOutlines} {}
+
+  /**
+   * A copy constructor that lets the caller override the BorderBackground
+   * list.
+   */
+  nsDisplayListSet(const nsDisplayListSet& aLists,
+                   nsDisplayList* aBorderBackground)
+      : mLists(aLists.mLists) {
+    mLists[0] = aBorderBackground;
+  }
+
+  /**
+   * Returns true if all the display lists in the display list set are empty.
+   */
+  bool IsEmpty() const {
+    for (auto* list : mLists) {
+      if (!list->IsEmpty()) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  /**
+   * Calls the function |aFn| for each display item in the display list set.
+   */
+  void ForEach(const std::function<void(nsDisplayItem*)>& aFn) const {
+    for (auto* list : mLists) {
+      list->ForEach(aFn);
+    }
+  }
+
+  /**
+   * Creates a shallow copy of this display list set to |aDestination|.
+   */
+  void CopyTo(const nsDisplayListSet& aDestination) const;
+
+  /**
+   * Move all display items in our lists to top of the corresponding lists in
+   * the destination.
+   */
+  void MoveTo(const nsDisplayListSet& aDestination) const;
+
+ private:
+  // This class is only used on stack, so we don't have to worry about leaking
+  // it.  Don't let us be heap-allocated!
+  void* operator new(size_t sz) noexcept(true);
+
+  std::array<nsDisplayList*, 6> mLists;
+};
+
+/**
+ * A specialization of nsDisplayListSet where the lists are actually internal
+ * to the object, and all distinct.
+ */
+struct nsDisplayListCollection : public nsDisplayListSet {
+  explicit nsDisplayListCollection(nsDisplayListBuilder* aBuilder)
+      : nsDisplayListSet(&mLists[0], &mLists[1], &mLists[2], &mLists[3],
+                         &mLists[4], &mLists[5]),
+        mLists{nsDisplayList{aBuilder}, nsDisplayList{aBuilder},
+               nsDisplayList{aBuilder}, nsDisplayList{aBuilder},
+               nsDisplayList{aBuilder}, nsDisplayList{aBuilder}} {}
+
+  /**
+   * Sort all lists by content order.
+   */
+  void SortAllByContentOrder(nsIContent* aCommonAncestor) {
+    for (auto& mList : mLists) {
+      mList.SortByContentOrder(aCommonAncestor);
+    }
+  }
+
+  /**
+   * Serialize this display list collection into a display list with the items
+   * in the correct Z order.
+   * @param aOutList the result display list
+   * @param aContent the content element to use for content ordering
+   */
+  void SerializeWithCorrectZOrder(nsDisplayList* aOutResultList,
+                                  nsIContent* aContent);
+
+ private:
+  // This class is only used on stack, so we don't have to worry about leaking
+  // it.  Don't let us be heap-allocated!
+  void* operator new(size_t sz) noexcept(true);
+
+  nsDisplayList mLists[6];
+};
+
+/**
+ * A display list that also retains the partial build
+ * information (in the form of a DAG) used to create it.
+ *
+ * Display lists built from a partial list aren't necessarily
+ * in the same order as a full build, and the DAG retains
+ * the information needing to interpret the current
+ * order correctly.
+ */
+class RetainedDisplayList : public nsDisplayList {
+ public:
+  explicit RetainedDisplayList(nsDisplayListBuilder* aBuilder)
+      : nsDisplayList(aBuilder) {}
+
+  RetainedDisplayList(RetainedDisplayList&& aOther)
+      : nsDisplayList(std::move(aOther)), mDAG(std::move(aOther.mDAG)) {}
+
+  RetainedDisplayList(const RetainedDisplayList&) = delete;
+  RetainedDisplayList& operator=(const RetainedDisplayList&) = delete;
+
+  ~RetainedDisplayList() override {
+    MOZ_ASSERT(mOldItems.IsEmpty(), "Must empty list before destroying");
+  }
+
+  RetainedDisplayList& operator=(RetainedDisplayList&& aOther) {
+    MOZ_ASSERT(IsEmpty(), "Can only move into an empty list!");
+    MOZ_ASSERT(mOldItems.IsEmpty(), "Can only move into an empty list!");
+
+    nsDisplayList::operator=(std::move(aOther));
+    mDAG = std::move(aOther.mDAG);
+    mOldItems = std::move(aOther.mOldItems);
+    return *this;
+  }
+
+  RetainedDisplayList& operator=(nsDisplayList&& aOther) {
+    MOZ_ASSERT(IsEmpty(), "Can only move into an empty list!");
+    MOZ_ASSERT(mOldItems.IsEmpty(), "Can only move into an empty list!");
+    nsDisplayList::operator=(std::move(aOther));
+    return *this;
+  }
+
+  void DeleteAll(nsDisplayListBuilder* aBuilder) override {
+    for (OldItemInfo& i : mOldItems) {
+      if (i.mItem && i.mOwnsItem) {
+        i.mItem->Destroy(aBuilder);
+        MOZ_ASSERT(!GetBottom(),
+                   "mOldItems should not be owning items if we also have items "
+                   "in the normal list");
+      }
+    }
+    mOldItems.Clear();
+    mDAG.Clear();
+    nsDisplayList::DeleteAll(aBuilder);
+  }
+
+  void AddSizeOfExcludingThis(nsWindowSizes&) const;
+
+  DirectedAcyclicGraph<MergedListUnits> mDAG;
+
+  // Temporary state initialized during the preprocess pass
+  // of RetainedDisplayListBuilder and then used during merging.
+  nsTArray<OldItemInfo> mOldItems;
+};
+
+class nsDisplayContainer final : public nsDisplayItem {
+ public:
+  nsDisplayContainer(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     const ActiveScrolledRoot* aActiveScrolledRoot,
+                     nsDisplayList* aList);
+
+  ~nsDisplayContainer() override { MOZ_COUNT_DTOR(nsDisplayContainer); }
+
+  NS_DISPLAY_DECL_NAME("nsDisplayContainer", TYPE_CONTAINER)
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    mChildren.DeleteAll(aBuilder);
+    nsDisplayItem::Destroy(aBuilder);
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+
+  nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const override;
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override {
+    return Nothing();
+  }
+
+  RetainedDisplayList* GetChildren() const override { return &mChildren; }
+  RetainedDisplayList* GetSameCoordinateSystemChildren() const override {
+    return GetChildren();
+  }
+
+  Maybe<nsRect> GetClipWithRespectToASR(
+      nsDisplayListBuilder* aBuilder,
+      const ActiveScrolledRoot* aASR) const override;
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return true;
+  }
+
+  void SetClipChain(const DisplayItemClipChain* aClipChain,
+                    bool aStore) override {
+    MOZ_ASSERT_UNREACHABLE("nsDisplayContainer does not support clipping");
+  }
+
+  void UpdateBounds(nsDisplayListBuilder* aBuilder) override;
+
+ private:
+  mutable RetainedDisplayList mChildren;
+  nsRect mBounds;
+};
+
+/**
+ * Use this class to implement not-very-frequently-used display items
+ * that are not opaque, do not receive events, and are bounded by a frame's
+ * border-rect.
+ *
+ * This should not be used for display items which are created frequently,
+ * because each item is one or two pointers bigger than an item from a
+ * custom display item class could be, and fractionally slower. However it does
+ * save code size. We use this for infrequently-used item types.
+ */
+class nsDisplayGeneric : public nsPaintedDisplayItem {
+ public:
+  typedef void (*PaintCallback)(nsIFrame* aFrame, gfx::DrawTarget* aDrawTarget,
+                                const nsRect& aDirtyRect, nsPoint aFramePt);
+
+  // XXX: should be removed eventually
+  typedef void (*OldPaintCallback)(nsIFrame* aFrame, gfxContext* aCtx,
+                                   const nsRect& aDirtyRect, nsPoint aFramePt);
+
+  nsDisplayGeneric(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                   PaintCallback aPaint, const char* aName,
+                   DisplayItemType aType)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mPaint(aPaint),
+        mOldPaint(nullptr),
+        mName(aName) {
+    MOZ_COUNT_CTOR(nsDisplayGeneric);
+    SetType(aType);
+  }
+
+  // XXX: should be removed eventually
+  nsDisplayGeneric(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                   OldPaintCallback aOldPaint, const char* aName,
+                   DisplayItemType aType)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mPaint(nullptr),
+        mOldPaint(aOldPaint),
+        mName(aName) {
+    MOZ_COUNT_CTOR(nsDisplayGeneric);
+    SetType(aType);
+  }
+
+  constexpr static DisplayItemType ItemType() {
+    return DisplayItemType::TYPE_GENERIC;
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayGeneric)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    MOZ_ASSERT(!!mPaint != !!mOldPaint);
+    if (mPaint) {
+      mPaint(mFrame, aCtx->GetDrawTarget(), GetPaintRect(aBuilder, aCtx),
+             ToReferenceFrame());
+    } else {
+      mOldPaint(mFrame, aCtx, GetPaintRect(aBuilder, aCtx), ToReferenceFrame());
+    }
+  }
+
+  const char* Name() const override { return mName; }
+
+  // This override is needed because GetType() for nsDisplayGeneric subclasses
+  // does not match TYPE_GENERIC that was used to allocate the object.
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    this->~nsDisplayGeneric();
+    aBuilder->Destroy(DisplayItemType::TYPE_GENERIC, this);
+  }
+
+ protected:
+  void* operator new(size_t aSize, nsDisplayListBuilder* aBuilder) {
+    return aBuilder->Allocate(aSize, DisplayItemType::TYPE_GENERIC);
+  }
+
+  template <typename T, typename F, typename... Args>
+  friend T* MakeDisplayItemWithIndex(nsDisplayListBuilder* aBuilder, F* aFrame,
+                                     const uint16_t aIndex, Args&&... aArgs);
+
+  PaintCallback mPaint;
+  OldPaintCallback mOldPaint;  // XXX: should be removed eventually
+  const char* mName;
+};
+
+#if defined(MOZ_REFLOW_PERF_DSP) && defined(MOZ_REFLOW_PERF)
+/**
+ * This class implements painting of reflow counts.  Ideally, we would simply
+ * make all the frame names be those returned by nsIFrame::GetFrameName
+ * (except that tosses in the content tag name!)  and support only one color
+ * and eliminate this class altogether in favor of nsDisplayGeneric, but for
+ * the time being we can't pass args to a PaintCallback, so just have a
+ * separate class to do the right thing.  Sadly, this alsmo means we need to
+ * hack all leaf frame classes to handle this.
+ *
+ * XXXbz the color thing is a bit of a mess, but 0 basically means "not set"
+ * here...  I could switch it all to nscolor, but why bother?
+ */
+class nsDisplayReflowCount : public nsPaintedDisplayItem {
+ public:
+  nsDisplayReflowCount(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       const char* aFrameName, uint32_t aColor = 0)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mFrameName(aFrameName),
+        mColor(aColor) {
+    MOZ_COUNT_CTOR(nsDisplayReflowCount);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayReflowCount)
+
+  NS_DISPLAY_DECL_NAME("nsDisplayReflowCount", TYPE_REFLOW_COUNT)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+ protected:
+  const char* mFrameName;
+  nscolor mColor;
+};
+
+#  define DO_GLOBAL_REFLOW_COUNT_DSP(_name)                                 \
+    PR_BEGIN_MACRO                                                          \
+    if (!aBuilder->IsBackgroundOnly() && !aBuilder->IsForEventDelivery() && \
+        PresShell()->IsPaintingFrameCounts()) {                             \
+      aLists.Outlines()->AppendNewToTop<mozilla::nsDisplayReflowCount>(     \
+          aBuilder, this, _name);                                           \
+    }                                                                       \
+    PR_END_MACRO
+
+#  define DO_GLOBAL_REFLOW_COUNT_DSP_COLOR(_name, _color)                   \
+    PR_BEGIN_MACRO                                                          \
+    if (!aBuilder->IsBackgroundOnly() && !aBuilder->IsForEventDelivery() && \
+        PresShell()->IsPaintingFrameCounts()) {                             \
+      aLists.Outlines()->AppendNewToTop<mozilla::nsDisplayReflowCount>(     \
+          aBuilder, this, _name, _color);                                   \
+    }                                                                       \
+    PR_END_MACRO
+
+/*
+  Macro to be used for classes that don't actually implement BuildDisplayList
+ */
+#  define DECL_DO_GLOBAL_REFLOW_COUNT_DSP(_class, _super)     \
+    void BuildDisplayList(nsDisplayListBuilder* aBuilder,     \
+                          const nsRect& aDirtyRect,           \
+                          const nsDisplayListSet& aLists) {   \
+      DO_GLOBAL_REFLOW_COUNT_DSP(#_class);                    \
+      _super::BuildDisplayList(aBuilder, aDirtyRect, aLists); \
+    }
+
+#else  // MOZ_REFLOW_PERF_DSP && MOZ_REFLOW_PERF
+
+#  define DO_GLOBAL_REFLOW_COUNT_DSP(_name)
+#  define DO_GLOBAL_REFLOW_COUNT_DSP_COLOR(_name, _color)
+#  define DECL_DO_GLOBAL_REFLOW_COUNT_DSP(_class, _super)
+
+#endif  // MOZ_REFLOW_PERF_DSP && MOZ_REFLOW_PERF
+
+class nsDisplayCaret : public nsPaintedDisplayItem {
+ public:
+  nsDisplayCaret(nsDisplayListBuilder* aBuilder, nsIFrame* aCaretFrame);
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+  ~nsDisplayCaret() override;
+#endif
+
+  NS_DISPLAY_DECL_NAME("Caret", TYPE_CARET)
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+ protected:
+  RefPtr<nsCaret> mCaret;
+  nsRect mBounds;
+};
+
+/**
+ * The standard display item to paint the CSS borders of a frame.
+ */
+class nsDisplayBorder : public nsPaintedDisplayItem {
+ public:
+  nsDisplayBorder(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBorder)
+
+  NS_DISPLAY_DECL_NAME("Border", TYPE_BORDER)
+
+  bool IsInvisibleInRect(const nsRect& aRect) const override;
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override;
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+
+  nsRegion GetTightBounds(nsDisplayListBuilder* aBuilder,
+                          bool* aSnap) const override {
+    *aSnap = true;
+    return CalculateBounds<nsRegion>(*mFrame->StyleBorder());
+  }
+
+ protected:
+  template <typename T>
+  T CalculateBounds(const nsStyleBorder& aStyleBorder) const {
+    nsRect borderBounds(ToReferenceFrame(), mFrame->GetSize());
+    if (aStyleBorder.IsBorderImageSizeAvailable()) {
+      borderBounds.Inflate(aStyleBorder.GetImageOutset());
+      return borderBounds;
+    }
+
+    nsMargin border = aStyleBorder.GetComputedBorder();
+    T result;
+    if (border.top > 0) {
+      result = nsRect(borderBounds.X(), borderBounds.Y(), borderBounds.Width(),
+                      border.top);
+    }
+    if (border.right > 0) {
+      result.OrWith(nsRect(borderBounds.XMost() - border.right,
+                           borderBounds.Y(), border.right,
+                           borderBounds.Height()));
+    }
+    if (border.bottom > 0) {
+      result.OrWith(nsRect(borderBounds.X(),
+                           borderBounds.YMost() - border.bottom,
+                           borderBounds.Width(), border.bottom));
+    }
+    if (border.left > 0) {
+      result.OrWith(nsRect(borderBounds.X(), borderBounds.Y(), border.left,
+                           borderBounds.Height()));
+    }
+
+    nscoord radii[8];
+    if (mFrame->GetBorderRadii(radii)) {
+      if (border.left > 0 || border.top > 0) {
+        nsSize cornerSize(radii[eCornerTopLeftX], radii[eCornerTopLeftY]);
+        result.OrWith(nsRect(borderBounds.TopLeft(), cornerSize));
+      }
+      if (border.top > 0 || border.right > 0) {
+        nsSize cornerSize(radii[eCornerTopRightX], radii[eCornerTopRightY]);
+        result.OrWith(
+            nsRect(borderBounds.TopRight() - nsPoint(cornerSize.width, 0),
+                   cornerSize));
+      }
+      if (border.right > 0 || border.bottom > 0) {
+        nsSize cornerSize(radii[eCornerBottomRightX],
+                          radii[eCornerBottomRightY]);
+        result.OrWith(nsRect(borderBounds.BottomRight() -
+                                 nsPoint(cornerSize.width, cornerSize.height),
+                             cornerSize));
+      }
+      if (border.bottom > 0 || border.left > 0) {
+        nsSize cornerSize(radii[eCornerBottomLeftX], radii[eCornerBottomLeftY]);
+        result.OrWith(
+            nsRect(borderBounds.BottomLeft() - nsPoint(0, cornerSize.height),
+                   cornerSize));
+      }
+    }
+    return result;
+  }
+
+  nsRect mBounds;
+};
+
+/**
+ * A simple display item that just renders a solid color across the
+ * specified bounds. For canvas frames (in the CSS sense) we split off the
+ * drawing of the background color into this class (from nsDisplayBackground
+ * via nsDisplayCanvasBackground). This is done so that we can always draw a
+ * background color to avoid ugly flashes of white when we can't draw a full
+ * frame tree (ie when a page is loading). The bounds can differ from the
+ * frame's bounds -- this is needed when a frame/iframe is loading and there
+ * is not yet a frame tree to go in the frame/iframe so we use the subdoc
+ * frame of the parent document as a standin.
+ */
+class nsDisplaySolidColorBase : public nsPaintedDisplayItem {
+ public:
+  nsDisplaySolidColorBase(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                          nscolor aColor)
+      : nsPaintedDisplayItem(aBuilder, aFrame), mColor(aColor) {}
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplaySolidColorGeometry(this, aBuilder, mColor);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    const nsDisplaySolidColorGeometry* geometry =
+        static_cast<const nsDisplaySolidColorGeometry*>(aGeometry);
+    if (mColor != geometry->mColor) {
+      bool dummy;
+      aInvalidRegion->Or(geometry->mBounds, GetBounds(aBuilder, &dummy));
+      return;
+    }
+    ComputeInvalidationRegionDifference(aBuilder, geometry, aInvalidRegion);
+  }
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override {
+    *aSnap = false;
+    nsRegion result;
+    if (NS_GET_A(mColor) == 255) {
+      result = GetBounds(aBuilder, aSnap);
+    }
+    return result;
+  }
+
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override {
+    return Some(mColor);
+  }
+
+ protected:
+  nscolor mColor;
+};
+
+class nsDisplaySolidColor : public nsDisplaySolidColorBase {
+ public:
+  nsDisplaySolidColor(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                      const nsRect& aBounds, nscolor aColor,
+                      bool aCanBeReused = true)
+      : nsDisplaySolidColorBase(aBuilder, aFrame, aColor),
+        mBounds(aBounds),
+        mIsCheckerboardBackground(false) {
+    NS_ASSERTION(NS_GET_A(aColor) > 0,
+                 "Don't create invisible nsDisplaySolidColors!");
+    MOZ_COUNT_CTOR(nsDisplaySolidColor);
+    if (!aCanBeReused) {
+      SetCantBeReused();
+    }
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplaySolidColor)
+
+  NS_DISPLAY_DECL_NAME("SolidColor", TYPE_SOLID_COLOR)
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  void WriteDebugInfo(std::stringstream& aStream) override;
+  void SetIsCheckerboardBackground() { mIsCheckerboardBackground = true; }
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  int32_t ZIndex() const override {
+    if (mOverrideZIndex) {
+      return mOverrideZIndex.value();
+    }
+    return nsDisplaySolidColorBase::ZIndex();
+  }
+
+  void SetOverrideZIndex(int32_t aZIndex) { mOverrideZIndex = Some(aZIndex); }
+
+ private:
+  nsRect mBounds;
+  bool mIsCheckerboardBackground;
+  Maybe<int32_t> mOverrideZIndex;
+};
+
+/**
+ * A display item that renders a solid color over a region. This is not
+ * exposed through CSS, its only purpose is efficient invalidation of
+ * the find bar highlighter dimmer.
+ */
+class nsDisplaySolidColorRegion : public nsPaintedDisplayItem {
+ public:
+  nsDisplaySolidColorRegion(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                            const nsRegion& aRegion, nscolor aColor)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mRegion(aRegion),
+        mColor(gfx::sRGBColor::FromABGR(aColor)) {
+    NS_ASSERTION(NS_GET_A(aColor) > 0,
+                 "Don't create invisible nsDisplaySolidColorRegions!");
+    MOZ_COUNT_CTOR(nsDisplaySolidColorRegion);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplaySolidColorRegion)
+
+  NS_DISPLAY_DECL_NAME("SolidColorRegion", TYPE_SOLID_COLOR_REGION)
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplaySolidColorRegionGeometry(this, aBuilder, mRegion,
+                                                 mColor);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    const nsDisplaySolidColorRegionGeometry* geometry =
+        static_cast<const nsDisplaySolidColorRegionGeometry*>(aGeometry);
+    if (mColor == geometry->mColor) {
+      aInvalidRegion->Xor(geometry->mRegion, mRegion);
+    } else {
+      aInvalidRegion->Or(geometry->mRegion.GetBounds(), mRegion.GetBounds());
+    }
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+ protected:
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  void WriteDebugInfo(std::stringstream& aStream) override;
+
+ private:
+  nsRegion mRegion;
+  gfx::sRGBColor mColor;
+};
+
+enum class AppendedBackgroundType : uint8_t {
+  None,
+  Background,
+  ThemedBackground,
+};
+
+/**
+ * A display item to paint one background-image for a frame. Each background
+ * image layer gets its own nsDisplayBackgroundImage.
+ */
+class nsDisplayBackgroundImage : public nsPaintedDisplayItem {
+ public:
+  struct InitData {
+    nsDisplayListBuilder* builder;
+    const ComputedStyle* backgroundStyle;
+    nsCOMPtr<imgIContainer> image;
+    nsRect backgroundRect;
+    nsRect fillArea;
+    nsRect destArea;
+    uint32_t layer;
+    bool isRasterImage;
+    bool shouldFixToViewport;
+  };
+
+  /**
+   * aLayer signifies which background layer this item represents.
+   * aIsThemed should be the value of aFrame->IsThemed.
+   * aBackgroundStyle should be the result of
+   * nsCSSRendering::FindBackground, or null if FindBackground returned false.
+   * aBackgroundRect is relative to aFrame.
+   */
+  static InitData GetInitData(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                              uint16_t aLayer, const nsRect& aBackgroundRect,
+                              const ComputedStyle* aBackgroundStyle);
+
+  explicit nsDisplayBackgroundImage(nsDisplayListBuilder* aBuilder,
+                                    nsIFrame* aFrame, const InitData& aInitData,
+                                    nsIFrame* aFrameForBounds = nullptr);
+  ~nsDisplayBackgroundImage() override;
+
+  NS_DISPLAY_DECL_NAME("Background", TYPE_BACKGROUND)
+
+  /**
+   * This will create and append new items for all the layers of the
+   * background. Returns the type of background that was appended.
+   * aAllowWillPaintBorderOptimization should usually be left at true, unless
+   * aFrame has special border drawing that causes opaque borders to not
+   * actually be opaque.
+   */
+  static AppendedBackgroundType AppendBackgroundItemsToTop(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+      const nsRect& aBackgroundRect, nsDisplayList* aList,
+      bool aAllowWillPaintBorderOptimization = true,
+      const nsRect& aBackgroundOriginRect = nsRect(),
+      nsIFrame* aSecondaryReferenceFrame = nullptr,
+      Maybe<nsDisplayListBuilder::AutoBuildingDisplayList>*
+          aAutoBuildingDisplayList = nullptr);
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override;
+
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const override;
+
+  /**
+   * GetBounds() returns the background painting area.
+   */
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  /**
+   * Return the background positioning area.
+   * (GetBounds() returns the background painting area.)
+   * Can be called only when mBackgroundStyle is non-null.
+   */
+  nsRect GetPositioningArea() const;
+
+  /**
+   * Returns true if existing rendered pixels of this display item may need
+   * to be redrawn if the positioning area size changes but its position does
+   * not.
+   * If false, only the changed painting area needs to be redrawn when the
+   * positioning area size changes but its position does not.
+   */
+  bool RenderingMightDependOnPositioningAreaSizeChange() const;
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayBackgroundGeometry(this, aBuilder);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+  bool ShouldFixToViewport(nsDisplayListBuilder* aBuilder) const override {
+    return mShouldFixToViewport;
+  }
+
+  nsRect GetDestRect() const { return mDestRect; }
+
+  nsIFrame* GetDependentFrame() override { return mDependentFrame; }
+
+  void SetDependentFrame(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame) {
+    if (!aBuilder->IsRetainingDisplayList() || mDependentFrame == aFrame) {
+      return;
+    }
+    mDependentFrame = aFrame;
+    if (aFrame) {
+      mDependentFrame->AddDisplayItem(this);
+    }
+  }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mDependentFrame) {
+      mDependentFrame = nullptr;
+    }
+    nsPaintedDisplayItem::RemoveFrame(aFrame);
+  }
+
+  // Match https://w3c.github.io/paint-timing/#contentful-image
+  bool IsContentful() const override {
+    const auto& styleImage =
+        mBackgroundStyle->StyleBackground()->mImage.mLayers[mLayer].mImage;
+
+    return styleImage.IsSizeAvailable() && styleImage.FinalImage().IsUrl();
+  }
+
+ protected:
+  bool CanBuildWebRenderDisplayItems(layers::WebRenderLayerManager* aManager,
+                                     nsDisplayListBuilder* aBuilder) const;
+  nsRect GetBoundsInternal(nsDisplayListBuilder* aBuilder,
+                           nsIFrame* aFrameForBounds = nullptr);
+
+  void PaintInternal(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+                     const nsRect& aBounds, nsRect* aClipRect);
+
+  // Cache the result of nsCSSRendering::FindBackground. Always null if
+  // mIsThemed is true or if FindBackground returned false.
+  RefPtr<const ComputedStyle> mBackgroundStyle;
+  nsCOMPtr<imgIContainer> mImage;
+  nsIFrame* mDependentFrame;
+  nsRect mBackgroundRect;  // relative to the reference frame
+  nsRect mFillRect;
+  nsRect mDestRect;
+  /* Bounds of this display item */
+  nsRect mBounds;
+  uint16_t mLayer;
+  bool mIsRasterImage;
+  /* Whether the image should be treated as fixed to the viewport. */
+  bool mShouldFixToViewport;
+};
+
+/**
+ * A display item to paint background image for table. For table parts, such
+ * as row, row group, col, col group, when drawing its background, we'll
+ * create separate background image display item for its containning cell.
+ * Those background image display items will reference to same DisplayItemData
+ * if we keep the mFrame point to cell's ancestor frame. We don't want to this
+ * happened bacause share same DisplatItemData will cause many bugs. So that
+ * we let mFrame point to cell frame and store the table type of the ancestor
+ * frame. And use mFrame and table type as key to generate DisplayItemData to
+ * avoid sharing DisplayItemData.
+ *
+ * Also store ancestor frame as mStyleFrame for all rendering informations.
+ */
+class nsDisplayTableBackgroundImage : public nsDisplayBackgroundImage {
+ public:
+  nsDisplayTableBackgroundImage(nsDisplayListBuilder* aBuilder,
+                                nsIFrame* aFrame, const InitData& aData,
+                                nsIFrame* aCellFrame);
+  ~nsDisplayTableBackgroundImage() override;
+
+  NS_DISPLAY_DECL_NAME("TableBackgroundImage", TYPE_TABLE_BACKGROUND_IMAGE)
+
+  bool IsInvalid(nsRect& aRect) const override;
+
+  nsIFrame* FrameForInvalidation() const override { return mStyleFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mStyleFrame) {
+      mStyleFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayBackgroundImage::RemoveFrame(aFrame);
+  }
+
+ protected:
+  nsIFrame* StyleFrame() const override { return mStyleFrame; }
+  nsIFrame* mStyleFrame;
+};
+
+/**
+ * A display item to paint the native theme background for a frame.
+ */
+class nsDisplayThemedBackground : public nsPaintedDisplayItem {
+ public:
+  nsDisplayThemedBackground(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                            const nsRect& aBackgroundRect);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayThemedBackground)
+
+  NS_DISPLAY_DECL_NAME("ThemedBackground", TYPE_THEMED_BACKGROUND)
+
+  void Init(nsDisplayListBuilder* aBuilder);
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    aBuilder->UnregisterThemeGeometry(this);
+    nsPaintedDisplayItem::Destroy(aBuilder);
+  }
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  bool ShouldUseBlobRenderingForFallback() const override {
+    return !XRE_IsParentProcess();
+  }
+
+  /**
+   * GetBounds() returns the background painting area.
+   */
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  /**
+   * Return the background positioning area.
+   * (GetBounds() returns the background painting area.)
+   * Can be called only when mBackgroundStyle is non-null.
+   */
+  nsRect GetPositioningArea() const;
+
+  /**
+   * Return whether our frame's document does not have the state
+   * NS_DOCUMENT_STATE_WINDOW_INACTIVE.
+   */
+  bool IsWindowActive() const;
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayThemedBackgroundGeometry(this, aBuilder);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+
+  void WriteDebugInfo(std::stringstream& aStream) override;
+
+ protected:
+  nsRect GetBoundsInternal();
+
+  void PaintInternal(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+                     const nsRect& aBounds, nsRect* aClipRect);
+
+  nsRect mBackgroundRect;
+  nsRect mBounds;
+  nsITheme::Transparency mThemeTransparency;
+  StyleAppearance mAppearance;
+};
+
+class nsDisplayTableThemedBackground : public nsDisplayThemedBackground {
+ public:
+  nsDisplayTableThemedBackground(nsDisplayListBuilder* aBuilder,
+                                 nsIFrame* aFrame,
+                                 const nsRect& aBackgroundRect,
+                                 nsIFrame* aAncestorFrame)
+      : nsDisplayThemedBackground(aBuilder, aFrame, aBackgroundRect),
+        mAncestorFrame(aAncestorFrame) {
+    if (aBuilder->IsRetainingDisplayList()) {
+      mAncestorFrame->AddDisplayItem(this);
+    }
+  }
+
+  ~nsDisplayTableThemedBackground() override {
+    if (mAncestorFrame) {
+      mAncestorFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  NS_DISPLAY_DECL_NAME("TableThemedBackground",
+                       TYPE_TABLE_THEMED_BACKGROUND_IMAGE)
+
+  nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mAncestorFrame) {
+      mAncestorFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayThemedBackground::RemoveFrame(aFrame);
+  }
+
+ protected:
+  nsIFrame* StyleFrame() const override { return mAncestorFrame; }
+  nsIFrame* mAncestorFrame;
+};
+
+class nsDisplayBackgroundColor : public nsPaintedDisplayItem {
+ public:
+  nsDisplayBackgroundColor(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                           const nsRect& aBackgroundRect,
+                           const ComputedStyle* aBackgroundStyle,
+                           const nscolor& aColor)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mBackgroundRect(aBackgroundRect),
+        mHasStyle(aBackgroundStyle),
+        mDependentFrame(nullptr),
+        mColor(gfx::sRGBColor::FromABGR(aColor)) {
+    if (mHasStyle) {
+      mBottomLayerClip =
+          aBackgroundStyle->StyleBackground()->BottomLayer().mClip;
+    } else {
+      MOZ_ASSERT(aBuilder->IsForEventDelivery());
+    }
+  }
+
+  ~nsDisplayBackgroundColor() override {
+    if (mDependentFrame) {
+      mDependentFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  NS_DISPLAY_DECL_NAME("BackgroundColor", TYPE_BACKGROUND_COLOR)
+
+  bool HasBackgroundClipText() const {
+    MOZ_ASSERT(mHasStyle);
+    return mBottomLayerClip == StyleGeometryBox::Text;
+  }
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  void PaintWithClip(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+                     const DisplayItemClip& aClip) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override;
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const override;
+
+  float GetOpacity() const { return mColor.a; }
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override {
+    *aSnap = true;
+    return mBackgroundRect;
+  }
+
+  bool CanPaintWithClip(const DisplayItemClip& aClip) override {
+    if (HasBackgroundClipText()) {
+      return false;
+    }
+
+    if (aClip.GetRoundedRectCount() > 1) {
+      return false;
+    }
+
+    return true;
+  }
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplaySolidColorGeometry(this, aBuilder, mColor.ToABGR());
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    const nsDisplaySolidColorGeometry* geometry =
+        static_cast<const nsDisplaySolidColorGeometry*>(aGeometry);
+
+    if (mColor.ToABGR() != geometry->mColor) {
+      bool dummy;
+      aInvalidRegion->Or(geometry->mBounds, GetBounds(aBuilder, &dummy));
+      return;
+    }
+    ComputeInvalidationRegionDifference(aBuilder, geometry, aInvalidRegion);
+  }
+
+  nsIFrame* GetDependentFrame() override { return mDependentFrame; }
+
+  void SetDependentFrame(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame) {
+    if (!aBuilder->IsRetainingDisplayList() || mDependentFrame == aFrame) {
+      return;
+    }
+    mDependentFrame = aFrame;
+    if (aFrame) {
+      mDependentFrame->AddDisplayItem(this);
+    }
+  }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mDependentFrame) {
+      mDependentFrame = nullptr;
+    }
+
+    nsPaintedDisplayItem::RemoveFrame(aFrame);
+  }
+
+  void WriteDebugInfo(std::stringstream& aStream) override;
+
+  bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) override;
+
+ protected:
+  const nsRect mBackgroundRect;
+  const bool mHasStyle;
+  StyleGeometryBox mBottomLayerClip;
+  nsIFrame* mDependentFrame;
+  gfx::sRGBColor mColor;
+};
+
+class nsDisplayTableBackgroundColor : public nsDisplayBackgroundColor {
+ public:
+  nsDisplayTableBackgroundColor(nsDisplayListBuilder* aBuilder,
+                                nsIFrame* aFrame, const nsRect& aBackgroundRect,
+                                const ComputedStyle* aBackgroundStyle,
+                                const nscolor& aColor, nsIFrame* aAncestorFrame)
+      : nsDisplayBackgroundColor(aBuilder, aFrame, aBackgroundRect,
+                                 aBackgroundStyle, aColor),
+        mAncestorFrame(aAncestorFrame) {
+    if (aBuilder->IsRetainingDisplayList()) {
+      mAncestorFrame->AddDisplayItem(this);
+    }
+  }
+
+  ~nsDisplayTableBackgroundColor() override {
+    if (mAncestorFrame) {
+      mAncestorFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  NS_DISPLAY_DECL_NAME("TableBackgroundColor", TYPE_TABLE_BACKGROUND_COLOR)
+
+  nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mAncestorFrame) {
+      mAncestorFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayBackgroundColor::RemoveFrame(aFrame);
+  }
+
+  bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+ protected:
+  nsIFrame* mAncestorFrame;
+};
+
+/**
+ * The standard display item to paint the outer CSS box-shadows of a frame.
+ */
+class nsDisplayBoxShadowOuter final : public nsPaintedDisplayItem {
+ public:
+  nsDisplayBoxShadowOuter(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayBoxShadowOuter);
+    mBounds = GetBoundsInternal();
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBoxShadowOuter)
+
+  NS_DISPLAY_DECL_NAME("BoxShadowOuter", TYPE_BOX_SHADOW_OUTER)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  bool IsInvisibleInRect(const nsRect& aRect) const override;
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const override {
+    return CanBuildWebRenderDisplayItems();
+  }
+
+  bool CanBuildWebRenderDisplayItems() const;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  nsRect GetBoundsInternal();
+
+ private:
+  nsRect mBounds;
+};
+
+/**
+ * The standard display item to paint the inner CSS box-shadows of a frame.
+ */
+class nsDisplayBoxShadowInner : public nsPaintedDisplayItem {
+ public:
+  nsDisplayBoxShadowInner(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayBoxShadowInner);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBoxShadowInner)
+
+  NS_DISPLAY_DECL_NAME("BoxShadowInner", TYPE_BOX_SHADOW_INNER)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayBoxShadowInnerGeometry(this, aBuilder);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    const nsDisplayBoxShadowInnerGeometry* geometry =
+        static_cast<const nsDisplayBoxShadowInnerGeometry*>(aGeometry);
+    if (!geometry->mPaddingRect.IsEqualInterior(GetPaddingRect())) {
+      // nsDisplayBoxShadowInner is based around the padding rect, but it can
+      // touch pixels outside of this. We should invalidate the entire bounds.
+      bool snap;
+      aInvalidRegion->Or(geometry->mBounds, GetBounds(aBuilder, &snap));
+    }
+  }
+
+  static bool CanCreateWebRenderCommands(nsDisplayListBuilder* aBuilder,
+                                         nsIFrame* aFrame,
+                                         const nsPoint& aReferenceOffset);
+  static void CreateInsetBoxShadowWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, const StackingContextHelper& aSc,
+      nsRect& aVisibleRect, nsIFrame* aFrame, const nsRect& aBorderRect);
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+};
+
+/**
+ * The standard display item to paint the CSS outline of a frame.
+ */
+class nsDisplayOutline final : public nsPaintedDisplayItem {
+ public:
+  nsDisplayOutline(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayOutline);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayOutline)
+
+  NS_DISPLAY_DECL_NAME("Outline", TYPE_OUTLINE)
+
+  bool ShouldUseBlobRenderingForFallback() const override {
+    MOZ_ASSERT(IsThemedOutline(),
+               "The only fallback path we have is for themed outlines");
+    return !XRE_IsParentProcess();
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  bool IsInvisibleInRect(const nsRect& aRect) const override;
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+ private:
+  nsRect GetInnerRect() const;
+  bool IsThemedOutline() const;
+  bool HasRadius() const;
+};
+
+/**
+ * A class that lets you receive events within the frame bounds but never
+ * paints.
+ */
+class nsDisplayEventReceiver final : public nsDisplayItem {
+ public:
+  nsDisplayEventReceiver(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayEventReceiver);
+  }
+
+  MOZ_COUNTED_DTOR_FINAL(nsDisplayEventReceiver)
+
+  NS_DISPLAY_DECL_NAME("EventReceiver", TYPE_EVENT_RECEIVER)
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) final;
+};
+
+/**
+ * Similar to nsDisplayEventReceiver in that it is used for hit-testing. However
+ * this gets built when we're doing widget painting and we need to send the
+ * compositor some hit-test info for a frame. This is effectively a dummy item
+ * whose sole purpose is to carry the hit-test info to the compositor.
+ */
+class nsDisplayCompositorHitTestInfo final : public nsDisplayItem {
+ public:
+  nsDisplayCompositorHitTestInfo(nsDisplayListBuilder* aBuilder,
+                                 nsIFrame* aFrame)
+      : nsDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayCompositorHitTestInfo);
+    mHitTestInfo.Initialize(aBuilder, aFrame);
+    SetHasHitTestInfo();
+  }
+
+  nsDisplayCompositorHitTestInfo(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, const nsRect& aArea,
+      const gfx::CompositorHitTestInfo& aHitTestFlags)
+      : nsDisplayItem(aBuilder, aFrame) {
+    MOZ_COUNT_CTOR(nsDisplayCompositorHitTestInfo);
+    mHitTestInfo.SetAreaAndInfo(aArea, aHitTestFlags);
+    mHitTestInfo.InitializeScrollTarget(aBuilder);
+    SetHasHitTestInfo();
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayCompositorHitTestInfo)
+
+  NS_DISPLAY_DECL_NAME("CompositorHitTestInfo", TYPE_COMPOSITOR_HITTEST_INFO)
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  bool isInvisible() const { return true; }
+
+  int32_t ZIndex() const override;
+  void SetOverrideZIndex(int32_t aZIndex);
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override {
+    *aSnap = false;
+    return nsRect();
+  }
+
+  const HitTestInfo& GetHitTestInfo() final { return mHitTestInfo; }
+
+ private:
+  HitTestInfo mHitTestInfo;
+  Maybe<int32_t> mOverrideZIndex;
+};
+
+class nsDisplayWrapper;
+
+/**
+ * A class that lets you wrap a display list as a display item.
+ *
+ * GetUnderlyingFrame() is troublesome for wrapped lists because if the wrapped
+ * list has many items, it's not clear which one has the 'underlying frame'.
+ * Thus we force the creator to specify what the underlying frame is. The
+ * underlying frame should be the root of a stacking context, because sorting
+ * a list containing this item will not get at the children.
+ *
+ * In some cases (e.g., clipping) we want to wrap a list but we don't have a
+ * particular underlying frame that is a stacking context root. In that case
+ * we allow the frame to be nullptr. Callers to GetUnderlyingFrame must
+ * detect and handle this case.
+ */
+class nsDisplayWrapList : public nsPaintedDisplayItem {
+ public:
+  /**
+   * Takes all the items from aList and puts them in our list.
+   */
+  nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                    nsDisplayList* aList);
+
+  nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                    nsDisplayItem* aItem);
+
+  nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                    nsDisplayList* aList,
+                    const ActiveScrolledRoot* aActiveScrolledRoot,
+                    bool aClearClipChain = false);
+
+  nsDisplayWrapList(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mList(aBuilder),
+        mFrameActiveScrolledRoot(aBuilder->CurrentActiveScrolledRoot()),
+        mOverrideZIndex(0),
+        mHasZIndexOverride(false) {
+    MOZ_COUNT_CTOR(nsDisplayWrapList);
+    mBaseBuildingRect = GetBuildingRect();
+    mListPtr = &mList;
+    mOriginalClipChain = mClipChain;
+  }
+
+  nsDisplayWrapList() = delete;
+
+  /**
+   * A custom copy-constructor that does not copy mList, as this would mutate
+   * the other item.
+   */
+  nsDisplayWrapList(const nsDisplayWrapList& aOther) = delete;
+  nsDisplayWrapList(nsDisplayListBuilder* aBuilder,
+                    const nsDisplayWrapList& aOther)
+      : nsPaintedDisplayItem(aBuilder, aOther),
+        mList(aBuilder),
+        mListPtr(&mList),
+        mFrameActiveScrolledRoot(aOther.mFrameActiveScrolledRoot),
+        mMergedFrames(aOther.mMergedFrames.Clone()),
+        mBounds(aOther.mBounds),
+        mBaseBuildingRect(aOther.mBaseBuildingRect),
+        mOriginalClipChain(aOther.mClipChain),
+        mOverrideZIndex(aOther.mOverrideZIndex),
+        mHasZIndexOverride(aOther.mHasZIndexOverride),
+        mClearingClipChain(aOther.mClearingClipChain) {
+    MOZ_COUNT_CTOR(nsDisplayWrapList);
+  }
+
+  ~nsDisplayWrapList() override;
+
+  const nsDisplayWrapList* AsDisplayWrapList() const final { return this; }
+  nsDisplayWrapList* AsDisplayWrapList() final { return this; }
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    mList.DeleteAll(aBuilder);
+    nsPaintedDisplayItem::Destroy(aBuilder);
+  }
+
+  /**
+   * Creates a new nsDisplayWrapper that holds a pointer to the display list
+   * owned by the given nsDisplayItem.
+   */
+  nsDisplayWrapper* CreateShallowCopy(nsDisplayListBuilder* aBuilder);
+
+  /**
+   * Call this if the wrapped list is changed.
+   */
+  void UpdateBounds(nsDisplayListBuilder* aBuilder) override {
+    // Clear the clip chain up to the asr, but don't store it, so that we'll
+    // recover it when we reuse the item.
+    if (mClearingClipChain) {
+      const DisplayItemClipChain* clip = mOriginalClipChain;
+      while (clip && ActiveScrolledRoot::IsAncestor(GetActiveScrolledRoot(),
+                                                    clip->mASR)) {
+        clip = clip->mParent;
+      }
+      SetClipChain(clip, false);
+    }
+
+    nsRect buildingRect;
+    mBounds = mListPtr->GetClippedBoundsWithRespectToASR(
+        aBuilder, mActiveScrolledRoot, &buildingRect);
+    // The display list may contain content that's visible outside the visible
+    // rect (i.e. the current dirty rect) passed in when the item was created.
+    // This happens when the dirty rect has been restricted to the visual
+    // overflow rect of a frame for some reason (e.g. when setting up dirty
+    // rects in nsDisplayListBuilder::MarkOutOfFlowFrameForDisplay), but that
+    // frame contains placeholders for out-of-flows that aren't descendants of
+    // the frame.
+    buildingRect.UnionRect(mBaseBuildingRect, buildingRect);
+    SetBuildingRect(buildingRect);
+  }
+
+  void SetClipChain(const DisplayItemClipChain* aClipChain,
+                    bool aStore) override {
+    nsDisplayItem::SetClipChain(aClipChain, aStore);
+
+    if (aStore) {
+      mOriginalClipChain = mClipChain;
+    }
+  }
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  Maybe<nscolor> IsUniform(nsDisplayListBuilder* aBuilder) const override;
+
+  /**
+   * Try to merge with the other item (which is below us in the display
+   * list). This gets used by nsDisplayClip to coalesce clipping operations
+   * (optimization), by nsDisplayOpacity to merge rendering for the same
+   * content element into a single opacity group (correctness), and will be
+   * used by nsDisplayOutline to merge multiple outlines for the same element
+   * (also for correctness).
+   */
+  virtual void Merge(const nsDisplayItem* aItem) {
+    MOZ_ASSERT(CanMerge(aItem));
+    MOZ_ASSERT(Frame() != aItem->Frame());
+    MergeFromTrackingMergedFrames(static_cast<const nsDisplayWrapList*>(aItem));
+  }
+
+  /**
+   * Returns the underlying frames of all display items that have been
+   * merged into this one (excluding this item's own underlying frame)
+   * to aFrames.
+   */
+  const nsTArray<nsIFrame*>& GetMergedFrames() const { return mMergedFrames; }
+
+  bool HasMergedFrames() const { return !mMergedFrames.IsEmpty(); }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return true;
+  }
+
+  bool IsInvalid(nsRect& aRect) const override {
+    if (mFrame->IsInvalid(aRect) && aRect.IsEmpty()) {
+      return true;
+    }
+    nsRect temp;
+    for (uint32_t i = 0; i < mMergedFrames.Length(); i++) {
+      if (mMergedFrames[i]->IsInvalid(temp) && temp.IsEmpty()) {
+        aRect.SetEmpty();
+        return true;
+      }
+      aRect = aRect.Union(temp);
+    }
+    aRect += ToReferenceFrame();
+    return !aRect.IsEmpty();
+  }
+
+  nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const override;
+
+  RetainedDisplayList* GetSameCoordinateSystemChildren() const override {
+    return mListPtr;
+  }
+
+  RetainedDisplayList* GetChildren() const override { return mListPtr; }
+
+  int32_t ZIndex() const override {
+    return (mHasZIndexOverride) ? mOverrideZIndex
+                                : nsPaintedDisplayItem::ZIndex();
+  }
+
+  void SetOverrideZIndex(int32_t aZIndex) {
+    mHasZIndexOverride = true;
+    mOverrideZIndex = aZIndex;
+  }
+
+  /**
+   * This creates a copy of this item, but wrapping aItem instead of
+   * our existing list. Only gets called if this item returned nullptr
+   * for GetUnderlyingFrame(). aItem is guaranteed to return non-null from
+   * GetUnderlyingFrame().
+   */
+  nsDisplayWrapList* WrapWithClone(nsDisplayListBuilder* aBuilder,
+                                   nsDisplayItem* aItem) {
+    MOZ_ASSERT_UNREACHABLE("We never returned nullptr for GetUnderlyingFrame!");
+    return nullptr;
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override {
+    return CreateWebRenderCommandsNewClipListOption(
+        aBuilder, aResources, aSc, aManager, aDisplayListBuilder, true);
+  }
+
+  // Same as the above but with the option to pass the aNewClipList argument to
+  // WebRenderCommandBuilder::CreateWebRenderCommandsFromDisplayList.
+  bool CreateWebRenderCommandsNewClipListOption(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder, bool aNewClipList);
+
+  const ActiveScrolledRoot* GetFrameActiveScrolledRoot() {
+    return mFrameActiveScrolledRoot;
+  }
+
+ protected:
+  void MergeFromTrackingMergedFrames(const nsDisplayWrapList* aOther) {
+    mBounds.UnionRect(mBounds, aOther->mBounds);
+    nsRect buildingRect;
+    buildingRect.UnionRect(GetBuildingRect(), aOther->GetBuildingRect());
+    SetBuildingRect(buildingRect);
+    mMergedFrames.AppendElement(aOther->mFrame);
+    mMergedFrames.AppendElements(aOther->mMergedFrames.Clone());
+  }
+
+  RetainedDisplayList mList;
+  RetainedDisplayList* mListPtr;
+  // The active scrolled root for the frame that created this
+  // wrap list.
+  RefPtr<const ActiveScrolledRoot> mFrameActiveScrolledRoot;
+  // The frames from items that have been merged into this item, excluding
+  // this item's own frame.
+  nsTArray<nsIFrame*> mMergedFrames;
+  nsRect mBounds;
+  // Displaylist building rect contributed by this display item itself.
+  // Our mBuildingRect may include the visible areas of children.
+  nsRect mBaseBuildingRect;
+  RefPtr<const DisplayItemClipChain> mOriginalClipChain;
+  int32_t mOverrideZIndex;
+  bool mHasZIndexOverride;
+  bool mClearingClipChain = false;
+};
+
+class nsDisplayWrapper : public nsDisplayWrapList {
+ public:
+  NS_DISPLAY_DECL_NAME("WrapList", TYPE_WRAP_LIST)
+
+  nsDisplayWrapper(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                   nsDisplayList* aList,
+                   const ActiveScrolledRoot* aActiveScrolledRoot,
+                   bool aClearClipChain = false)
+      : nsDisplayWrapList(aBuilder, aFrame, aList, aActiveScrolledRoot,
+                          aClearClipChain) {}
+
+  nsDisplayWrapper(const nsDisplayWrapper& aOther) = delete;
+  nsDisplayWrapper(nsDisplayListBuilder* aBuilder,
+                   const nsDisplayWrapList& aOther)
+      : nsDisplayWrapList(aBuilder, aOther) {}
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+  friend class nsDisplayListBuilder;
+  friend class nsDisplayWrapList;
+};
+
+/**
+ * We call WrapDisplayList on the in-flow lists: BorderBackground(),
+ * BlockBorderBackgrounds() and Content().
+ * We call WrapDisplayItem on each item of Outlines(), PositionedDescendants(),
+ * and Floats(). This is done to support special wrapping processing for frames
+ * that may not be in-flow descendants of the current frame.
+ */
+class nsDisplayItemWrapper {
+ public:
+  // This is never instantiated directly (it has pure virtual methods), so no
+  // need to count constructors and destructors.
+
+  bool WrapBorderBackground() { return true; }
+  virtual nsDisplayItem* WrapList(nsDisplayListBuilder* aBuilder,
+                                  nsIFrame* aFrame, nsDisplayList* aList) = 0;
+  virtual nsDisplayItem* WrapItem(nsDisplayListBuilder* aBuilder,
+                                  nsDisplayItem* aItem) = 0;
+
+  nsresult WrapLists(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     const nsDisplayListSet& aIn, const nsDisplayListSet& aOut);
+  nsresult WrapListsInPlace(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                            const nsDisplayListSet& aLists);
+
+ protected:
+  nsDisplayItemWrapper() = default;
+};
+
+/**
+ * The standard display item to paint a stacking context with translucency
+ * set by the stacking context root frame's 'opacity' style.
+ */
+class nsDisplayOpacity : public nsDisplayWrapList {
+ public:
+  nsDisplayOpacity(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                   nsDisplayList* aList,
+                   const ActiveScrolledRoot* aActiveScrolledRoot,
+                   bool aForEventsOnly, bool aNeedsActiveLayer,
+                   bool aWrapsBackdropFilter);
+
+  nsDisplayOpacity(nsDisplayListBuilder* aBuilder,
+                   const nsDisplayOpacity& aOther)
+      : nsDisplayWrapList(aBuilder, aOther),
+        mOpacity(aOther.mOpacity),
+        mForEventsOnly(aOther.mForEventsOnly),
+        mNeedsActiveLayer(aOther.mNeedsActiveLayer),
+        mChildOpacityState(ChildOpacityState::Unknown),
+        mWrapsBackdropFilter(aOther.mWrapsBackdropFilter) {
+    MOZ_COUNT_CTOR(nsDisplayOpacity);
+    // We should not try to merge flattened opacities.
+    MOZ_ASSERT(aOther.mChildOpacityState != ChildOpacityState::Applied);
+  }
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayOpacity)
+
+  NS_DISPLAY_DECL_NAME("Opacity", TYPE_OPACITY)
+
+  void InvalidateCachedChildInfo(nsDisplayListBuilder* aBuilder) override {
+    mChildOpacityState = ChildOpacityState::Unknown;
+  }
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  bool CanMerge(const nsDisplayItem* aItem) const override {
+    // items for the same content element should be merged into a single
+    // compositing group
+    // aItem->GetUnderlyingFrame() returns non-null because it's
+    // nsDisplayOpacity
+    return HasDifferentFrame(aItem) && HasSameTypeAndClip(aItem) &&
+           HasSameContent(aItem);
+  }
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayOpacityGeometry(this, aBuilder, mOpacity);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+
+  bool IsInvalid(nsRect& aRect) const override {
+    if (mForEventsOnly) {
+      return false;
+    }
+    return nsDisplayWrapList::IsInvalid(aRect);
+  }
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const override;
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  bool CanApplyOpacityToChildren(WebRenderLayerManager* aManager,
+                                 nsDisplayListBuilder* aBuilder,
+                                 float aInheritedOpacity);
+
+  bool NeedsGeometryUpdates() const override {
+    // For flattened nsDisplayOpacity items, ComputeInvalidationRegion() only
+    // handles invalidation for changed |mOpacity|. In order to keep track of
+    // the current bounds of the item for invalidation, nsDisplayOpacityGeometry
+    // for the corresponding DisplayItemData needs to be updated, even if the
+    // reported invalidation region is empty.
+    return mChildOpacityState == ChildOpacityState::Deferred;
+  }
+
+  /**
+   * Returns true if ShouldFlattenAway() applied opacity to children.
+   */
+  bool OpacityAppliedToChildren() const {
+    return mChildOpacityState == ChildOpacityState::Applied;
+  }
+
+  static bool NeedsActiveLayer(nsDisplayListBuilder* aBuilder,
+                               nsIFrame* aFrame);
+  void WriteDebugInfo(std::stringstream& aStream) override;
+  bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  float GetOpacity() const { return mOpacity; }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+
+  bool CanApplyToChildren(WebRenderLayerManager* aManager,
+                          nsDisplayListBuilder* aBuilder);
+  bool ApplyToMask();
+
+  float mOpacity;
+  bool mForEventsOnly : 1;
+  enum class ChildOpacityState : uint8_t {
+    // Our child list has changed since the last time ApplyToChildren was
+    // called.
+    Unknown,
+    // Our children defer opacity handling to us.
+    Deferred,
+    // Opacity is applied to our children.
+    Applied
+  };
+  bool mNeedsActiveLayer : 1;
+#ifndef __GNUC__
+  ChildOpacityState mChildOpacityState : 2;
+#else
+  ChildOpacityState mChildOpacityState;
+#endif
+  bool mWrapsBackdropFilter;
+};
+
+class nsDisplayBlendMode : public nsDisplayWrapList {
+ public:
+  nsDisplayBlendMode(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     nsDisplayList* aList, StyleBlend aBlendMode,
+                     const ActiveScrolledRoot* aActiveScrolledRoot,
+                     const bool aIsForBackground);
+  nsDisplayBlendMode(nsDisplayListBuilder* aBuilder,
+                     const nsDisplayBlendMode& aOther)
+      : nsDisplayWrapList(aBuilder, aOther),
+        mBlendMode(aOther.mBlendMode),
+        mIsForBackground(aOther.mIsForBackground) {
+    MOZ_COUNT_CTOR(nsDisplayBlendMode);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBlendMode)
+
+  NS_DISPLAY_DECL_NAME("BlendMode", TYPE_BLEND_MODE)
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    // We don't need to compute an invalidation region since we have
+    // LayerTreeInvalidation
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  bool CanMerge(const nsDisplayItem* aItem) const override;
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  gfx::CompositionOp BlendMode();
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ protected:
+  StyleBlend mBlendMode;
+  bool mIsForBackground;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+class nsDisplayTableBlendMode : public nsDisplayBlendMode {
+ public:
+  nsDisplayTableBlendMode(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                          nsDisplayList* aList, StyleBlend aBlendMode,
+                          const ActiveScrolledRoot* aActiveScrolledRoot,
+                          nsIFrame* aAncestorFrame, const bool aIsForBackground)
+      : nsDisplayBlendMode(aBuilder, aFrame, aList, aBlendMode,
+                           aActiveScrolledRoot, aIsForBackground),
+        mAncestorFrame(aAncestorFrame) {
+    if (aBuilder->IsRetainingDisplayList()) {
+      mAncestorFrame->AddDisplayItem(this);
+    }
+  }
+
+  nsDisplayTableBlendMode(nsDisplayListBuilder* aBuilder,
+                          const nsDisplayTableBlendMode& aOther)
+      : nsDisplayBlendMode(aBuilder, aOther),
+        mAncestorFrame(aOther.mAncestorFrame) {
+    if (aBuilder->IsRetainingDisplayList()) {
+      mAncestorFrame->AddDisplayItem(this);
+    }
+  }
+
+  ~nsDisplayTableBlendMode() override {
+    if (mAncestorFrame) {
+      mAncestorFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  NS_DISPLAY_DECL_NAME("TableBlendMode", TYPE_TABLE_BLEND_MODE)
+
+  nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mAncestorFrame) {
+      mAncestorFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayBlendMode::RemoveFrame(aFrame);
+  }
+
+ protected:
+  nsIFrame* mAncestorFrame;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+class nsDisplayBlendContainer : public nsDisplayWrapList {
+ public:
+  static nsDisplayBlendContainer* CreateForMixBlendMode(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
+      const ActiveScrolledRoot* aActiveScrolledRoot);
+
+  static nsDisplayBlendContainer* CreateForBackgroundBlendMode(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+      nsIFrame* aSecondaryFrame, nsDisplayList* aList,
+      const ActiveScrolledRoot* aActiveScrolledRoot);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBlendContainer)
+
+  NS_DISPLAY_DECL_NAME("BlendContainer", TYPE_BLEND_CONTAINER)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  bool CanMerge(const nsDisplayItem* aItem) const override {
+    // Items for the same content element should be merged into a single
+    // compositing group.
+    return HasDifferentFrame(aItem) && HasSameTypeAndClip(aItem) &&
+           HasSameContent(aItem) &&
+           mIsForBackground ==
+               static_cast<const nsDisplayBlendContainer*>(aItem)
+                   ->mIsForBackground;
+  }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ protected:
+  nsDisplayBlendContainer(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                          nsDisplayList* aList,
+                          const ActiveScrolledRoot* aActiveScrolledRoot,
+                          bool aIsForBackground);
+  nsDisplayBlendContainer(nsDisplayListBuilder* aBuilder,
+                          const nsDisplayBlendContainer& aOther)
+      : nsDisplayWrapList(aBuilder, aOther),
+        mIsForBackground(aOther.mIsForBackground) {
+    MOZ_COUNT_CTOR(nsDisplayBlendContainer);
+  }
+
+  // Used to distinguish containers created at building stacking
+  // context or appending background.
+  bool mIsForBackground;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+class nsDisplayTableBlendContainer : public nsDisplayBlendContainer {
+ public:
+  NS_DISPLAY_DECL_NAME("TableBlendContainer", TYPE_TABLE_BLEND_CONTAINER)
+
+  nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mAncestorFrame) {
+      mAncestorFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayBlendContainer::RemoveFrame(aFrame);
+  }
+
+ protected:
+  nsDisplayTableBlendContainer(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                               nsDisplayList* aList,
+                               const ActiveScrolledRoot* aActiveScrolledRoot,
+                               bool aIsForBackground, nsIFrame* aAncestorFrame)
+      : nsDisplayBlendContainer(aBuilder, aFrame, aList, aActiveScrolledRoot,
+                                aIsForBackground),
+        mAncestorFrame(aAncestorFrame) {
+    if (aBuilder->IsRetainingDisplayList()) {
+      mAncestorFrame->AddDisplayItem(this);
+    }
+  }
+
+  nsDisplayTableBlendContainer(nsDisplayListBuilder* aBuilder,
+                               const nsDisplayTableBlendContainer& aOther)
+      : nsDisplayBlendContainer(aBuilder, aOther),
+        mAncestorFrame(aOther.mAncestorFrame) {}
+
+  ~nsDisplayTableBlendContainer() override {
+    if (mAncestorFrame) {
+      mAncestorFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  nsIFrame* mAncestorFrame;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+/**
+ * nsDisplayOwnLayer constructor flags. If we nest this class inside
+ * nsDisplayOwnLayer then we can't forward-declare it up at the top of this
+ * file and that makes it hard to use in all the places that we need to use it.
+ */
+enum class nsDisplayOwnLayerFlags {
+  None = 0,
+  GenerateSubdocInvalidations = 1 << 0,
+  GenerateScrollableLayer = 1 << 1,
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(nsDisplayOwnLayerFlags)
+
+/**
+ * A display item that has no purpose but to ensure its contents get
+ * their own layer.
+ */
+class nsDisplayOwnLayer : public nsDisplayWrapList {
+ public:
+  enum OwnLayerType {
+    OwnLayerForTransformWithRoundedClip,
+    OwnLayerForStackingContext,
+    OwnLayerForScrollbar,
+    OwnLayerForScrollThumb,
+    OwnLayerForSubdoc,
+    OwnLayerForBoxFrame
+  };
+
+  /**
+   * @param aFlags eGenerateSubdocInvalidations :
+   * Add UserData to the created ContainerLayer, so that invalidations
+   * for this layer are send to our nsPresContext.
+   * eGenerateScrollableLayer : only valid on nsDisplaySubDocument (and
+   * subclasses), indicates this layer is to be a scrollable layer, so call
+   * ComputeFrameMetrics, etc.
+   * @param aScrollTarget when eVerticalScrollbar or eHorizontalScrollbar
+   * is set in the flags, this parameter should be the ViewID of the
+   * scrollable content this scrollbar is for.
+   */
+  nsDisplayOwnLayer(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsDisplayList* aList,
+      const ActiveScrolledRoot* aActiveScrolledRoot,
+      nsDisplayOwnLayerFlags aFlags = nsDisplayOwnLayerFlags::None,
+      const layers::ScrollbarData& aScrollbarData = layers::ScrollbarData{},
+      bool aForceActive = true, bool aClearClipChain = false);
+
+  nsDisplayOwnLayer(nsDisplayListBuilder* aBuilder,
+                    const nsDisplayOwnLayer& aOther)
+      : nsDisplayWrapList(aBuilder, aOther),
+        mFlags(aOther.mFlags),
+        mScrollbarData(aOther.mScrollbarData),
+        mForceActive(aOther.mForceActive),
+        mWrAnimationId(aOther.mWrAnimationId) {
+    MOZ_COUNT_CTOR(nsDisplayOwnLayer);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayOwnLayer)
+
+  NS_DISPLAY_DECL_NAME("OwnLayer", TYPE_OWN_LAYER)
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    GetChildren()->Paint(aBuilder, aCtx,
+                         mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+
+  bool CanMerge(const nsDisplayItem* aItem) const override {
+    // Don't allow merging, each sublist must have its own layer
+    return false;
+  }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  void WriteDebugInfo(std::stringstream& aStream) override;
+  nsDisplayOwnLayerFlags GetFlags() { return mFlags; }
+  bool IsScrollThumbLayer() const;
+  bool IsScrollbarContainer() const;
+  bool IsRootScrollbarContainer() const;
+  bool IsScrollbarLayerForRoot() const;
+  bool IsZoomingLayer() const;
+  bool IsFixedPositionLayer() const;
+  bool IsStickyPositionLayer() const;
+  bool HasDynamicToolbar() const;
+  virtual bool ShouldGetFixedOrStickyAnimationId() { return false; }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ protected:
+  nsDisplayOwnLayerFlags mFlags;
+
+  /**
+   * If this nsDisplayOwnLayer represents a scroll thumb layer or a
+   * scrollbar container layer, mScrollbarData stores information
+   * about the scrollbar. Otherwise, mScrollbarData will be
+   * default-constructed (in particular with mDirection == Nothing())
+   * and can be ignored.
+   */
+  layers::ScrollbarData mScrollbarData;
+  bool mForceActive;
+  uint64_t mWrAnimationId;
+};
+
+/**
+ * A display item for subdocuments. This is more or less the same as
+ * nsDisplayOwnLayer, except that it always populates the FrameMetrics instance
+ * on the ContainerLayer it builds.
+ */
+class nsDisplaySubDocument : public nsDisplayOwnLayer {
+ public:
+  nsDisplaySubDocument(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       nsSubDocumentFrame* aSubDocFrame, nsDisplayList* aList,
+                       nsDisplayOwnLayerFlags aFlags);
+  ~nsDisplaySubDocument() override;
+
+  NS_DISPLAY_DECL_NAME("SubDocument", TYPE_SUBDOCUMENT)
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+
+  virtual nsSubDocumentFrame* SubDocumentFrame() { return mSubDocFrame; }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return mShouldFlatten;
+  }
+
+  void SetShouldFlattenAway(bool aShouldFlatten) {
+    mShouldFlatten = aShouldFlatten;
+  }
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+
+  nsIFrame* FrameForInvalidation() const override;
+  void RemoveFrame(nsIFrame* aFrame) override;
+
+ protected:
+  ViewID mScrollParentId;
+  bool mForceDispatchToContentRegion{};
+  bool mShouldFlatten;
+  nsSubDocumentFrame* mSubDocFrame;
+};
+
+/**
+ * A display item used to represent sticky position elements. The contents
+ * gets its own layer and creates a stacking context, and the layer will have
+ * position-related metadata set on it.
+ */
+class nsDisplayStickyPosition : public nsDisplayOwnLayer {
+ public:
+  nsDisplayStickyPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                          nsDisplayList* aList,
+                          const ActiveScrolledRoot* aActiveScrolledRoot,
+                          const ActiveScrolledRoot* aContainerASR,
+                          bool aClippedToDisplayPort);
+  nsDisplayStickyPosition(nsDisplayListBuilder* aBuilder,
+                          const nsDisplayStickyPosition& aOther)
+      : nsDisplayOwnLayer(aBuilder, aOther),
+        mContainerASR(aOther.mContainerASR),
+        mClippedToDisplayPort(aOther.mClippedToDisplayPort),
+        mShouldFlatten(false) {
+    MOZ_COUNT_CTOR(nsDisplayStickyPosition);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayStickyPosition)
+
+  const DisplayItemClip& GetClip() const override {
+    return DisplayItemClip::NoClip();
+  }
+  bool IsClippedToDisplayPort() const { return mClippedToDisplayPort; }
+
+  NS_DISPLAY_DECL_NAME("StickyPosition", TYPE_STICKY_POSITION)
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    GetChildren()->Paint(aBuilder, aCtx,
+                         mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+  bool ShouldGetFixedOrStickyAnimationId() override;
+
+  const ActiveScrolledRoot* GetContainerASR() const { return mContainerASR; }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+  bool CanMoveAsync() override { return true; }
+
+  void SetShouldFlatten(bool aShouldFlatten) {
+    mShouldFlatten = aShouldFlatten;
+  }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) final {
+    return mShouldFlatten;
+  }
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+
+  void CalculateLayerScrollRanges(StickyScrollContainer* aStickyScrollContainer,
+                                  float aAppUnitsPerDevPixel, float aScaleX,
+                                  float aScaleY,
+                                  LayerRectAbsolute& aStickyOuter,
+                                  LayerRectAbsolute& aStickyInner);
+
+  StickyScrollContainer* GetStickyScrollContainer();
+
+  // This stores the ASR that this sticky container item would have assuming it
+  // has no fixed descendants. This may be the same as the ASR returned by
+  // GetActiveScrolledRoot(), or it may be a descendant of that.
+  RefPtr<const ActiveScrolledRoot> mContainerASR;
+  // This flag tracks if this sticky item is just clipped to the enclosing
+  // scrollframe's displayport, or if there are additional clips in play. In
+  // the former case, we can skip setting the displayport clip as the scrolled-
+  // clip of the corresponding layer. This allows sticky items to remain
+  // unclipped when the enclosing scrollframe is scrolled past the displayport.
+  // i.e. when the rest of the scrollframe checkerboards, the sticky item will
+  // not. This makes sense to do because the sticky item has abnormal scrolling
+  // behavior and may still be visible even if the rest of the scrollframe is
+  // checkerboarded. Note that the sticky item will still be subject to the
+  // scrollport clip.
+  bool mClippedToDisplayPort;
+
+  // True if this item should be flattened away.
+  bool mShouldFlatten;
+};
+
+class nsDisplayFixedPosition : public nsDisplayOwnLayer {
+ public:
+  nsDisplayFixedPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                         nsDisplayList* aList,
+                         const ActiveScrolledRoot* aActiveScrolledRoot,
+                         const ActiveScrolledRoot* aScrollTargetASR);
+  nsDisplayFixedPosition(nsDisplayListBuilder* aBuilder,
+                         const nsDisplayFixedPosition& aOther)
+      : nsDisplayOwnLayer(aBuilder, aOther),
+        mScrollTargetASR(aOther.mScrollTargetASR),
+        mIsFixedBackground(aOther.mIsFixedBackground) {
+    MOZ_COUNT_CTOR(nsDisplayFixedPosition);
+  }
+
+  static nsDisplayFixedPosition* CreateForFixedBackground(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+      nsIFrame* aSecondaryFrame, nsDisplayBackgroundImage* aImage,
+      const uint16_t aIndex, const ActiveScrolledRoot* aScrollTargetASR);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayFixedPosition)
+
+  NS_DISPLAY_DECL_NAME("FixedPosition", TYPE_FIXED_POSITION)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    GetChildren()->Paint(aBuilder, aCtx,
+                         mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+
+  bool ShouldFixToViewport(nsDisplayListBuilder* aBuilder) const override {
+    return mIsFixedBackground;
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+  bool ShouldGetFixedOrStickyAnimationId() override;
+  void WriteDebugInfo(std::stringstream& aStream) override;
+
+ protected:
+  // For background-attachment:fixed
+  nsDisplayFixedPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                         nsDisplayList* aList,
+                         const ActiveScrolledRoot* aScrollTargetASR);
+  ViewID GetScrollTargetId() const;
+
+  RefPtr<const ActiveScrolledRoot> mScrollTargetASR;
+  bool mIsFixedBackground;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+class nsDisplayTableFixedPosition : public nsDisplayFixedPosition {
+ public:
+  NS_DISPLAY_DECL_NAME("TableFixedPosition", TYPE_TABLE_FIXED_POSITION)
+
+  nsIFrame* FrameForInvalidation() const override { return mAncestorFrame; }
+
+  void RemoveFrame(nsIFrame* aFrame) override {
+    if (aFrame == mAncestorFrame) {
+      mAncestorFrame = nullptr;
+      SetDeletedFrame();
+    }
+    nsDisplayFixedPosition::RemoveFrame(aFrame);
+  }
+
+ protected:
+  nsDisplayTableFixedPosition(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                              nsDisplayList* aList, nsIFrame* aAncestorFrame,
+                              const ActiveScrolledRoot* aScrollTargetASR);
+
+  nsDisplayTableFixedPosition(nsDisplayListBuilder* aBuilder,
+                              const nsDisplayTableFixedPosition& aOther)
+      : nsDisplayFixedPosition(aBuilder, aOther),
+        mAncestorFrame(aOther.mAncestorFrame) {}
+
+  ~nsDisplayTableFixedPosition() override {
+    if (mAncestorFrame) {
+      mAncestorFrame->RemoveDisplayItem(this);
+    }
+  }
+
+  nsIFrame* mAncestorFrame;
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+};
+
+/**
+ * This creates an empty scrollable layer. It has no child layers.
+ * It is used to record the existence of a scrollable frame in the layer
+ * tree.
+ */
+class nsDisplayScrollInfoLayer : public nsDisplayWrapList {
+ public:
+  nsDisplayScrollInfoLayer(nsDisplayListBuilder* aBuilder,
+                           nsIFrame* aScrolledFrame, nsIFrame* aScrollFrame,
+                           const gfx::CompositorHitTestInfo& aHitInfo,
+                           const nsRect& aHitArea);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayScrollInfoLayer)
+
+  NS_DISPLAY_DECL_NAME("ScrollInfoLayer", TYPE_SCROLL_INFO_LAYER)
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override {
+    *aSnap = false;
+    return nsRegion();
+  }
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    return;
+  }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  void WriteDebugInfo(std::stringstream& aStream) override;
+  UniquePtr<layers::ScrollMetadata> ComputeScrollMetadata(
+      nsDisplayListBuilder* aBuilder,
+      layers::WebRenderLayerManager* aLayerManager);
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+ protected:
+  nsIFrame* mScrollFrame;
+  nsIFrame* mScrolledFrame;
+  ViewID mScrollParentId;
+  gfx::CompositorHitTestInfo mHitInfo;
+  nsRect mHitArea;
+};
+
+/**
+ * nsDisplayZoom is used for subdocuments that have a different full zoom than
+ * their parent documents. This item creates a container layer.
+ */
+class nsDisplayZoom : public nsDisplaySubDocument {
+ public:
+  /**
+   * @param aFrame is the root frame of the subdocument.
+   * @param aList contains the display items for the subdocument.
+   * @param aAPD is the app units per dev pixel ratio of the subdocument.
+   * @param aParentAPD is the app units per dev pixel ratio of the parent
+   * document.
+   * @param aFlags eGenerateSubdocInvalidations :
+   * Add UserData to the created ContainerLayer, so that invalidations
+   * for this layer are send to our nsPresContext.
+   */
+  nsDisplayZoom(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                nsSubDocumentFrame* aSubDocFrame, nsDisplayList* aList,
+                int32_t aAPD, int32_t aParentAPD,
+                nsDisplayOwnLayerFlags aFlags = nsDisplayOwnLayerFlags::None);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayZoom)
+
+  NS_DISPLAY_DECL_NAME("Zoom", TYPE_ZOOM)
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  // Get the app units per dev pixel ratio of the child document.
+  int32_t GetChildAppUnitsPerDevPixel() { return mAPD; }
+  // Get the app units per dev pixel ratio of the parent document.
+  int32_t GetParentAppUnitsPerDevPixel() { return mParentAPD; }
+
+ private:
+  int32_t mAPD, mParentAPD;
+};
+
+/**
+ * nsDisplayAsyncZoom is used for APZ zooming. It wraps the contents of the
+ * root content document's scroll frame, including fixed position content. It
+ * does not contain the scroll frame's scrollbars. It is clipped to the scroll
+ * frame's scroll port clip. It is not scrolled; only its non-fixed contents
+ * are scrolled. This item creates a container layer.
+ */
+class nsDisplayAsyncZoom : public nsDisplayOwnLayer {
+ public:
+  nsDisplayAsyncZoom(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     nsDisplayList* aList,
+                     const ActiveScrolledRoot* aActiveScrolledRoot,
+                     layers::FrameMetrics::ViewID aViewID);
+  nsDisplayAsyncZoom(nsDisplayListBuilder* aBuilder,
+                     const nsDisplayAsyncZoom& aOther)
+      : nsDisplayOwnLayer(aBuilder, aOther), mViewID(aOther.mViewID) {
+    MOZ_COUNT_CTOR(nsDisplayAsyncZoom);
+  }
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+  virtual ~nsDisplayAsyncZoom();
+#endif
+
+  NS_DISPLAY_DECL_NAME("AsyncZoom", TYPE_ASYNC_ZOOM)
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+
+ protected:
+  layers::FrameMetrics::ViewID mViewID;
+};
+
+/**
+ * A base class for different effects types.
+ */
+class nsDisplayEffectsBase : public nsDisplayWrapList {
+ public:
+  nsDisplayEffectsBase(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       nsDisplayList* aList,
+                       const ActiveScrolledRoot* aActiveScrolledRoot,
+                       bool aClearClipChain = false);
+  nsDisplayEffectsBase(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       nsDisplayList* aList);
+
+  nsDisplayEffectsBase(nsDisplayListBuilder* aBuilder,
+                       const nsDisplayEffectsBase& aOther)
+      : nsDisplayWrapList(aBuilder, aOther),
+        mEffectsBounds(aOther.mEffectsBounds) {
+    MOZ_COUNT_CTOR(nsDisplayEffectsBase);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayEffectsBase)
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return false;
+  }
+
+  gfxRect BBoxInUserSpace() const;
+  gfxPoint UserSpaceOffset() const;
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+
+ protected:
+  bool ValidateSVGFrame();
+
+  // relative to mFrame
+  nsRect mEffectsBounds;
+};
+
+/**
+ * A display item to paint a stacking context with 'mask' and 'clip-path'
+ * effects set by the stacking context root frame's style.  The 'mask' and
+ * 'clip-path' properties may both contain multiple masks and clip paths,
+ * respectively.
+ *
+ * Note that 'mask' and 'clip-path' may just contain CSS simple-images and CSS
+ * basic shapes, respectively.  That is, they don't necessarily reference
+ * resources such as SVG 'mask' and 'clipPath' elements.
+ */
+class nsDisplayMasksAndClipPaths : public nsDisplayEffectsBase {
+ public:
+  nsDisplayMasksAndClipPaths(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                             nsDisplayList* aList,
+                             const ActiveScrolledRoot* aActiveScrolledRoot,
+                             bool aWrapsBackdropFilter);
+  nsDisplayMasksAndClipPaths(nsDisplayListBuilder* aBuilder,
+                             const nsDisplayMasksAndClipPaths& aOther)
+      : nsDisplayEffectsBase(aBuilder, aOther),
+        mDestRects(aOther.mDestRects.Clone()),
+        mWrapsBackdropFilter(aOther.mWrapsBackdropFilter) {
+    MOZ_COUNT_CTOR(nsDisplayMasksAndClipPaths);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayMasksAndClipPaths)
+
+  NS_DISPLAY_DECL_NAME("Mask", TYPE_MASK)
+
+  bool CanMerge(const nsDisplayItem* aItem) const override;
+
+  void Merge(const nsDisplayItem* aItem) override {
+    nsDisplayWrapList::Merge(aItem);
+
+    const nsDisplayMasksAndClipPaths* other =
+        static_cast<const nsDisplayMasksAndClipPaths*>(aItem);
+    mEffectsBounds.UnionRect(
+        mEffectsBounds,
+        other->mEffectsBounds + other->mFrame->GetOffsetTo(mFrame));
+  }
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayMasksAndClipPathsGeometry(this, aBuilder);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override;
+#ifdef MOZ_DUMP_PAINTING
+  void PrintEffects(nsACString& aTo);
+#endif
+
+  bool IsValidMask();
+
+  void PaintWithContentsPaintCallback(
+      nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+      const std::function<void()>& aPaintChildren);
+
+  /*
+   * Paint mask onto aMaskContext in mFrame's coordinate space and
+   * return whether the mask layer was painted successfully.
+   */
+  bool PaintMask(nsDisplayListBuilder* aBuilder, gfxContext* aMaskContext,
+                 bool aHandleOpacity, bool* aMaskPainted = nullptr);
+
+  const nsTArray<nsRect>& GetDestRects() { return mDestRects; }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  Maybe<nsRect> GetClipWithRespectToASR(
+      nsDisplayListBuilder* aBuilder,
+      const ActiveScrolledRoot* aASR) const override;
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+
+  nsTArray<nsRect> mDestRects;
+  bool mWrapsBackdropFilter;
+};
+
+class nsDisplayBackdropFilters : public nsDisplayWrapList {
+ public:
+  nsDisplayBackdropFilters(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                           nsDisplayList* aList, const nsRect& aBackdropRect,
+                           nsIFrame* aStyleFrame)
+      : nsDisplayWrapList(aBuilder, aFrame, aList),
+        mStyle(aFrame == aStyleFrame ? nullptr : aStyleFrame->Style()),
+        mBackdropRect(aBackdropRect) {
+    MOZ_COUNT_CTOR(nsDisplayBackdropFilters);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayBackdropFilters)
+
+  NS_DISPLAY_DECL_NAME("BackdropFilter", TYPE_BACKDROP_FILTER)
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
+    return !aBuilder->IsPaintingForWebRender();
+  }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+
+ private:
+  RefPtr<ComputedStyle> mStyle;
+  nsRect mBackdropRect;
+};
+
+/**
+ * A display item to paint a stacking context with filter effects set by the
+ * stacking context root frame's style.
+ *
+ * Note that the filters may just be simple CSS filter functions.  That is,
+ * they won't necessarily be references to SVG 'filter' elements.
+ */
+class nsDisplayFilters : public nsDisplayEffectsBase {
+ public:
+  nsDisplayFilters(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                   nsDisplayList* aList, nsIFrame* aStyleFrame,
+                   bool aWrapsBackdropFilter);
+
+  nsDisplayFilters(nsDisplayListBuilder* aBuilder,
+                   const nsDisplayFilters& aOther)
+      : nsDisplayEffectsBase(aBuilder, aOther),
+        mStyle(aOther.mStyle),
+        mEffectsBounds(aOther.mEffectsBounds),
+        mWrapsBackdropFilter(aOther.mWrapsBackdropFilter) {
+    MOZ_COUNT_CTOR(nsDisplayFilters);
+  }
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayFilters)
+
+  NS_DISPLAY_DECL_NAME("Filter", TYPE_FILTER)
+
+  bool CanMerge(const nsDisplayItem* aItem) const override {
+    // Items for the same content element should be merged into a single
+    // compositing group.
+    return HasDifferentFrame(aItem) && HasSameTypeAndClip(aItem) &&
+           HasSameContent(aItem);
+  }
+
+  void Merge(const nsDisplayItem* aItem) override {
+    nsDisplayWrapList::Merge(aItem);
+
+    const nsDisplayFilters* other = static_cast<const nsDisplayFilters*>(aItem);
+    mEffectsBounds.UnionRect(
+        mEffectsBounds,
+        other->mEffectsBounds + other->mFrame->GetOffsetTo(mFrame));
+  }
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override {
+    *aSnap = false;
+    return mEffectsBounds + ToReferenceFrame();
+  }
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplaySVGEffectGeometry(this, aBuilder);
+  }
+
+#ifdef MOZ_DUMP_PAINTING
+  void PrintEffects(nsACString& aTo);
+#endif
+
+  void PaintWithContentsPaintCallback(
+      nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+      const std::function<void(gfxContext* aContext)>& aPaintChildren);
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  bool CanCreateWebRenderCommands() const;
+
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const override {
+    return CanCreateWebRenderCommands();
+  }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ private:
+  NS_DISPLAY_ALLOW_CLONING()
+
+  RefPtr<ComputedStyle> mStyle;
+  // relative to mFrame
+  nsRect mEffectsBounds;
+  nsRect mVisibleRect;
+  bool mWrapsBackdropFilter;
+};
+
+/* A display item that applies a transformation to all of its descendant
+ * elements.  This wrapper should only be used if there is a transform applied
+ * to the root element.
