Redraw only dirty area for accelerated 2D Canvas.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1696 matching lines @@
 }
 
 void CompositedLayerMapping::contentChanged(ContentChangeType changeType)
 {
     if ((changeType == ImageChanged) && isDirectlyCompositedImage()) {
         updateImageContents();
         return;
     }
 
     if ((changeType == CanvasChanged || changeType == CanvasPixelsChanged) && isAcceleratedCanvas(renderer())) {
-        m_graphicsLayer->setContentsNeedsDisplay();
+        m_graphicsLayer->setContentsNeedsDisplayInRect(toHTMLCanvasElement(renderer()->node())->dirtyRect());
         return;

[abarth-chromium, 2014/06/20 15:19:47]

Rather than using this awkward notification pipeline, can you refactor the code
to call a function on CompositedLayerMapping that passes the dirtyRect as an
actual argument?  Currently, this sends the control flow through a one path and
the data through another path.  It would be much better to send the control and
data through the same path.  Also, that would save you from having to store
m_dirtyRect on HTMLCanvasElement.

     }
 }
 
 void CompositedLayerMapping::updateImageContents()
 {
     ASSERT(renderer()->isImage());
     RenderImage* imageRenderer = toRenderImage(renderer());
 
     ImageResource* cachedImage = imageRenderer->cachedImage();
     if (!cachedImage)
@@ skipping 508 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore