Clear scrollbar damages in time

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 189 matching lines @@
     if (scrollRect.isEmpty())
         return;
 
     LayoutRect repaintRect = scrollRect;
     box().flipForWritingMode(repaintRect);
 
     IntRect intRect = pixelSnappedIntRect(repaintRect);
 
     if (box().frameView()->isInPerformLayout()) {
         if (scrollbar == m_vBar.get()) {
-            m_verticalBarDamage = intRect;
+            m_verticalBarDamage.unite(intRect);
             m_hasVerticalBarDamage = true;
         } else {
-            m_horizontalBarDamage = intRect;
+            m_horizontalBarDamage.unite(intRect);
             m_hasHorizontalBarDamage = true;
         }
-
+        box().setMayNeedPaintInvalidation(true);
     } else {
         box().invalidatePaintRectangle(intRect);
     }
 }
 
 void RenderLayerScrollableArea::invalidateScrollCornerRect(const IntRect& rect)
 {
     if (GraphicsLayer* layer = layerForScrollCorner()) {
         layer->setNeedsDisplayInRect(rect);
         return;
@@ skipping 1240 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 WebCore