Only update composited scrolling state when necessary

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 473 matching lines @@
 
     // A stacking context is a layer that has a non-auto z-index.
     bool isStackingContext() const { return isStackingContext(renderer()->style()); }
 
     // A stacking container can have z-order lists. All stacking contexts are
     // 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(); }
 
-    // Gets the enclosing stacking container for this layer, excluding this
-    // layer itself.
-    RenderLayer* stackingContainer() const;
+    RenderLayer* ancestorStackingContainer() const;
+    RenderLayer* ancestorStackingContext() 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(); }
+    RenderLayer* enclosingStackingContainer() { return isStackingContainer() ? this : ancestorStackingContainer(); }
 
     void dirtyZOrderLists();
     void dirtyStackingContainerZOrderLists();
 
     Vector<RenderLayer*>* posZOrderList() const
     {
         ASSERT(!m_zOrderListsDirty);
         ASSERT(isStackingContainer() || !m_posZOrderList);
         return m_posZOrderList.get();
     }
@@ skipping 328 matching lines @@
     void computeRepaintRectsIncludingDescendants();
     void clearRepaintRects();
 
     void clipToRect(RenderLayer* rootLayer, GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&,
                     BorderRadiusClippingRule = IncludeSelfForBorderRadius);
     void restoreClip(GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&);
 
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
+    void updateIsNormalFlowOnly();
+    void updateVisibilityAfterStyleChange(const RenderStyle* oldStyle);
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
     void updateScrollbarsAfterLayout();
 
     void setAncestorChainHasOutOfFlowPositionedDescendant(RenderObject* containingBlock);
     void dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void updateNeedsCompositedScrolling();
@@ skipping 261 matching lines @@
     // stacking context.
     bool m_hasOutOfFlowPositionedDescendant : 1;
     bool m_hasOutOfFlowPositionedDescendantDirty : 1;
 
     bool m_needsCompositedScrolling : 1;
 
     // If this is true, then no non-descendant appears between any of our
     // 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;
 
     bool m_usedTransparency : 1; // Tracks whether we need to close a transparent layer, i.e., whether
                                  // we ended up painting this layer or any descendants (and therefore need to
                                  // blend).
     bool m_paintingInsideReflection : 1;  // A state bit tracking if we are painting inside a replica.
     bool m_inOverflowRelayout : 1;
     unsigned m_repaintStatus : 2; // RepaintStatus
 
@@ skipping 144 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h