Only update composited scrolling state when necessary

Source/WebCore/rendering/RenderView.h

Index: Source/WebCore/rendering/RenderView.h
diff --git a/Source/WebCore/rendering/RenderView.h b/Source/WebCore/rendering/RenderView.h
index 33d8e3c4ca249b06004e92c0a54b31dbd826e126..b69cb47fd50a89c4ebe0a317af6a421a25578976 100644
--- a/Source/WebCore/rendering/RenderView.h
+++ b/Source/WebCore/rendering/RenderView.h
@@ -39,8 +39,28 @@ class RenderWidget;
 class CustomFilterGlobalContext;
 #endif
 
+enum CompositedScrollingUpdateMode {
+    UpdateNeedsCompositedScrolling,
+    UpdateDescendantsAreContiguousInStackingOrder
+};
+
+// This object allows postponing calls to RenderLayer::updateNeedsCompositedScrolling
+// while any instance is in scope. When the last instance is destructed, all
+// updates are processed. This allows us to postpone updates during layer list
+// updates. (If we didn't postpone the updates, they would cause errors since
+// they require clean lists).
+struct ScopedCompositedScrollingUpdater {

[jamesr, 2013/04/09 21:10:00]

in particular this class seems very hacky

+    ScopedCompositedScrollingUpdater(RenderView*);
+    ScopedCompositedScrollingUpdater(RenderLayer*, CompositedScrollingUpdateMode);
+    ~ScopedCompositedScrollingUpdater();
+private:
+    RenderView* m_renderView;
+};
+
 class RenderView : public RenderBlock {
 public:
+    friend struct ScopedCompositedScrollingUpdater;
+
     explicit RenderView(Document*);
     virtual ~RenderView();
 
@@ -214,7 +234,9 @@ public:
     void addRenderCounter() { m_renderCounterCount++; }
     void removeRenderCounter() { ASSERT(m_renderCounterCount > 0); m_renderCounterCount--; }
     bool hasRenderCounters() { return m_renderCounterCount; }
-    
+
+    void cancelScheduledCompositedScrollingUpdatesForLayer(RenderLayer*);
+
     virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0) OVERRIDE;
 
 protected:
@@ -316,6 +338,11 @@ private:
     LayoutState* m_layoutState;
     unsigned m_layoutStateDisableCount;
     OwnPtr<RenderLayerCompositor> m_compositor;
+
+    unsigned m_scopedCompositedScrollingUpdatersCount;
+    HashSet<RenderLayer*> m_pendingCompositedScrollingUpdates;
+    HashSet<RenderLayer*> m_pendingContiguityUpdates;
+
 #if ENABLE(CSS_SHADERS) && USE(3D_GRAPHICS)
     OwnPtr<CustomFilterGlobalContext> m_customFilterGlobalContext;
 #endif