Fix & clarify some comments in compositing. Rename clippedByAncestor to more accurately

Source/core/rendering/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 338 matching lines @@
 {
     // Scrollbar layers use this layer for relative positioning, so don't clip.
     if (layerForHorizontalScrollbar() || layerForVerticalScrollbar())
         return false;
 
     if (layerOrAncestorIsTransformedOrUsingCompositedScrolling(&m_owningLayer))
         return false;
 
     // Scrolled composited layers are clipped by their ancestor clipping layer,
     // so don't clip these, either.
-    if (!compositor()->clippedByAncestor(&m_owningLayer))
-        return true;
+    if (compositor()->clippedByNonAncestorInStackingTree(&m_owningLayer)
+        && m_owningLayer.renderer()->containingBlock()->enclosingLayer() == m_owningLayer.ancestorScrollingLayer())
+        return false;
 
-    if (m_owningLayer.renderer()->containingBlock()->enclosingLayer() != m_owningLayer.ancestorScrollingLayer())
-        return true;
-
-    return false;
+    return true;
 }
 
 void CompositedLayerMapping::updateCompositedBounds(GraphicsLayerUpdater::UpdateType updateType)
 {
     if (!shouldUpdateGraphicsLayer(updateType))
         return;
 
     // We need to know if we draw content in order to update our bounds (this has an effect
     // on whether or not descendands will paint into our backing). Update this value now.
     updateDrawsContent();
@@ skipping 77 matching lines @@
     if (updateForegroundLayer(compositor->needsContentsCompositingLayer(&m_owningLayer)))
         layerConfigChanged = true;
 
     bool needsDescendantsClippingLayer = compositor->clipsCompositingDescendants(&m_owningLayer);
 
     // Our scrolling layer will clip.
     if (m_owningLayer.needsCompositedScrolling())
         needsDescendantsClippingLayer = false;
 
     RenderLayer* scrollParent = compositor->acceleratedCompositingForOverflowScrollEnabled() ? m_owningLayer.scrollParent() : 0;
-    bool needsAncestorClip = compositor->clippedByAncestor(&m_owningLayer);
+    bool needsAncestorClip = compositor->clippedByNonAncestorInStackingTree(&m_owningLayer);
     if (scrollParent) {
         // If our containing block is our ancestor scrolling layer, then we'll already be clipped
         // to it via our scroll parent and we don't need an ancestor clipping layer.
         if (m_owningLayer.renderer()->containingBlock()->enclosingLayer() == m_owningLayer.ancestorCompositedScrollingLayer())
             needsAncestorClip = false;
     }
 
     if (updateClippingLayers(needsAncestorClip, needsDescendantsClippingLayer))
         layerConfigChanged = true;
 
@@ skipping 1683 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore