Let dirty rect inflate after its unite with the affected area

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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.
@@ skipping 251 matching lines @@
 }
 
 void HTMLCanvasElement::didDraw(const FloatRect& rect)
 {
     if (rect.isEmpty())
         return;
     m_imageBufferIsClear = false;
     clearCopiedImage();
     if (layoutObject())
         layoutObject()->setMayNeedPaintInvalidation();
+    m_dirtyRect.unite(rect);
     if (m_context && m_context->is2d() && m_context->shouldAntialias() && page() && page()->deviceScaleFactor() > 1.0f)
         m_dirtyRect.inflate(1);

[Justin Novosad, 2016/05/18 19:11:42]

what you meant to do is inflate "rect" before the union. As opposed to inflate
m_dirtyRect, which contains an accumulation

-    m_dirtyRect.unite(rect);
     if (m_context && m_context->is2d() && hasImageBuffer())
         buffer()->didDraw(rect);
 }
 
 void HTMLCanvasElement::didFinalizeFrame()
 {
     notifyListenersCanvasChanged();
 
     if (m_dirtyRect.isEmpty())
         return;
@@ skipping 841 matching lines @@
 }
 
 String HTMLCanvasElement::getIdFromControl(const Element* element)
 {
     if (m_context)
         return m_context->getIdFromControl(element);
     return String();
 }
 
 } // namespace blink