Only update composited scrolling state when necessary

Source/WebCore/rendering/RenderLayer.h

Index: Source/WebCore/rendering/RenderLayer.h
diff --git a/Source/WebCore/rendering/RenderLayer.h b/Source/WebCore/rendering/RenderLayer.h
index 21a6c919dc01fd742dd8ee8f5dc6b7e0f7ca60b8..d77dca4eb11342f1ee9eaea0ba555f3d920560a0 100644
--- a/Source/WebCore/rendering/RenderLayer.h
+++ b/Source/WebCore/rendering/RenderLayer.h
@@ -315,6 +315,7 @@ typedef Vector<LayerFragment, 1> LayerFragments;
 class RenderLayer : public ScrollableArea {
 public:
     friend class RenderReplica;
+    friend struct ScopedCompositedScrollingUpdater;
 
     RenderLayer(RenderLayerModelObject*);
     ~RenderLayer();
@@ -497,10 +498,18 @@ public:
     // layer itself.
     RenderLayer* stackingContainer() const;
 
+    // Gets the enclosing stacking context for this layer, excluding this
+    // layer itself.
+    RenderLayer* stackingContext() const;
+
     // Gets the enclosing stacking container for this layer, possibly the layer
     // itself, if it is a stacking container.
     RenderLayer* enclosingStackingContainer() { return isStackingContainer() ? this : stackingContainer(); }
 
+    // Gets the enclosing stacking context for this layer, possibly the layer
+    // itself, if it is a stacking context.
+    RenderLayer* enclosingStackingContext() { return isStackingContext() ? this : stackingContext(); }
+
     void dirtyZOrderLists();
     void dirtyStackingContainerZOrderLists();
 
@@ -866,6 +875,8 @@ private:
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
+    void updateIsNormalFlowOnly();
+    void updateVisibilityAfterStyleChange(const RenderStyle* oldStyle);
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
@@ -876,6 +887,11 @@ private:
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void updateNeedsCompositedScrolling();
+    void setNeedsCompositedScrolling(bool);
+
+    void requestNeedsCompositedScrollingUpdate();
+    void requestDescendantsAreContiguousInStackingOrderUpdate();
+    void cancelScheduledCompositedScrollingUpdates() const;
 
     // Returns true if the position changed.
     bool updateLayerPosition();
@@ -1151,6 +1167,7 @@ protected:
     // descendants in stacking order. This is one of the requirements of being
     // able to safely become a stacking context.
     bool m_descendantsAreContiguousInStackingOrder : 1;
+    bool m_descendantsAreContiguousInStackingOrderDirty : 1;
 
     const bool m_isRootLayer : 1;
 
@@ -1272,20 +1289,6 @@ inline void RenderLayer::clearZOrderLists()
     m_negZOrderList.clear();
 }
 
-inline void RenderLayer::updateZOrderLists()
-{
-    if (!m_zOrderListsDirty)
-        return;
-
-    if (!isStackingContainer()) {
-        clearZOrderLists();
-        m_zOrderListsDirty = false;
-        return;
-    }
-
-    rebuildZOrderLists();
-}
-
 #if !ASSERT_DISABLED
 class LayerListMutationDetector {
 public: