Land layer squashing behind a flag

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 211 matching lines @@
     bool allocateOrClearCompositedLayerMapping(RenderLayer*);
 
     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);
 
+    struct SquashingState {
+        SquashingState()
+            : mostRecentMapping(0)
+            , hasMostRecentMapping(false)
+            , nextSquashedLayerIndex(0) { }
+
+        void updateSquashingStateForNewMapping(CompositedLayerMappingPtr, bool hasNewCompositedLayerMapping, IntPoint newOffsetFromAbsolute);
+
+        // The most recent composited backing that the layer should squash onto if needed.
+        CompositedLayerMappingPtr mostRecentMapping;
+        bool hasMostRecentMapping;
+
+        // Offset in absolute coordinates of the compositedLayerMapping's owning layer.
+        IntPoint offsetFromAbsolute;
+
+        // Counter that tracks what index the next RenderLayer would be if it gets squashed to the current squashing layer.
+        size_t nextSquashedLayerIndex;
+    };
+
     // 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*);
 
     // Returns true if any layer's compositing changed
     void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap*, struct CompositingRecursionData&, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants);
 
     // Defines which RenderLayers will paint into which composited backings, by allocating and destroying CompositedLayerMappings as needed.
     void assignLayersToBackings(RenderLayer*, bool& layersChanged);
-    void assignLayersToBackingsInternal(RenderLayer*, bool& layersChanged);
+    void assignLayersToBackingsInternal(RenderLayer*, SquashingState&, bool& layersChanged);
 
     // 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*);
 
     // Hook compositing layers together
     void setCompositingParent(RenderLayer* childLayer, RenderLayer* parentLayer);
     void removeCompositedChildren(RenderLayer*);
@@ skipping 92 matching lines @@
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h