Canvas element that use the alpha attribute don't display correctly.

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 297 matching lines @@
     // Clear the dirty rect
     m_dirtyRect = FloatRect();
 
     if (context->paintingDisabled())
         return;
 
     if (m_context) {
         if (!paintsIntoCanvasBuffer() && !document().printing())
             return;
         m_context->paintRenderingResultsToCanvas();
+
+        // When alpha is false, we should initialize to opaque black.
+        if (!hasImageBuffer() && !m_context->hasAlpha())
+            createImageBuffer();

[Justin Novosad, 2014/03/17 17:24:23]

I think you can solve this problem without creating an ImageBuffer.

     }
 
     if (hasImageBuffer()) {
         ImageBuffer* imageBuffer = buffer();
         if (imageBuffer) {
             CompositeOperator compositeOperator = !m_context || m_context->hasAlpha() ? CompositeSourceOver : CompositeCopy;
             if (m_presentedImage)
                 context->drawImage(m_presentedImage.get(), pixelSnappedIntRect(r), compositeOperator, DoNotRespectImageOrientation, useLowQualityScale);
             else
                 context->drawImageBuffer(imageBuffer, pixelSnappedIntRect(r), compositeOperator, blink::WebBlendModeNormal, useLowQualityScale);
         }
     }

[Justin Novosad, 2014/03/17 17:24:23]

You could add an else clause here that just draws a black rectangle

 
     if (is3D())
         toWebGLRenderingContext(m_context.get())->markLayerComposited();
 }
 
 bool HTMLCanvasElement::is3D() const
 {
     return m_context && m_context->is3d();
 }
 
@@ skipping 313 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }