Only update composited-scrolling state once after layout.

Source/core/rendering/RenderLayerCompositor.h

 /*
  * Copyright (C) 2009 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 98 matching lines @@
     // Copy the accelerated compositing related flags from Settings
     void cacheAcceleratedCompositingFlags();
 
     // Called when the layer hierarchy needs to be updated (compositing layers have been
     // created, destroyed or re-parented).
     void setCompositingLayersNeedRebuild(bool needRebuild = true);
     bool compositingLayersNeedRebuild() const { return m_compositingLayersNeedRebuild; }
 
     // Called when something outside WebKit affects the visible rect (e.g. delegated scrolling). Might schedule a layer flush.
     void didChangeVisibleRect();
-    
+
+    // Updating properties required for determining if compositing is necessary.
+    void updateCompositingRequirementsState();
+    void setNeedsUpdateCompositingRequirementsState() { m_needsUpdateCompositingRequirementsState = true; }
+
     // Rebuild the tree of compositing layers
     void updateCompositingLayers(CompositingUpdateType, RenderLayer* updateRoot = 0);
     
     // Update the compositing state of the given layer. Returns true if that state changed.
     enum CompositingChangeRepaint { CompositingChangeRepaintNow, CompositingChangeWillRepaintLater };
     bool updateLayerCompositingState(RenderLayer*, CompositingChangeRepaint = CompositingChangeRepaintNow);
 
     // Update the geometry for compositing children of compositingAncestor.
     void updateCompositingDescendantGeometry(RenderLayer* compositingAncestor, RenderLayer*, bool compositedChildrenOnly);
     
@@ skipping 88 matching lines @@
 #if ENABLE(RUBBER_BANDING)
     GraphicsLayer* layerForOverhangAreas() const { return m_layerForOverhangAreas.get(); }
 
     GraphicsLayer* updateLayerForTopOverhangArea(bool wantsLayer);
     GraphicsLayer* updateLayerForBottomOverhangArea(bool wantsLayer);
 #endif
 
     void updateViewportConstraintStatus(RenderLayer*);
     void removeViewportConstrainedLayer(RenderLayer*);
 
+    void addOutOfFlowPositionedLayer(RenderLayer*);
+    void removeOutOfFlowPositionedLayer(RenderLayer*);
+
     void resetTrackedRepaintRects();
     void setTracksRepaints(bool);
 
     void reportMemoryUsage(MemoryObjectInfo*) const;
     void setShouldReevaluateCompositingAfterLayout() { m_reevaluateCompositingAfterLayout = true; }
 
     bool viewHasTransparentBackground(Color* backgroundColor = 0) const;
 
     CompositingReasons reasonsForCompositing(const RenderLayer*) const;
     
@@ skipping 105 matching lines @@
     bool m_showRepaintCounter;
 
     // When true, we have to wait until layout has happened before we can decide whether to enter compositing mode,
     // because only then do we know the final size of plugins and iframes.
     mutable bool m_reevaluateCompositingAfterLayout;
 
     bool m_compositing;
     bool m_compositingLayersNeedRebuild;
     bool m_forceCompositingMode;
     bool m_inPostLayoutUpdate; // true when it's OK to trust layout information (e.g. layer sizes and positions)
+    bool m_needsUpdateCompositingRequirementsState;
 
     bool m_isTrackingRepaints; // Used for testing.
     
     RootLayerAttachment m_rootLayerAttachment;
 
     // Enclosing clipping layer for iframe content
     OwnPtr<GraphicsLayer> m_clipLayer;
     OwnPtr<GraphicsLayer> m_scrollLayer;
 
     HashSet<RenderLayer*> m_viewportConstrainedLayers;
     HashSet<RenderLayer*> m_viewportConstrainedLayersNeedingUpdate;
 
+    // This is used in updateCompositingRequirementsState to avoid full tree
+    // walks while determining if layers have unclipped descendants.
+    HashSet<RenderLayer*> m_outOfFlowPositionedLayers;
+
     // Enclosing layer for overflow controls and the clipping layer
     OwnPtr<GraphicsLayer> m_overflowControlsHostLayer;
 
     // Layers for overflow controls
     OwnPtr<GraphicsLayer> m_layerForHorizontalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if ENABLE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangAreas;
     OwnPtr<GraphicsLayer> m_contentShadowLayer;
     OwnPtr<GraphicsLayer> m_layerForTopOverhangArea;
     OwnPtr<GraphicsLayer> m_layerForBottomOverhangArea;
 #endif
 
 #if !LOG_DISABLED
     int m_rootLayerUpdateCount;
     int m_obligateCompositedLayerCount; // count of layer that have to be composited.
     int m_secondaryCompositedLayerCount; // count of layers that have to be composited because of stacking or overlap.
     double m_obligatoryBackingStoreBytes;
     double m_secondaryBackingStoreBytes;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h