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,
+    CompositingUpdateFinishAllDeferredWork
 };
 
 // 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 24 matching lines @@
     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
+    // Main entry point for a full update. As needed, this function will compute compositing requirements,
+    // rebuild the composited layer tree, and/or update all the properties assocaited with each layer of the
+    // composited layer tree.
     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);
 
     // Whether layer's compositedLayerMapping needs a GraphicsLayer to do clipping by an ancestor (non-stacking-context parent with overflow).
@@ skipping 88 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 setNeedsToRecomputeCompositingRequirements() { m_needsToRecomputeCompositingRequirements = true; }
 
     // 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 17 matching lines @@
     bool allocateOrClearCompositedLayerMapping(RenderLayer*, CompositingChangeRepaint shouldRepaint);
 
     void clearMappingForRenderLayerIncludingDescendants(RenderLayer*);
 
     // Repaint the given rect (which is layer's coords), and regions of child layers that intersect that rect.
     void recursiveRepaintLayer(RenderLayer*, const IntRect* = 0);
 
     void addToOverlapMap(OverlapMap&, RenderLayer*, IntRect& layerBounds, bool& boundsComputed);
     void addToOverlapMapRecursive(OverlapMap&, RenderLayer*, RenderLayer* ancestorLayer = 0);
 
+    // Forces an update for all frames of frame tree recursively. Used only when the mainFrame compositor is ready to
+    // finish all deferred work.
+    static void finishCompositingUpdateForFrameTree(Frame*);

[jamesr, 2013/10/18 15:56:42]

why is this static?  anything that has a Frame* can walk to its FrameView to get
to its RenderLayerCompositor instance

[shawnsingh, 2013/10/18 18:37:30]

I made this static because earlier I was accidentally using isMainFrame()
instead of compositor->isMainFrame.  This was a source of many of my iframe
failures, and it made sense to prevent an error like that from happening again
by making it static.   (I mentioned this in Comment #7)

I can change this if you want.  We could make it a non-member static if we make
isMainFrame public, for example.

+
     // Returns true if any layer's compositing changed
     void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap*, struct CompositingRecursionData&, bool& layersChanged, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants);
 
     // Recurses down the tree, parenting descendant compositing layers and collecting an array of child layers for the current compositing layer.
     void rebuildCompositingLayerTree(RenderLayer*, Vector<GraphicsLayer*>& childGraphicsLayersOfEnclosingLayer, int depth);
 
     // Recurses down the tree, updating layer geometry only.
     void updateLayerTreeGeometry(RenderLayer*, int depth);
 
     // Hook compositing layers together
@@ skipping 59 matching lines @@
 private:
     RenderView* m_renderView;
     OwnPtr<GraphicsLayer> m_rootContentLayer;
 
     bool m_hasAcceleratedCompositing;
     ChromeClient::CompositingTriggerFlags m_compositingTriggers;
 
     int m_compositedLayerCount;
     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;
+    // FIXME: This should absolutely not be mutable.
+    mutable bool m_needsToRecomputeCompositingRequirements;
+    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