Implement a Hit Test Cache.

Source/core/paint/DeprecatedPaintLayerScrollableArea.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 417 matching lines @@
     // FIXME: This invalidation will be unnecessary in slimming paint phase 2.
     if (requiresPaintInvalidation)
         box().setShouldDoFullPaintInvalidation();
 
     // Schedule the scroll DOM event.
     if (box().node())
         box().node()->document().enqueueScrollEventForNode(box().node());
 
     if (AXObjectCache* cache = box().document().existingAXObjectCache())
         cache->handleScrollPositionChanged(&box());
+    box().view()->clearHitTestCache();
 }
 
 IntPoint DeprecatedPaintLayerScrollableArea::scrollPosition() const
 {
     return IntPoint(flooredIntSize(m_scrollOffset));
 }
 
 DoublePoint DeprecatedPaintLayerScrollableArea::scrollPositionDouble() const
 {
     return DoublePoint(m_scrollOffset);
@@ skipping 950 matching lines @@
 void DeprecatedPaintLayerScrollableArea::setTopmostScrollChild(DeprecatedPaintLayer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink