Always use a separate front texture on Mac OS to work around graphics driver bugs.

Source/core/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 115 matching lines @@
     , m_multisampleColorBuffer(0)
     , m_contentsChanged(true)
     , m_internalColorFormat(0)
     , m_colorFormat(0)
     , m_internalRenderbufferFormat(0)
     , m_maxTextureSize(0)
     , m_contextEvictionManager(contextEvictionManager)
 {
     // Used by browser tests to detect the use of a DrawingBuffer.
     TRACE_EVENT_INSTANT0("test_gpu", "DrawingBufferCreation");
+    // Before enabling mailboxes for canvas, make sure the scenarios
+    // from http://crbug.com/234428 do not reproduce!
 #if !ENABLE(CANVAS_USES_MAILBOX)
     // We need a separate front and back textures if ...
     m_separateFrontTexture = m_preserveDrawingBuffer == Preserve // ... we have to preserve contents after compositing, which is done with a copy or ...
                              || WebKit::Platform::current()->isThreadedCompositingEnabled(); // ... if we're in threaded mode and need to double buffer.
+
+#if OS(DARWIN)
+    // http://crbug.com/234428 : always use a separate front texture
+    // on Mac OS. Doing this on all GPUs, not just NVIDIA GPUs,
+    // ensures consistent behavior and is much less code.
+    m_separateFrontTexture = true;
+#endif // OS(DARWIN)
+
 #endif // !ENABLE(CANVAS_USES_MAILBOX)
     initialize(size);
 }
 
 DrawingBuffer::~DrawingBuffer()
 {
     releaseResources();
 }
 
 unsigned DrawingBuffer::prepareTexture(WebKit::WebTextureUpdater& updater)
@@ skipping 658 matching lines @@
 
 void DrawingBuffer::bind()
 {
     if (!m_context)
         return;
 
     m_context->bindFramebuffer(GraphicsContext3D::FRAMEBUFFER, m_multisampleFBO ? m_multisampleFBO : m_fbo);
 }
 
 } // namespace WebCore