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diff --git a/layout/painting/RetainedDisplayListBuilder.h b/layout/painting/RetainedDisplayListBuilder.h
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+++ b/layout/painting/RetainedDisplayListBuilder.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef RETAINEDDISPLAYLISTBUILDER_H_
+#define RETAINEDDISPLAYLISTBUILDER_H_
+
+#include "nsDisplayList.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/TypedEnumBits.h"
+
+class nsWindowSizes;
+
+/**
+ * RetainedDisplayListData contains frame invalidation information. It is stored
+ * in root frames, and used by RetainedDisplayListBuilder.
+ * Currently this is implemented as a map of frame pointers to flags.
+ */
+struct RetainedDisplayListData {
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(DisplayListData, RetainedDisplayListData)
+
+  enum class FrameFlags : uint8_t {
+    None = 0,
+    Modified = 1 << 0,
+    HasProps = 1 << 1,
+    HadWillChange = 1 << 2
+  };
+
+  RetainedDisplayListData() : mModifiedFramesCount(0) {}
+
+  /**
+   * Adds the frame to modified frames list.
+   */
+  void AddModifiedFrame(nsIFrame* aFrame);
+
+  /**
+   * Removes all the frames from this RetainedDisplayListData.
+   */
+  void Clear() {
+    mFrames.Clear();
+    mModifiedFramesCount = 0;
+  }
+
+  /**
+   * Returns a mutable reference to flags set for the given |aFrame|. If the
+   * frame does not exist in this RetainedDisplayListData, it is added with
+   * default constructible flags FrameFlags::None.
+   */
+  FrameFlags& Flags(nsIFrame* aFrame) { return mFrames.GetOrInsert(aFrame); }
+
+  /**
+   * Returns flags set for the given |aFrame|, or FrameFlags::None if the frame
+   * is not in this RetainedDisplayListData.
+   */
+  FrameFlags GetFlags(nsIFrame* aFrame) const { return mFrames.Get(aFrame); }
+
+  /**
+   * Returns an iterator to the underlying frame storage.
+   */
+  auto Iterator() { return mFrames.Iter(); }
+
+  /**
+   * Returns the count of modified frames in this RetainedDisplayListData.
+   */
+  uint32_t ModifiedFramesCount() const { return mModifiedFramesCount; }
+
+  /**
+   * Removes the given |aFrame| from this RetainedDisplayListData.
+   */
+  bool Remove(nsIFrame* aFrame) { return mFrames.Remove(aFrame); }
+
+ private:
+  nsDataHashtable<nsPtrHashKey<nsIFrame>, FrameFlags> mFrames;
+  uint32_t mModifiedFramesCount;
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(RetainedDisplayListData::FrameFlags)
+
+/**
+ * Returns RetainedDisplayListData property for the given |aRootFrame|, or
+ * nullptr if the property is not set.
+ */
+RetainedDisplayListData* GetRetainedDisplayListData(nsIFrame* aRootFrame);
+
+/**
+ * Returns RetainedDisplayListData property for the given |aRootFrame|. Creates
+ * and sets a new RetainedDisplayListData property if it is not already set.
+ */
+RetainedDisplayListData* GetOrSetRetainedDisplayListData(nsIFrame* aRootFrame);
+
+enum class PartialUpdateResult { Failed, NoChange, Updated };
+
+enum class PartialUpdateFailReason {
+  NA,
+  EmptyList,
+  RebuildLimit,
+  FrameType,
+  Disabled,
+  Content,
+  VisibleRect,
+};
+
+struct RetainedDisplayListMetrics {
+  RetainedDisplayListMetrics() { Reset(); }
+
+  void Reset() {
+    mNewItems = 0;
+    mRebuiltItems = 0;
+    mRemovedItems = 0;
+    mReusedItems = 0;
+    mTotalItems = 0;
+    mPartialBuildDuration = 0;
+    mFullBuildDuration = 0;
+    mPartialUpdateFailReason = PartialUpdateFailReason::NA;
+    mPartialUpdateResult = PartialUpdateResult::NoChange;
+  }
+
+  void StartBuild() { mStartTime = mozilla::TimeStamp::Now(); }
+
+  void EndFullBuild() { mFullBuildDuration = Elapsed(); }
+
+  void EndPartialBuild(PartialUpdateResult aResult) {
+    mPartialBuildDuration = Elapsed();
+    mPartialUpdateResult = aResult;
+  }
+
+  double Elapsed() {
+    return (mozilla::TimeStamp::Now() - mStartTime).ToMilliseconds();
+  }
+
+  const char* FailReasonString() const {
+    switch (mPartialUpdateFailReason) {
+      case PartialUpdateFailReason::NA:
+        return "N/A";
+      case PartialUpdateFailReason::EmptyList:
+        return "Empty list";
+      case PartialUpdateFailReason::RebuildLimit:
+        return "Rebuild limit";
+      case PartialUpdateFailReason::FrameType:
+        return "Frame type";
+      case PartialUpdateFailReason::Disabled:
+        return "Disabled";
+      case PartialUpdateFailReason::Content:
+        return "Content";
+      case PartialUpdateFailReason::VisibleRect:
+        return "VisibleRect";
+      default:
+        MOZ_ASSERT_UNREACHABLE("Enum value not handled!");
+    }
+  }
+
+  unsigned int mNewItems;
+  unsigned int mRebuiltItems;
+  unsigned int mRemovedItems;
+  unsigned int mReusedItems;
+  unsigned int mTotalItems;
+
+  mozilla::TimeStamp mStartTime;
+  double mPartialBuildDuration;
+  double mFullBuildDuration;
+  PartialUpdateFailReason mPartialUpdateFailReason;
+  PartialUpdateResult mPartialUpdateResult;
+};
+
+struct RetainedDisplayListBuilder {
+  RetainedDisplayListBuilder(nsIFrame* aReferenceFrame,
+                             nsDisplayListBuilderMode aMode, bool aBuildCaret)
+      : mBuilder(aReferenceFrame, aMode, aBuildCaret, true) {}
+  ~RetainedDisplayListBuilder() { mList.DeleteAll(&mBuilder); }
+
+  nsDisplayListBuilder* Builder() { return &mBuilder; }
+
+  nsDisplayList* List() { return &mList; }
+
+  RetainedDisplayListMetrics* Metrics() { return &mMetrics; }
+
+  PartialUpdateResult AttemptPartialUpdate(nscolor aBackstop);
+
+  /**
+   * Iterates through the display list builder reference frame document and
+   * subdocuments, and clears the modified frame lists from the root frames.
+   * Also clears the frame properties set by RetainedDisplayListBuilder for all
+   * the frames in the modified frame lists.
+   */
+  void ClearFramesWithProps();
+  void AddSizeOfIncludingThis(nsWindowSizes&) const;
+
+  NS_DECLARE_FRAME_PROPERTY_DELETABLE(Cached, RetainedDisplayListBuilder)
+
+ private:
+  void IncrementSubDocPresShellPaintCount(nsDisplayItem* aItem);
+
+  /**
+   * Invalidates the current and previous caret frame if they have changed.
+   */
+  void InvalidateCaretFramesIfNeeded();
+
+  /**
+   * A simple early exit heuristic to avoid slow partial display list rebuilds.
+   * Returns true if a partial display list build should be attempted.
+   */
+  bool ShouldBuildPartial(nsTArray<nsIFrame*>& aModifiedFrames);
+
+  /**
+   * Recursively pre-processes the old display list tree before building the
+   * new partial display lists, and serializes the old list into an array,
+   * recording indices on items for fast lookup during merging. Builds an
+   * initial linear DAG for the list if we don't have an existing one. Finds
+   * items that have a different AGR from the specified one, and marks them to
+   * also be built so that we get relative ordering correct. Passes
+   * aKeepLinked=true internally for sub-lists that can't be changed to keep the
+   * original list structure linked for fast re-use.
+   */
+  bool PreProcessDisplayList(RetainedDisplayList* aList,
+                             AnimatedGeometryRoot* aAGR,
+                             PartialUpdateResult& aUpdated,
+                             nsIFrame* aOuterFrame = nullptr,
+                             uint32_t aCallerKey = 0,
+                             uint32_t aNestingDepth = 0,
+                             bool aKeepLinked = false);
+
+  /**
+   * Merges items from aNewList into non-invalidated items from aOldList and
+   * stores the result in aOutList.
+   *
+   * aOuterItem is a pointer to an item that owns one of the lists, if
+   * available. If both lists are populated, then both outer items must not be
+   * invalidated, and identical, so either can be passed here.
+   *
+   * Returns true if changes were made, and the resulting display list (in
+   * aOutList) is different from aOldList.
+   */
+  bool MergeDisplayLists(
+      nsDisplayList* aNewList, RetainedDisplayList* aOldList,
+      RetainedDisplayList* aOutList,
+      mozilla::Maybe<const mozilla::ActiveScrolledRoot*>& aOutContainerASR,
+      nsDisplayItem* aOuterItem = nullptr);
+
+  bool ComputeRebuildRegion(nsTArray<nsIFrame*>& aModifiedFrames,
+                            nsRect* aOutDirty,
+                            AnimatedGeometryRoot** aOutModifiedAGR,
+                            nsTArray<nsIFrame*>& aOutFramesWithProps);
+
+  bool ProcessFrame(nsIFrame* aFrame, nsDisplayListBuilder* aBuilder,
+                    nsIFrame* aStopAtFrame,
+                    nsTArray<nsIFrame*>& aOutFramesWithProps,
+                    const bool aStopAtStackingContext, nsRect* aOutDirty,
+                    AnimatedGeometryRoot** aOutModifiedAGR);
+
+  friend class MergeState;
+
+  nsDisplayListBuilder mBuilder;
+  RetainedDisplayList mList;
+  nsRect mPreviousVisibleRect;
+  WeakFrame mPreviousCaret;
+  RetainedDisplayListMetrics mMetrics;
+};
+
+#endif  // RETAINEDDISPLAYLISTBUILDER_H_