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diff --git a/gfx/layers/composite/ImageComposite.h b/gfx/layers/composite/ImageComposite.h
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+++ b/gfx/layers/composite/ImageComposite.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 MOZILLA_GFX_IMAGECOMPOSITE_H
+#define MOZILLA_GFX_IMAGECOMPOSITE_H
+
+#include "CompositableHost.h"  // for CompositableTextureHostRef
+#include "mozilla/gfx/2D.h"
+#include "mozilla/TimeStamp.h"  // for TimeStamp
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * Implements Image selection logic.
+ */
+class ImageComposite {
+ public:
+  static const float BIAS_TIME_MS;
+
+  explicit ImageComposite();
+  virtual ~ImageComposite();
+
+  int32_t GetFrameID() {
+    const TimedImage* img = ChooseImage();
+    return img ? img->mFrameID : -1;
+  }
+
+  int32_t GetProducerID() {
+    const TimedImage* img = ChooseImage();
+    return img ? img->mProducerID : -1;
+  }
+
+  int32_t GetLastFrameID() const { return mLastFrameID; }
+  int32_t GetLastProducerID() const { return mLastProducerID; }
+  uint32_t GetDroppedFramesAndReset() {
+    uint32_t dropped = mDroppedFrames;
+    mDroppedFrames = 0;
+    return dropped;
+  }
+
+  enum Bias {
+    // Don't apply bias to frame times
+    BIAS_NONE,
+    // Apply a negative bias to frame times to keep them before the vsync time
+    BIAS_NEGATIVE,
+    // Apply a positive bias to frame times to keep them after the vsync time
+    BIAS_POSITIVE,
+  };
+
+ protected:
+  virtual TimeStamp GetCompositionTime() const = 0;
+  virtual CompositionOpportunityId GetCompositionOpportunityId() const = 0;
+  virtual void AppendImageCompositeNotification(
+      const ImageCompositeNotificationInfo& aInfo) const = 0;
+
+  struct TimedImage {
+    CompositableTextureHostRef mTextureHost;
+    TimeStamp mTimeStamp;
+    gfx::IntRect mPictureRect;
+    int32_t mFrameID;
+    int32_t mProducerID;
+  };
+
+  /**
+   * ChooseImage is guaranteed to return the same TimedImage every time it's
+   * called during the same composition, up to the end of Composite() ---
+   * it depends only on mImages, mCompositor->GetCompositionTime(), and mBias.
+   * mBias is updated at the end of Composite().
+   */
+  const TimedImage* ChooseImage();
+  int ChooseImageIndex();
+  const TimedImage* GetImage(size_t aIndex) const;
+  size_t ImagesCount() const { return mImages.Length(); }
+  const nsTArray<TimedImage>& Images() const { return mImages; }
+
+  void RemoveImagesWithTextureHost(TextureHost* aTexture);
+  void ClearImages();
+  void SetImages(nsTArray<TimedImage>&& aNewImages);
+
+ protected:
+  // Send ImageComposite notifications and update the ChooseImage bias.
+  void OnFinishRendering(int aImageIndex, const TimedImage* aImage,
+                         base::ProcessId aProcessId,
+                         const CompositableHandle& aHandle);
+
+  int32_t mLastFrameID = -1;
+  int32_t mLastProducerID = -1;
+
+ private:
+  nsTArray<TimedImage> mImages;
+  TimeStamp GetBiasedTime(const TimeStamp& aInput) const;
+
+  // Called when we know that frames older than aImage will never get another
+  // chance to be displayed.
+  // Counts frames in mImages that were skipped, and adds the skipped count to
+  // mSkippedFramesSinceLastComposite.
+  void CountSkippedFrames(const TimedImage* aImage);
+
+  // Update mLastFrameID and mLastProducerID, and report dropped frames.
+  // Returns whether the frame changed.
+  bool UpdateCompositedFrame(int aImageIndex,
+                             bool aWasVisibleAtPreviousComposition);
+
+  void UpdateBias(size_t aImageIndex, bool aFrameUpdated);
+
+  // Emit a profiler marker about video frame timestamp jitter.
+  void DetectTimeStampJitter(const TimedImage* aNewImage);
+
+  /**
+   * Bias to apply to the next frame.
+   */
+  Bias mBias = BIAS_NONE;
+
+  // Video frames that were in mImages but that will never reach the screen.
+  // This count is reset in UpdateCompositedFrame.
+  int32_t mSkippedFramesSinceLastComposite = 0;
+
+  // The number of dropped frames since the last call to
+  // GetDroppedFramesAndReset(). Not all skipped frames are considered "dropped"
+  // - for example, videos that are scrolled out of view or videos in background
+  // tabs are expected to skip frames, and those skipped frames are not counted
+  // as dropped frames.
+  uint32_t mDroppedFrames = 0;
+
+  // The composition opportunity IDs of the last calls to ChooseImageIndex and
+  // UpdateCompositedFrame, respectively.
+  CompositionOpportunityId mLastChooseImageIndexComposition;
+  CompositionOpportunityId mLastFrameUpdateComposition;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // MOZILLA_GFX_IMAGECOMPOSITE_H