+ *
+ * The reason that a "bounds" rect is involved in transform calculations is
+ * because CSS-transforms allow percentage values for the x and y components
+ * of <translation-value>s, where percentages are percentages of the element's
+ * border box.
+ *
+ * INVARIANT: The wrapped frame is transformed or we supplied a transform getter
+ * function.
+ * INVARIANT: The wrapped frame is non-null.
+ */
+class nsDisplayTransform : public nsPaintedDisplayItem {
+  using Matrix4x4 = gfx::Matrix4x4;
+  using Matrix4x4Flagged = gfx::Matrix4x4Flagged;
+  using TransformReferenceBox = nsStyleTransformMatrix::TransformReferenceBox;
+
+ public:
+  enum class PrerenderDecision : uint8_t { No, Full, Partial };
+
+  enum {
+    WithTransformGetter,
+  };
+
+  /* Constructor accepts a display list, empties it, and wraps it up.  It also
+   * ferries the underlying frame to the nsDisplayItem constructor.
+   */
+  nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     nsDisplayList* aList, const nsRect& aChildrenBuildingRect);
+
+  nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     nsDisplayList* aList, const nsRect& aChildrenBuildingRect,
+                     PrerenderDecision aPrerenderDecision);
+
+  nsDisplayTransform(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                     nsDisplayList* aList, const nsRect& aChildrenBuildingRect,
+                     decltype(WithTransformGetter));
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayTransform)
+
+  NS_DISPLAY_DECL_NAME("nsDisplayTransform", TYPE_TRANSFORM)
+
+  void UpdateBounds(nsDisplayListBuilder* aBuilder) override;
+
+  /**
+   * This function updates bounds for items with a frame establishing
+   * 3D rendering context.
+   */
+  void UpdateBoundsFor3D(nsDisplayListBuilder* aBuilder);
+
+  void DoUpdateBoundsPreserves3D(nsDisplayListBuilder* aBuilder) override;
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    GetChildren()->DeleteAll(aBuilder);
+    nsPaintedDisplayItem::Destroy(aBuilder);
+  }
+
+  nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const override;
+
+  RetainedDisplayList* GetChildren() const override { return &mChildren; }
+
+  nsRect GetUntransformedBounds(nsDisplayListBuilder* aBuilder,
+                                bool* aSnap) const override {
+    *aSnap = false;
+    return mChildBounds;
+  }
+
+  const nsRect& GetUntransformedPaintRect() const override {
+    return mChildrenBuildingRect;
+  }
+
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override;
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override;
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override;
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx,
+             const Maybe<gfx::Polygon>& aPolygon);
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+  bool UpdateScrollData(layers::WebRenderScrollData* aData,
+                        layers::WebRenderLayerScrollData* aLayerData) override;
+
+  nsDisplayItemGeometry* AllocateGeometry(
+      nsDisplayListBuilder* aBuilder) override {
+    return new nsDisplayTransformGeometry(
+        this, aBuilder, GetTransformForRendering(),
+        mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {
+    const nsDisplayTransformGeometry* geometry =
+        static_cast<const nsDisplayTransformGeometry*>(aGeometry);
+
+    // This code is only called for flattened, inactive transform items.
+    // Only check if the transform has changed. The bounds invalidation should
+    // be handled by the children themselves.
+    if (!geometry->mTransform.FuzzyEqual(GetTransformForRendering())) {
+      bool snap;
+      aInvalidRegion->Or(GetBounds(aBuilder, &snap), geometry->mBounds);
+    }
+  }
+
+  const nsIFrame* ReferenceFrameForChildren() const override {
+    // If we were created using a transform-getter, then we don't
+    // belong to a transformed frame, and aren't a reference frame
+    // for our children.
+    if (!mHasTransformGetter) {
+      return mFrame;
+    }
+    return nsPaintedDisplayItem::ReferenceFrameForChildren();
+  }
+
+  const nsRect& GetBuildingRectForChildren() const override {
+    return mChildrenBuildingRect;
+  }
+
+  enum { INDEX_MAX = UINT32_MAX >> TYPE_BITS };
+
+  /**
+   * We include the perspective matrix from our containing block for the
+   * purposes of visibility calculations, but we exclude it from the transform
+   * we set on the layer (for rendering), since there will be an
+   * nsDisplayPerspective created for that.
+   */
+  const Matrix4x4Flagged& GetTransform() const;
+  const Matrix4x4Flagged& GetInverseTransform() const;
+
+  bool ShouldSkipTransform(nsDisplayListBuilder* aBuilder) const;
+  Matrix4x4 GetTransformForRendering(
+      LayoutDevicePoint* aOutOrigin = nullptr) const;
+
+  /**
+   * Return the transform that is aggregation of all transform on the
+   * preserves3d chain.
+   */
+  const Matrix4x4& GetAccumulatedPreserved3DTransform(
+      nsDisplayListBuilder* aBuilder);
+
+  float GetHitDepthAtPoint(nsDisplayListBuilder* aBuilder,
+                           const nsPoint& aPoint);
+  /**
+   * TransformRect takes in as parameters a rectangle (in aFrame's coordinate
+   * space) and returns the smallest rectangle (in aFrame's coordinate space)
+   * containing the transformed image of that rectangle.  That is, it takes
+   * the four corners of the rectangle, transforms them according to the
+   * matrix associated with the specified frame, then returns the smallest
+   * rectangle containing the four transformed points.
+   *
+   * @param untransformedBounds The rectangle (in app units) to transform.
+   * @param aFrame The frame whose transformation should be applied.  This
+   *        function raises an assertion if aFrame is null or doesn't have a
+   *        transform applied to it.
+   * @param aRefBox the reference box to use, which would usually be just
+   *        TransformReferemceBox(aFrame), but callers may override it if
+   *        needed.
+   */
+  static nsRect TransformRect(const nsRect& aUntransformedBounds,
+                              const nsIFrame* aFrame,
+                              TransformReferenceBox& aRefBox);
+
+  /* UntransformRect is like TransformRect, except that it inverts the
+   * transform.
+   */
+  static bool UntransformRect(const nsRect& aTransformedBounds,
+                              const nsRect& aChildBounds,
+                              const nsIFrame* aFrame, nsRect* aOutRect);
+  static bool UntransformRect(const nsRect& aTransformedBounds,
+                              const nsRect& aChildBounds,
+                              const Matrix4x4& aMatrix, float aAppUnitsPerPixel,
+                              nsRect* aOutRect);
+
+  bool UntransformRect(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+                       nsRect* aOutRect) const;
+
+  bool UntransformBuildingRect(nsDisplayListBuilder* aBuilder,
+                               nsRect* aOutRect) const {
+    return UntransformRect(aBuilder, GetBuildingRect(), aOutRect);
+  }
+
+  static gfx::Point3D GetDeltaToTransformOrigin(const nsIFrame* aFrame,
+                                                TransformReferenceBox&,
+                                                float aAppUnitsPerPixel);
+
+  /*
+   * Returns true if aFrame has perspective applied from its containing
+   * block.
+   * Returns the matrix to append to apply the persective (taking
+   * perspective-origin into account), relative to aFrames coordinate
+   * space).
+   * aOutMatrix is assumed to be the identity matrix, and isn't explicitly
+   * cleared.
+   */
+  static bool ComputePerspectiveMatrix(const nsIFrame* aFrame,
+                                       float aAppUnitsPerPixel,
+                                       Matrix4x4& aOutMatrix);
+
+  struct MOZ_STACK_CLASS FrameTransformProperties {
+    FrameTransformProperties(const nsIFrame* aFrame,
+                             TransformReferenceBox& aRefBox,
+                             float aAppUnitsPerPixel);
+    FrameTransformProperties(const StyleTranslate& aTranslate,
+                             const StyleRotate& aRotate,
+                             const StyleScale& aScale,
+                             const StyleTransform& aTransform,
+                             const Maybe<ResolvedMotionPathData>& aMotion,
+                             const gfx::Point3D& aToTransformOrigin)
+        : mFrame(nullptr),
+          mTranslate(aTranslate),
+          mRotate(aRotate),
+          mScale(aScale),
+          mTransform(aTransform),
+          mMotion(aMotion),
+          mToTransformOrigin(aToTransformOrigin) {}
+
+    bool HasTransform() const {
+      return !mTranslate.IsNone() || !mRotate.IsNone() || !mScale.IsNone() ||
+             !mTransform.IsNone() || mMotion.isSome();
+    }
+
+    const nsIFrame* mFrame;
+    const StyleTranslate& mTranslate;
+    const StyleRotate& mRotate;
+    const StyleScale& mScale;
+    const StyleTransform& mTransform;
+    const Maybe<ResolvedMotionPathData> mMotion;
+    const gfx::Point3D mToTransformOrigin;
+  };
+
+  /**
+   * Given a frame with the transform property or an SVG transform,
+   * returns the transformation matrix for that frame.
+   *
+   * @param aFrame The frame to get the matrix from.
+   * @param aOrigin Relative to which point this transform should be applied.
+   * @param aAppUnitsPerPixel The number of app units per graphics unit.
+   * @param aBoundsOverride [optional] If this is nullptr (the default), the
+   *        computation will use the value of TransformReferenceBox(aFrame).
+   *        Otherwise, it will use the value of aBoundsOverride.  This is
+   *        mostly for internal use and in most cases you will not need to
+   *        specify a value.
+   * @param aFlags OFFSET_BY_ORIGIN The resulting matrix will be translated
+   *        by aOrigin. This translation is applied *before* the CSS transform.
+   * @param aFlags INCLUDE_PRESERVE3D_ANCESTORS The computed transform will
+   *        include the transform of any ancestors participating in the same
+   *        3d rendering context.
+   * @param aFlags INCLUDE_PERSPECTIVE The resulting matrix will include the
+   *        perspective transform from the containing block if applicable.
+   */
+  enum {
+    OFFSET_BY_ORIGIN = 1 << 0,
+    INCLUDE_PRESERVE3D_ANCESTORS = 1 << 1,
+    INCLUDE_PERSPECTIVE = 1 << 2,
+  };
+  static constexpr uint32_t kTransformRectFlags =
+      INCLUDE_PERSPECTIVE | OFFSET_BY_ORIGIN | INCLUDE_PRESERVE3D_ANCESTORS;
+  static Matrix4x4 GetResultingTransformMatrix(const nsIFrame* aFrame,
+                                               const nsPoint& aOrigin,
+                                               float aAppUnitsPerPixel,
+                                               uint32_t aFlags);
+  static Matrix4x4 GetResultingTransformMatrix(
+      const FrameTransformProperties& aProperties, TransformReferenceBox&,
+      float aAppUnitsPerPixel);
+
+  struct PrerenderInfo {
+    bool CanUseAsyncAnimations() const {
+      return mDecision != PrerenderDecision::No && mHasAnimations;
+    }
+    PrerenderDecision mDecision = PrerenderDecision::No;
+    bool mHasAnimations = true;
+  };
+  /**
+   * Decide whether we should prerender some or all of the contents of the
+   * transformed frame even when it's not completely visible (yet).
+   * Return PrerenderDecision::Full if the entire contents should be
+   * prerendered, PrerenderDecision::Partial if some but not all of the
+   * contents should be prerendered, or PrerenderDecision::No if only the
+   * visible area should be rendered.
+   * |mNoAffectDecisionInPreserve3D| is set if the prerender decision should not
+   * affect the decision on other frames in the preserve 3d tree.
+   * |aDirtyRect| is updated to the area that should be prerendered.
+   */
+  static PrerenderInfo ShouldPrerenderTransformedContent(
+      nsDisplayListBuilder* aBuilder, nsIFrame* aFrame, nsRect* aDirtyRect);
+
+  bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) override;
+
+  bool MayBeAnimated(nsDisplayListBuilder* aBuilder) const;
+
+  void WriteDebugInfo(std::stringstream& aStream) override;
+
+  bool CanMoveAsync() override {
+    return EffectCompositor::HasAnimationsForCompositor(
+        mFrame, DisplayItemType::TYPE_TRANSFORM);
+  }
+
+  /**
+   * This item is an additional item as the boundary between parent
+   * and child 3D rendering context.
+   * \see nsIFrame::BuildDisplayListForStackingContext().
+   */
+  bool IsTransformSeparator() const { return mIsTransformSeparator; }
+  /**
+   * This item is the boundary between parent and child 3D rendering
+   * context.
+   */
+  bool IsLeafOf3DContext() const {
+    return (IsTransformSeparator() ||
+            (!mFrame->Extend3DContext() && Combines3DTransformWithAncestors()));
+  }
+  /**
+   * The backing frame of this item participates a 3D rendering
+   * context.
+   */
+  bool IsParticipating3DContext() const {
+    return mFrame->Extend3DContext() || Combines3DTransformWithAncestors();
+  }
+
+  bool IsPartialPrerender() const {
+    return mPrerenderDecision == PrerenderDecision::Partial;
+  }
+
+  /**
+   * Mark this item as created together with `nsDisplayPerspective`.
+   * \see nsIFrame::BuildDisplayListForStackingContext().
+   */
+  void MarkWithAssociatedPerspective() { mHasAssociatedPerspective = true; }
+
+  void AddSizeOfExcludingThis(nsWindowSizes&) const override;
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ private:
+  void ComputeBounds(nsDisplayListBuilder* aBuilder);
+  nsRect TransformUntransformedBounds(nsDisplayListBuilder* aBuilder,
+                                      const Matrix4x4Flagged& aMatrix) const;
+  void UpdateUntransformedBounds(nsDisplayListBuilder* aBuilder);
+
+  void SetReferenceFrameToAncestor(nsDisplayListBuilder* aBuilder);
+  void Init(nsDisplayListBuilder* aBuilder, nsDisplayList* aChildren);
+
+  static Matrix4x4 GetResultingTransformMatrixInternal(
+      const FrameTransformProperties& aProperties,
+      TransformReferenceBox& aRefBox, const nsPoint& aOrigin,
+      float aAppUnitsPerPixel, uint32_t aFlags);
+
+  void Collect3DTransformLeaves(nsDisplayListBuilder* aBuilder,
+                                nsTArray<nsDisplayTransform*>& aLeaves);
+  using TransformPolygon = layers::BSPPolygon<nsDisplayTransform>;
+  void CollectSorted3DTransformLeaves(nsDisplayListBuilder* aBuilder,
+                                      nsTArray<TransformPolygon>& aLeaves);
+
+  mutable RetainedDisplayList mChildren;
+  mutable Maybe<Matrix4x4Flagged> mTransform;
+  mutable Maybe<Matrix4x4Flagged> mInverseTransform;
+  // Accumulated transform of ancestors on the preserves-3d chain.
+  UniquePtr<Matrix4x4> mTransformPreserves3D;
+  nsRect mChildrenBuildingRect;
+
+  // The untransformed bounds of |mChildren|.
+  nsRect mChildBounds;
+  // The transformed bounds of this display item.
+  nsRect mBounds;
+  PrerenderDecision mPrerenderDecision : 8;
+  // This item is a separator between 3D rendering contexts, and
+  // mTransform have been presetted by the constructor.
+  // This also forces us not to extend the 3D context.  Since we don't create a
+  // transform item, a container layer, for every frame in a preserves3d
+  // context, the transform items of a child preserves3d context may extend the
+  // parent context unintendedly if the root of the child preserves3d context
+  // doesn't create a transform item.
+  bool mIsTransformSeparator : 1;
+  // True if we have a transform getter.
+  bool mHasTransformGetter : 1;
+  // True if this item is created together with `nsDisplayPerspective`
+  // from the same CSS stacking context.
+  bool mHasAssociatedPerspective : 1;
+};
+
+/* A display item that applies a perspective transformation to a single
+ * nsDisplayTransform child item. We keep this as a separate item since the
+ * perspective-origin is relative to an ancestor of the transformed frame, and
+ * APZ can scroll the child separately.
+ */
+class nsDisplayPerspective : public nsPaintedDisplayItem {
+ public:
+  nsDisplayPerspective(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       nsDisplayList* aList);
+  ~nsDisplayPerspective() override = default;
+
+  NS_DISPLAY_DECL_NAME("nsDisplayPerspective", TYPE_PERSPECTIVE)
+
+  void Destroy(nsDisplayListBuilder* aBuilder) override {
+    mList.DeleteAll(aBuilder);
+    nsPaintedDisplayItem::Destroy(aBuilder);
+  }
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) override {
+    return GetChildren()->HitTest(aBuilder, aRect, aState, aOutFrames);
+  }
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const override {
+    *aSnap = false;
+    return GetChildren()->GetClippedBoundsWithRespectToASR(aBuilder,
+                                                           mActiveScrolledRoot);
+  }
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const override {}
+
+  nsRegion GetOpaqueRegion(nsDisplayListBuilder* aBuilder,
+                           bool* aSnap) const override;
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+  RetainedDisplayList* GetSameCoordinateSystemChildren() const override {
+    return &mList;
+  }
+
+  RetainedDisplayList* GetChildren() const override { return &mList; }
+
+  nsRect GetComponentAlphaBounds(
+      nsDisplayListBuilder* aBuilder) const override {
+    return GetChildren()->GetComponentAlphaBounds(aBuilder);
+  }
+
+  void DoUpdateBoundsPreserves3D(nsDisplayListBuilder* aBuilder) override {
+    if (GetChildren()->GetTop()) {
+      static_cast<nsDisplayTransform*>(GetChildren()->GetTop())
+          ->DoUpdateBoundsPreserves3D(aBuilder);
+    }
+  }
+
+  bool CreatesStackingContextHelper() override { return true; }
+
+ private:
+  mutable RetainedDisplayList mList;
+};
+
+class nsDisplayTextGeometry;
+
+/**
+ * This class adds basic support for limiting the rendering (in the inline axis
+ * of the writing mode) to the part inside the specified edges.
+ * The two members, mVisIStartEdge and mVisIEndEdge, are relative to the edges
+ * of the frame's scrollable overflow rectangle and are the amount to suppress
+ * on each side.
+ *
+ * Setting none, both or only one edge is allowed.
+ * The values must be non-negative.
+ * The default value for both edges is zero, which means everything is painted.
+ */
+class nsDisplayText final : public nsPaintedDisplayItem {
+ public:
+  nsDisplayText(nsDisplayListBuilder* aBuilder, nsTextFrame* aFrame);
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayText)
+
+  NS_DISPLAY_DECL_NAME("Text", TYPE_TEXT)
+
+  nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap) const final {
+    *aSnap = false;
+    return mBounds;
+  }
+
+  void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
+               HitTestState* aState, nsTArray<nsIFrame*>* aOutFrames) final {
+    if (nsRect(ToReferenceFrame(), mFrame->GetSize()).Intersects(aRect)) {
+      aOutFrames->AppendElement(mFrame);
+    }
+  }
+
+  bool CreateWebRenderCommands(wr::DisplayListBuilder& aBuilder,
+                               wr::IpcResourceUpdateQueue& aResources,
+                               const StackingContextHelper& aSc,
+                               layers::RenderRootStateManager* aManager,
+                               nsDisplayListBuilder* aDisplayListBuilder) final;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) final;
+
+  nsRect GetComponentAlphaBounds(nsDisplayListBuilder* aBuilder) const final {
+    if (gfxPlatform::GetPlatform()->RespectsFontStyleSmoothing()) {
+      // On OS X, web authors can turn off subpixel text rendering using the
+      // CSS property -moz-osx-font-smoothing. If they do that, we don't need
+      // to use component alpha layers for the affected text.
+      if (mFrame->StyleFont()->mFont.smoothing == NS_FONT_SMOOTHING_GRAYSCALE) {
+        return nsRect();
+      }
+    }
+    bool snap;
+    return GetBounds(aBuilder, &snap);
+  }
+
+  nsDisplayItemGeometry* AllocateGeometry(nsDisplayListBuilder* aBuilder) final;
+
+  void ComputeInvalidationRegion(nsDisplayListBuilder* aBuilder,
+                                 const nsDisplayItemGeometry* aGeometry,
+                                 nsRegion* aInvalidRegion) const final;
+
+  void RenderToContext(gfxContext* aCtx, nsDisplayListBuilder* aBuilder,
+                       const nsRect& aVisibleRect, float aOpacity = 1.0f,
+                       bool aIsRecording = false);
+
+  bool CanApplyOpacity(WebRenderLayerManager* aManager,
+                       nsDisplayListBuilder* aBuilder) const final;
+
+  void WriteDebugInfo(std::stringstream& aStream) final;
+
+  static nsDisplayText* CheckCast(nsDisplayItem* aItem) {
+    return (aItem->GetType() == DisplayItemType::TYPE_TEXT)
+               ? static_cast<nsDisplayText*>(aItem)
+               : nullptr;
+  }
+
+  nscoord& VisIStartEdge() { return mVisIStartEdge; }
+  nscoord& VisIEndEdge() { return mVisIEndEdge; }
+
+ private:
+  nsRect mBounds;
+  nsRect mVisibleRect;
+
+  // Lengths measured from the visual inline start and end sides
+  // (i.e. left and right respectively in horizontal writing modes,
+  // regardless of bidi directionality; top and bottom in vertical modes).
+  nscoord mVisIStartEdge;
+  nscoord mVisIEndEdge;
+};
+
+/**
+ * A display item that for webrender to handle SVG
+ */
+class nsDisplaySVGWrapper : public nsDisplayWrapList {
+ public:
+  nsDisplaySVGWrapper(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                      nsDisplayList* aList);
+
+  MOZ_COUNTED_DTOR_OVERRIDE(nsDisplaySVGWrapper)
+
+  NS_DISPLAY_DECL_NAME("SVGWrapper", TYPE_SVG_WRAPPER)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    GetChildren()->Paint(aBuilder, aCtx,
+                         mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+  bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override;
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+};
+
+/**
+ * A display item for webrender to handle SVG foreign object
+ */
+class nsDisplayForeignObject : public nsDisplayWrapList {
+ public:
+  nsDisplayForeignObject(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                         nsDisplayList* aList);
+#ifdef NS_BUILD_REFCNT_LOGGING
+  virtual ~nsDisplayForeignObject();
+#endif
+
+  NS_DISPLAY_DECL_NAME("ForeignObject", TYPE_FOREIGN_OBJECT)
+
+  virtual bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override;
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override {
+    GetChildren()->Paint(aBuilder, aCtx,
+                         mFrame->PresContext()->AppUnitsPerDevPixel());
+  }
+
+  bool CreateWebRenderCommands(
+      wr::DisplayListBuilder& aBuilder, wr::IpcResourceUpdateQueue& aResources,
+      const StackingContextHelper& aSc,
+      layers::RenderRootStateManager* aManager,
+      nsDisplayListBuilder* aDisplayListBuilder) override;
+};
+
+/**
+ * A display item to represent a hyperlink.
+ */
+class nsDisplayLink : public nsPaintedDisplayItem {
+ public:
+  nsDisplayLink(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                const char* aLinkSpec, const nsRect& aRect)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mLinkSpec(aLinkSpec),
+        mRect(aRect) {}
+
+  NS_DISPLAY_DECL_NAME("Link", TYPE_LINK)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+ private:
+  nsCString mLinkSpec;
+  nsRect mRect;
+};
+
+/**
+ * A display item to represent a destination within the document.
+ */
+class nsDisplayDestination : public nsPaintedDisplayItem {
+ public:
+  nsDisplayDestination(nsDisplayListBuilder* aBuilder, nsIFrame* aFrame,
+                       const char* aDestinationName, const nsPoint& aPosition)
+      : nsPaintedDisplayItem(aBuilder, aFrame),
+        mDestinationName(aDestinationName),
+        mPosition(aPosition) {}
+
+  NS_DISPLAY_DECL_NAME("Destination", TYPE_DESTINATION)
+
+  void Paint(nsDisplayListBuilder* aBuilder, gfxContext* aCtx) override;
+
+ private:
+  nsCString mDestinationName;
+  nsPoint mPosition;
+};
+
+class MOZ_STACK_CLASS FlattenedDisplayListIterator {
+ public:
+  FlattenedDisplayListIterator(nsDisplayListBuilder* aBuilder,
+                               nsDisplayList* aList)
+      : mBuilder(aBuilder), mStart(aList->begin()), mEnd(aList->end()) {
+    ResolveFlattening();
+  }
+
+  bool HasNext() const { return !AtEndOfCurrentList(); }
+
+  nsDisplayItem* GetNextItem() {
+    MOZ_ASSERT(HasNext());
+
+    nsDisplayItem* current = NextItem();
+    Advance();
+
+    if (!AtEndOfCurrentList() && current->CanMerge(NextItem())) {
+      // Since we can merge at least two display items, create an array and
+      // collect mergeable display items there.
+      AutoTArray<nsDisplayItem*, 2> willMerge{current};
+
+      auto it = mStart;
+      while (it != mEnd) {
+        nsDisplayItem* next = *it;
+        if (current->CanMerge(next)) {
+          willMerge.AppendElement(next);
+          ++it;
+        } else {
+          break;
+        }
+      }
+      mStart = it;
+
+      current = mBuilder->MergeItems(willMerge);
+    }
+
+    ResolveFlattening();
+    return current;
+  }
+
+ protected:
+  void Advance() { ++mStart; }
+
+  bool AtEndOfNestedList() const {
+    return AtEndOfCurrentList() && mStack.Length() > 0;
+  }
+
+  bool AtEndOfCurrentList() const { return mStart == mEnd; }
+
+  nsDisplayItem* NextItem() {
+    MOZ_ASSERT(HasNext());
+    return *mStart;
+  }
+
+  bool ShouldFlattenNextItem() {
+    return HasNext() && NextItem()->ShouldFlattenAway(mBuilder);
+  }
+
+  void ResolveFlattening() {
+    // Handle the case where we reach the end of a nested list, or the current
+    // item should start a new nested list. Repeat this until we find an actual
+    // item, or the very end of the outer list.
+    while (AtEndOfNestedList() || ShouldFlattenNextItem()) {
+      if (AtEndOfNestedList()) {
+        // We reached the end of the list, pop the next list from the stack.
+        std::tie(mStart, mEnd) = mStack.PopLastElement();
+      } else {
+        // The next item wants to be flattened. This means that we will skip the
+        // flattened item and directly iterate over its sublist.
+        MOZ_ASSERT(ShouldFlattenNextItem());
+
+        nsDisplayList* sublist = NextItem()->GetChildren();
+        MOZ_ASSERT(sublist);
+
+        // Skip the flattened item.
+        Advance();
+
+        // Store the current position on the stack.
+        if (!AtEndOfCurrentList()) {
+          mStack.AppendElement(std::make_pair(mStart, mEnd));
+        }
+
+        // Iterate over the sublist.
+        mStart = sublist->begin();
+        mEnd = sublist->end();
+      }
+    }
+  }
+
+ private:
+  nsDisplayListBuilder* mBuilder;
+  nsDisplayList::iterator mStart;
+  nsDisplayList::iterator mEnd;
+  AutoTArray<std::pair<nsDisplayList::iterator, nsDisplayList::iterator>, 3>
+      mStack;
+};
+
+class PaintTelemetry {
+ public:
+  class AutoRecordPaint {
+   public:
+    AutoRecordPaint();
+    ~AutoRecordPaint();
+
+   private:
+    TimeStamp mStart;
+  };
+
+ private:
+  static uint32_t sPaintLevel;
+};
+
+}  // namespace mozilla
+
+#endif /*NSDISPLAYLIST_H_*/