Paint solid color and local attachment backgrounds into scrolling layer.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 291 matching lines @@
         if (!context->hasAlpha())
             m_graphicsLayer->setContentsOpaque(true);
         else if (WebLayer* layer = context->platformLayer())
             m_graphicsLayer->setContentsOpaque(!Color(layer->backgroundColor()).hasAlpha());
         else
             m_graphicsLayer->setContentsOpaque(false);
     } else if (m_backgroundLayer) {
         m_graphicsLayer->setContentsOpaque(false);
         m_backgroundLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
     } else {
-        // For non-root layers, background is always painted by the primary graphics layer.
-        m_graphicsLayer->setContentsOpaque(m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
+        const bool backgroundOpaque = m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds());
+        // Whether or not the background is painted into the primary graphics layer, it will be
+        // entirely occluded by an opaque background.
+        m_graphicsLayer->setContentsOpaque(backgroundOpaque);
+
+        // For non-root layers, background is painted by the scrolling contents layer if all backgrounds
+        // are solid color or background attachment local, otherwise background is painted by
+        // the primary graphics layer.
+        if (hasScrollingLayer() && m_owningLayer.shouldPaintBackgroundOntoForeground())
+            m_scrollingContentsLayer->setContentsOpaque(backgroundOpaque);
     }
 }
 
 void CompositedLayerMapping::updateCompositedBounds()
 {
     ASSERT(m_owningLayer.compositor()->lifecycle().state() == DocumentLifecycle::InCompositingUpdate);
     // FIXME: if this is really needed for performance, it would be better to store it on Layer.
     m_compositedBounds = m_owningLayer.boundingBoxForCompositing();
     m_contentOffsetInCompositingLayerDirty = true;
 }
@@ skipping 2336 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink