Position overflow controls during layout.

Source/core/rendering/RenderLayerScrollableArea.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 632 matching lines @@
         if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar()) {
             int clientWidth = box().pixelSnappedClientWidth();
             horizontalScrollbar->setProportion(clientWidth, overflowRect().width());
         }
         if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
             int clientHeight = box().pixelSnappedClientHeight();
             verticalScrollbar->setProportion(clientHeight, overflowRect().height());
         }
     }
 
-    updateScrollableAreaSet(hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow());
+    bool hasOverflow = hasScrollableHorizontalOverflow() || hasScrollableVerticalOverflow();
+    updateScrollableAreaSet(hasOverflow);
+
+    if (hasOverflow) {
+        DisableCompositingQueryAsserts disabler;
+        positionOverflowControls(IntSize());
+    }
 }
 
 bool RenderLayerScrollableArea::hasHorizontalOverflow() const
 {
     ASSERT(!m_scrollDimensionsDirty);
 
     return pixelSnappedScrollWidth() > box().pixelSnappedClientWidth();
 }
 
 bool RenderLayerScrollableArea::hasVerticalOverflow() const
@@ skipping 787 matching lines @@
 void RenderLayerScrollableArea::setTopmostScrollChild(RenderLayer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink