Don't invalidate paint on background image change if it's obscured

third_party/WebKit/Source/platform/graphics/BitmapImage.cpp

 /*
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2008 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
@@ skipping 366 matching lines @@
     }
 
     return Image::imageForDefaultFrame();
 }
 
 bool BitmapImage::frameHasAlphaAtIndex(size_t index)
 {
     if (m_frames.size() <= index)
         return true;
 
+    if (m_frames[index].m_haveMetadata && !m_frames[index].m_hasAlpha)
+        return false;
+
+    // m_hasAlpha may change after m_haveMetadata is set to true, so always ask
+    // ImageSource for the value if the cached value is the default value.
+    bool hasAlpha = m_source.frameHasAlphaAtIndex(index);
+
     if (m_frames[index].m_haveMetadata)
-        return m_frames[index].m_hasAlpha;
+        m_frames[index].m_hasAlpha = hasAlpha;
 
-    return m_source.frameHasAlphaAtIndex(index);
+    return hasAlpha;
 }
 
 bool BitmapImage::currentFrameKnownToBeOpaque(MetadataMode metadataMode)
 {
     if (metadataMode == PreCacheMetadata) {
         // frameHasAlphaAtIndex() conservatively returns false for uncached frames. To increase the
         // chance of an accurate answer, pre-cache the current frame metadata.
         frameAtIndex(currentFrame());
     }
     return !frameHasAlphaAtIndex(currentFrame());
@@ skipping 209 matching lines @@
 
     // We need to draw this frame if we advanced to it while not skipping, or if
     // while trying to skip frames we hit the last frame and thus had to stop.
     if (skippingFrames != advancedAnimation)
         getImageObserver()->animationAdvanced(this);
 
     return advancedAnimation;
 }
 
 } // namespace blink