Rework composited scrolling updates.

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 e5fc1d71d9c3cc0d11508317eb7736991ae687af..6e5c0579da606dd0913c1ae3af033db5285698a9 100644
--- a/Source/WebCore/rendering/RenderLayer.h
+++ b/Source/WebCore/rendering/RenderLayer.h
@@ -491,11 +491,11 @@ public:
     // stacking containers, but the converse is not true. Layers that use
     // composited scrolling are stacking containers, but they may not
     // necessarily be stacking contexts.
-    bool isStackingContainer() const { return isStackingContext() || needsCompositedScrolling(); }
+    bool isStackingContainer(bool allowDirty = false) const { return isStackingContext() || needsCompositedScrolling(allowDirty); }
 
     // Gets the enclosing stacking container for this layer, excluding this
     // layer itself.
-    RenderLayer* stackingContainer() const;
+    RenderLayer* stackingContainer(bool allowDirty = false) const;
 
     // Gets the enclosing stacking container for this layer, possibly the layer
     // itself, if it is a stacking container.
@@ -507,6 +507,8 @@ public:
     Vector<RenderLayer*>* posZOrderList() const
     {
         ASSERT(!m_zOrderListsDirty);
+        ASSERT(!m_hasOutOfFlowPositionedDescendantDirty);
+        ASSERT(!m_descendantsAreContiguousInStackingOrderDirty);
         ASSERT(isStackingContainer() || !m_posZOrderList);
         return m_posZOrderList.get();
     }
@@ -516,6 +518,8 @@ public:
     Vector<RenderLayer*>* negZOrderList() const
     {
         ASSERT(!m_zOrderListsDirty);
+        ASSERT(!m_hasOutOfFlowPositionedDescendantDirty);
+        ASSERT(!m_descendantsAreContiguousInStackingOrderDirty);
         ASSERT(isStackingContainer() || !m_negZOrderList);
         return m_negZOrderList.get();
     }
@@ -757,7 +761,7 @@ public:
     virtual GraphicsLayer* layerForVerticalScrollbar() const;
     virtual GraphicsLayer* layerForScrollCorner() const;
     virtual bool usesCompositedScrolling() const OVERRIDE;
-    bool needsCompositedScrolling() const;
+    bool needsCompositedScrolling(bool allowDirty = false) const;
     bool needsCompositingLayersRebuiltForClip(const RenderStyle* oldStyle, const RenderStyle* newStyle) const;
     bool needsCompositingLayersRebuiltForOverflow(const RenderStyle* oldStyle, const RenderStyle* newStyle) const;
 #if ENABLE(CSS_FILTERS)
@@ -864,6 +868,7 @@ private:
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
+    void updateIsNormalFlowOnly();
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
@@ -1147,6 +1152,7 @@ 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;