Fix bug in scrollParent

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 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 1734 matching lines @@
     // ancestor, and we need not be composited. If, on the other hand, we reside in a z-order
     // list, and on our walk upwards to our scrolling ancestor we find no layer that is a stacking
     // context, then we know that in the stacking tree, we will not be in the subtree rooted at
     // our scrolling ancestor, and we will therefore not scroll with it. In this case, we must
     // be a composited layer since the compositor will need to take special measures to ensure
     // that we scroll with our scrolling ancestor and it cannot do this if we do not promote.
     if (m_ancestorDependentPropertyCache && !m_ancestorDependentPropertyCache->scrollParentDirty())
         return m_ancestorDependentPropertyCache->scrollParent();
 
     RenderLayer* scrollParent = ancestorCompositedScrollingLayer();
-    if (!scrollParent || scrollParent->stackingNode()->isStackingContainer())
+    if (!scrollParent || scrollParent->stackingNode()->isStackingContainer()) {
+        if (m_ancestorDependentPropertyCache)
+            m_ancestorDependentPropertyCache->setScrollParent(0);
         return 0;
+    }
 
     // If we hit a stacking context on our way up to the ancestor scrolling layer, it will already
     // be composited due to an overflow scrolling parent, so we don't need to.
     for (RenderLayer* ancestor = parent(); ancestor && ancestor != scrollParent; ancestor = ancestor->parent()) {
-        if (ancestor->stackingNode()->isStackingContainer())
-            return 0;
+        if (ancestor->stackingNode()->isStackingContainer()) {
+            scrollParent = 0;
+            break;
+        }
         if (!isInCompositingUpdate())
             continue;
         if (AncestorDependentPropertyCache* ancestorCache = ancestor->m_ancestorDependentPropertyCache.get()) {
             if (!ancestorCache->ancestorCompositedScrollingLayerDirty() && ancestorCache->ancestorCompositedScrollingLayer() == scrollParent) {
                 scrollParent = ancestorCache->scrollParent();
                 break;
             }
         }
     }
 
@@ skipping 2402 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif