Allow layers that need squashing to draw into the squashing layer in some cases.

Source/core/rendering/compositing/CompositedLayerMapping.h

 /*
  * 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 33 matching lines @@
     RenderLayer* renderLayer;
 
     LayoutRect compositedBounds;
 
     // At first, the m_squashingLayer's bounds/location are not known. The value offsetFromSquashingCLM is
     // an intermediate offset for a squashed RenderLayer, described with respect to the CompositedLayerMapping's
     // owning layer that would eventually have the m_squashingLayer. Once the shared GraphicsLayer's bounds are
     // known, then we can trivially convert this offset to m_squashingLayer's space.
     LayoutSize offsetFromSquashingCLM;
 
+    // The clip rect to apply, in the local coordinate space of the squashed layer, when painting it.
+    IntRect localClipRectForSquashedLayer;
+
     // Offset describing where this squashed RenderLayer paints into the shared GraphicsLayer backing.
     IntSize offsetFromRenderer;
     LayoutSize subpixelAccumulation;
 
     GraphicsLayerPaintingPhase paintingPhase;
 
     bool isBackgroundLayer;
 
     bool isEquivalentForSquashing(const GraphicsLayerPaintInfo& other)
     {
@@ skipping 134 matching lines @@
 
     virtual String debugName(const GraphicsLayer*) OVERRIDE;
 
     LayoutSize contentOffsetInCompositingLayer() const;
 
     LayoutPoint squashingOffsetFromTransformedAncestor()
     {
         return m_squashingLayerOffsetFromTransformedAncestor;
     }
 
+    // If there is a squashed layer painting into this CLM that is an ancestor of the given RenderObject, return it. Otherwise return 0.
+    const GraphicsLayerPaintInfo* containingSquashedLayer(const RenderObject*) const;
+
 private:
     void createPrimaryGraphicsLayer();
     void destroyGraphicsLayers();
 
     PassOwnPtr<GraphicsLayer> createGraphicsLayer(CompositingReasons);
     bool toggleScrollbarLayerIfNeeded(OwnPtr<GraphicsLayer>&, bool needsLayer, CompositingReasons);
 
     RenderLayerModelObject* renderer() const { return m_owningLayer.renderer(); }
     RenderLayerCompositor* compositor() const { return m_owningLayer.compositor(); }
 
@@ skipping 51 matching lines @@
     void updateContentsRect();
     void updateAfterWidgetResize();
     void updateCompositingReasons();
 
     bool hasVisibleNonCompositingDescendantLayers() const;
 
     void paintsIntoCompositedAncestorChanged();
 
     void doPaintTask(GraphicsLayerPaintInfo&, GraphicsContext*, const IntRect& clip);
 
+    // Computes the background clip rect for the given squashed layer, up to any containing layer that is squashed into the
+    // same squashing layer and contains this squashed layer's clipping ancestor.
+    // The clip rect is returned in the coordinate space of the given squashed layer.
+    // If there is no such containing layer, returns the infinite rect.
+    // FIXME: unify this code with the code that sets up m_ancestorClippingLayer. They are doing very similar things.
+    IntRect localClipRectForSquashedLayer(const GraphicsLayerPaintInfo&) const;
+
     RenderLayer& m_owningLayer;
 
     // The hierarchy of layers that is maintained by the CompositedLayerMapping looks like this:
     //
     //  + m_ancestorClippingLayer [OPTIONAL]
     //     + m_graphicsLayer
     //        + m_childContainmentLayer [OPTIONAL] <-OR-> m_scrollingLayer [OPTIONAL] <-OR-> m_childTransformLayer
     //                                                     + m_scrollingContentsLayer [Present iff m_scrollingLayer is present]
     //
     // We need an ancestor clipping layer if our clipping ancestor is not our ancestor in the
@@ skipping 84 matching lines @@
     bool m_requiresOwnBackingStoreForAncestorReasons : 1;
     bool m_canCompositeFilters : 1;
     bool m_backgroundLayerPaintsFixedRootBackground : 1;
     bool m_needToUpdateGraphicsLayer : 1;
     bool m_needToUpdateGraphicsLayerOfAllDecendants : 1;
 };
 
 } // namespace WebCore
 
 #endif // CompositedLayerMapping_h