Reconstruct customscrollbars if owningLayoutObject changes.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.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 998 matching lines @@
 
     return layoutObject;
 }
 
 bool PaintLayerScrollableArea::needsScrollbarReconstruction() const
 {
     const LayoutObject& actualLayoutObject = layoutObjectForScrollbar(box());
     bool shouldUseCustom = actualLayoutObject.isBox() && actualLayoutObject.styleRef().hasPseudoStyle(SCROLLBAR);
     bool hasAnyScrollbar = hasScrollbar();
     bool hasCustom = (hasHorizontalScrollbar() && horizontalScrollbar()->isCustomScrollbar()) || (hasVerticalScrollbar() && verticalScrollbar()->isCustomScrollbar());
-    return hasAnyScrollbar && (shouldUseCustom != hasCustom);
+    bool didCustomScrollbarOwnerChanged = false;
+
+    if (hasHorizontalScrollbar() && horizontalScrollbar()->isCustomScrollbar()) {
+        if (actualLayoutObject != toLayoutScrollbar(horizontalScrollbar())->owningLayoutObject())
+            didCustomScrollbarOwnerChanged = true;
+    }
+
+    if (hasVerticalScrollbar() && verticalScrollbar()->isCustomScrollbar()) {
+        if (actualLayoutObject != toLayoutScrollbar(verticalScrollbar())->owningLayoutObject())
+            didCustomScrollbarOwnerChanged = true;
+    }
+
+    return hasAnyScrollbar && ((shouldUseCustom != hasCustom) || (shouldUseCustom && didCustomScrollbarOwnerChanged));
 }
 
 void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar)
 {
     if (hasScrollbar == hasHorizontalScrollbar())
         return;
 
     if (!hasScrollbar && !layerForHorizontalScrollbar())
         horizontalScrollbar()->invalidate();
 
@@ skipping 532 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink