Set up clip and scroll parents on the blink side.

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index bee43c6984f7c488a0e866d41e5c44673a86f2c0..74f13a50428ccbced6f0016545252a2a968d2d71 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -1751,7 +1751,11 @@ bool RenderLayerCompositor::clippedByAncestor(RenderLayer* layer) const
     if (!layer->isComposited() || !layer->parent())
         return false;
 
-    RenderLayer* compositingAncestor = layer->ancestorCompositingLayer();
+    // Scroll children use their scrolling ancestor as their clip root.
+    RenderLayer* compositingAncestor = clippedByScrollingAncestor(layer)

[enne (OOO), 2013/08/22 20:54:30]

clippedByScrollingAncestor seems like a bunch of work to repeat in
clippedByAncestor and then immediately again by the caller of clippedByAncestor.
 Can this be simplified?

+        ? layer->ancestorScrollingLayer()
+        : layer->ancestorCompositingLayer();
+
     if (!compositingAncestor)
         return false;
 
@@ -1775,6 +1779,31 @@ bool RenderLayerCompositor::clippedByAncestor(RenderLayer* layer) const
     return layer->backgroundClipRect(RenderLayer::ClipRectsContext(computeClipRoot, 0, TemporaryClipRects)).rect() != PaintInfo::infiniteRect(); // FIXME: Incorrect for CSS regions.
 }
 
+bool RenderLayerCompositor::clippedByScrollingAncestor(RenderLayer* layer) const
+{
+    if (!layer->isComposited() || !layer->parent())
+        return false;
+
+    if (!requiresCompositingForOverflowScrollingParent(layer))
+        return false;
+
+    RenderLayer* compositingAncestor = layer->ancestorCompositingLayer();
+    if (!compositingAncestor)
+        return false;
+
+    RenderLayer* scrollingAncestor = layer->ancestorScrollingLayer();
+    if (!scrollingAncestor)
+        return false;
+
+    if (!scrollingAncestor->hasAncestor(compositingAncestor))
+        return false;
+
+    // FIXME: Incorrect for CSS regions.
+    RenderLayer::ClipRectsContext clipRectsContext(compositingAncestor, 0, TemporaryClipRects, IgnoreOverlayScrollbarSize, IgnoreOverflowClip, scrollingAncestor);
+    LayoutRect clipRect = layer->backgroundClipRect(clipRectsContext).rect();
+    return clipRect != PaintInfo::infiniteRect() && !clipRect.isEmpty();
+}
+
 // Return true if the given layer is a stacking context and has compositing child
 // layers that it needs to clip. In this case we insert a clipping GraphicsLayer
 // into the hierarchy between this layer and its children in the z-order hierarchy.