The position of opaque black rect is incorrect when alpha is false.

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 304 matching lines @@
         if (imageBuffer) {
             CompositeOperator compositeOperator = !m_context || m_context->hasAlpha() ? CompositeSourceOver : CompositeCopy;
             if (m_presentedImage)
                 context->drawImage(m_presentedImage.get(), pixelSnappedIntRect(r), compositeOperator, DoNotRespectImageOrientation);
             else
                 context->drawImageBuffer(imageBuffer, pixelSnappedIntRect(r), 0, compositeOperator);
         }
     } else {
         // When alpha is false, we should draw to opaque black.
         if (m_context && !m_context->hasAlpha())
-            context->fillRect(FloatRect(0, 0, width(), height()), Color(0, 0, 0));
+            context->fillRect(FloatRect(r), Color(0, 0, 0));
     }
 
     if (is3D())
         toWebGLRenderingContext(m_context.get())->markLayerComposited();
 }
 
 bool HTMLCanvasElement::is3D() const
 {
     return m_context && m_context->is3d();
 }
@@ skipping 362 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }