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

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 28 matching lines @@
 class RenderPart;
 class RenderVideo;
 class ScrollingCoordinator;
 class StickyPositionViewportConstraints;
 
 
 enum CompositingUpdateType {
     CompositingUpdateAfterStyleChange,
     CompositingUpdateAfterLayout,
     CompositingUpdateOnScroll,
-    CompositingUpdateOnCompositedScroll
+    CompositingUpdateOnCompositedScroll,
+    CompositingUpdateOnBeginDrawingFrame
 };
 
 // RenderLayerCompositor manages the hierarchy of
 // composited RenderLayers. It determines which RenderLayers
 // become compositing, and creates and maintains a hierarchy of
 // GraphicsLayers based on the RenderLayer painting order.
 //
 // There is one RenderLayerCompositor per RenderView.
 
 class RenderLayerCompositor : public GraphicsLayerClient {
@@ skipping 132 matching lines @@
 
     void updateViewportConstraintStatus(RenderLayer*);
     void removeViewportConstrainedLayer(RenderLayer*);
 
     void addOutOfFlowPositionedLayer(RenderLayer*);
     void removeOutOfFlowPositionedLayer(RenderLayer*);
 
     void resetTrackedRepaintRects();
     void setTracksRepaints(bool);
 
-    void setShouldReevaluateCompositingAfterLayout() { m_reevaluateCompositingAfterLayout = true; }
+    void setShouldReevaluateCompositingAfterLayout() { m_needsToRecomputeCompositingRequirements = true; }

[Ian Vollick, 2013/10/08 18:32:21]

nit: should this function be renamed as well, then?

 
     // Returns all reasons (direct, indirectly due to subtree, and indirectly due to overlap) that a layer should be composited.
     CompositingReasons reasonsForCompositing(const RenderLayer*) const;
 
     virtual String debugName(const GraphicsLayer*) OVERRIDE;
 
 private:
     class OverlapMap;
 
     // GraphicsLayerClient implementation
@@ skipping 96 matching lines @@
 private:
     RenderView* m_renderView;
     OwnPtr<GraphicsLayer> m_rootContentLayer;
 
     bool m_hasAcceleratedCompositing;
     ChromeClient::CompositingTriggerFlags m_compositingTriggers;
 
     int m_compositedLayerCount;
     bool m_showRepaintCounter;
 
+    // FIXME update this comment before landing the patch
     // 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;
+    mutable bool m_needsToRecomputeCompositingRequirements;
+    mutable bool m_needsToUpdateLayerTreeGeometry;
 
     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;
@@ skipping 27 matching lines @@
     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