RenderLayer::scrollParent should walk the containing block chain

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index be92d92417e0a84c5a96345eb64f1c16ec420a37..e399a93bd5506b537d5f783e25f74e61d0253d63 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1082,26 +1082,6 @@ RenderLayer* RenderLayer::enclosingCompositingLayerForRepaint(IncludeSelfOrNot i
     return 0;
 }
 
-RenderLayer* RenderLayer::ancestorCompositedScrollingLayer() const
-{
-    if (!compositor()->acceleratedCompositingForOverflowScrollEnabled())
-        return 0;
-
-    RenderObject* containingBlock = renderer()->containingBlock();
-    if (!containingBlock)
-        return 0;
-
-    RenderLayer* ancestorCompositedScrollingLayer = 0;
-    for (RenderLayer* ancestorLayer = containingBlock->enclosingLayer(); ancestorLayer; ancestorLayer = ancestorLayer->parent()) {
-        if (ancestorLayer->needsCompositedScrolling()) {
-            ancestorCompositedScrollingLayer = ancestorLayer;
-            break;
-        }
-    }
-
-    return ancestorCompositedScrollingLayer;
-}
-
 RenderLayer* RenderLayer::ancestorScrollingLayer() const
 {
     RenderObject* containingBlock = renderer()->containingBlock();
@@ -1602,33 +1582,15 @@ RenderLayer* RenderLayer::scrollParent() const
     if (stackingNode()->isNormalFlowOnly())
         return 0;
 
-    // A layer scrolls with its containing block. So to find the overflow scrolling layer
-    // that we scroll with respect to, we must ascend the layer tree until we reach the
-    // first overflow scrolling div at or above our containing block. I will refer to this
-    // layer as our 'scrolling ancestor'.
-    //
-    // Now, if we reside in a normal flow list, then we will naturally scroll with our scrolling
-    // 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.
-
-    RenderLayer* scrollParent = ancestorCompositedScrollingLayer();
-    if (!scrollParent || scrollParent->stackingNode()->isStackingContext())
+    if (!parent())
         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()->isStackingContext()) {
-            scrollParent = 0;
-            break;
-        }
+    for (RenderLayer* current = renderer()->containingBlock()->enclosingLayer(); !current->stackingNode()->isStackingContext(); current = current->renderer()->containingBlock()->enclosingLayer()) {
+        if (current->scrollsOverflow())
+            return current;
     }
 
-    return scrollParent;
+    return 0;
 }
 
 RenderLayer* RenderLayer::clipParent() const