GIF Animations "clear from cache related glitch" fix

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp

Index: third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
index cc5954edf9de97a6391a4c904dc4826cc341741d..bf46ddc90833f8c0fe7fa1b33465bbf42a2a3daf 100644
--- a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
@@ -220,15 +220,48 @@ bool GIFImageDecoder::frameComplete(size_t frameIndex)
 
 size_t GIFImageDecoder::clearCacheExceptFrame(size_t clearExceptFrame)
 {
-    // We need to preserve frames such that:
-    //  1. We don't clear |clearExceptFrame|;
-    //  2. We don't clear any frame from which a future initFrameBuffer() call
-    //     will copy bitmap data.
-    // All other frames can be cleared.
+    // We expect that after this call, we'll be asked to decode frames after
+    // this one.  So we want to avoid clearing frames such that those requests
+    // would force re-decoding from the beginning of the image.
+    //
+    // When |clearExceptFrame| is e.g. DisposeKeep, simply not clearing that
+    // frame is sufficient, as the next frame will be based on it, and in
+    // general future frames can't be based on anything previous.
+    //
+    // However, if this frame is DisposeOverwritePrevious, then subsequent
+    // frames will depend on this frame's required previous frame.  In this
+    // case, we need to preserve both this frame and that one.
+    size_t clearExceptFrame2 = kNotFound;
+    if (clearExceptFrame < m_frameBufferCache.size()) {
+        const ImageFrame& frame = m_frameBufferCache[clearExceptFrame];
+        if ((frame.status() != ImageFrame::FrameEmpty) && (frame.disposalMethod() == ImageFrame::DisposeOverwritePrevious)) {
+            clearExceptFrame2 = clearExceptFrame;
+            clearExceptFrame = frame.requiredPreviousFrameIndex();
+        }
+    }
+
+    // Now |clearExceptFrame| indicates the frame that future frames will
+    // depend on.  But if decoding is skipping forward past intermediate frames,
+    // this frame may be FrameEmpty.  So we need to keep traversing back through
+    // the required previous frames until we find the nearest non-empty
+    // ancestor.  Preserving that will minimize the amount of future decoding
+    // needed.
     while ((clearExceptFrame < m_frameBufferCache.size()) && (m_frameBufferCache[clearExceptFrame].status() == ImageFrame::FrameEmpty))
         clearExceptFrame = m_frameBufferCache[clearExceptFrame].requiredPreviousFrameIndex();
+    return clearCacheExceptTwoFrames(clearExceptFrame, clearExceptFrame2);
+}
+
 
-    return ImageDecoder::clearCacheExceptFrame(clearExceptFrame);
+size_t GIFImageDecoder::clearCacheExceptTwoFrames(size_t clearExceptFrame1, size_t clearExceptFrame2)
+{
+    size_t frameBytesCleared = 0;
+    for (size_t i = 0; i < m_frameBufferCache.size(); ++i) {
+        if (m_frameBufferCache[i].status() != ImageFrame::FrameEmpty && i != clearExceptFrame1 && i != clearExceptFrame2) {
+            frameBytesCleared += frameBytesAtIndex(i);
+            clearFrameBuffer(i);
+        }
+    }
+    return frameBytesCleared;
 }
 
 void GIFImageDecoder::clearFrameBuffer(size_t frameIndex)