Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/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 732 matching lines @@
         m_scrollingContentsLayer->setOffsetFromRenderer(scrollingContentsOffset, GraphicsLayer::DontSetNeedsDisplay);
 
         if (m_foregroundLayer) {
             if (m_foregroundLayer->size() != m_scrollingContentsLayer->size())
                 m_foregroundLayer->setSize(m_scrollingContentsLayer->size());
             m_foregroundLayer->setNeedsDisplay();
             m_foregroundLayer->setOffsetFromRenderer(m_scrollingContentsLayer->offsetFromRenderer());
         }
     }
 
+    m_owningLayer->positionOverflowControls();

[Ian Vollick, 2013/10/17 01:30:25]

What's the reason for this change?

[shawnsingh, 2013/10/17 01:44:04]

Because we defer computing compositing, we may not decide to composite a layer
before updateLayerPositions() is called.  updateLayerPositions is where
composited scrollbars were being positioned, so in that scenario, positioning
composited scrollbars was getting skipped.  The CompositedLayerMapping here,
which manages those scrollbars, should be the one responsible for re-positioning
composited scrollbars.   Eventually we should be able to remove the code that
positions composited scrollbars from updateLayerPositions, but for now to be
safe, I left it there.

+
     // If this layer was created just for clipping or to apply perspective, it doesn't need its own backing store.
     setRequiresOwnBackingStore(compositor()->requiresOwnBackingStore(m_owningLayer, compAncestor));
 
     updateContentsRect(isSimpleContainer);
     updateBackgroundColor(isSimpleContainer);
     updateDrawsContent(isSimpleContainer);
     updateContentsOpaque();
     updateAfterWidgetResize();
     registerScrollingLayers();
 
@@ skipping 1201 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore