Mark when we may have been invalidated to early out on repaintTreeAfterLayout.

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 332 matching lines @@
 {
     if (!box().isMarquee()) {
         // Ensure that the dimensions will be computed if they need to be (for overflow:hidden blocks).
         if (m_scrollDimensionsDirty)
             computeScrollDimensions();
     }
 
     if (scrollOffset() == toIntSize(newScrollOffset))
         return;
 
+    layer()->renderer()->setMayNeedInvalidation(true);
+
     setScrollOffset(toIntSize(newScrollOffset));
 
     LocalFrame* frame = box().frame();
     ASSERT(frame);
 
     RefPtr<FrameView> frameView = box().frameView();
 
     TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "ScrollLayer", "data", InspectorScrollLayerEvent::data(&box()));
     // FIXME(361045): remove InspectorInstrumentation calls once DevTools Timeline migrates to tracing.
     InspectorInstrumentation::willScrollLayer(&box());
@@ skipping 220 matching lines @@
     if (newScrollOffset != adjustedScrollOffset())
         scrollToOffsetWithoutAnimation(-scrollOrigin() + newScrollOffset);
 }
 
 void RenderLayerScrollableArea::updateAfterLayout()
 {
     // List box parts handle the scrollbars by themselves so we have nothing to do.
     if (box().style()->appearance() == ListboxPart)
         return;
 
+    box().setMayNeedInvalidation(true);
+
     m_scrollDimensionsDirty = true;
     IntSize originalScrollOffset = adjustedScrollOffset();
 
     computeScrollDimensions();
 
     if (!box().isMarquee()) {
         // Layout may cause us to be at an invalid scroll position. In this case we need
         // to pull our scroll offsets back to the max (or push them up to the min).
         IntSize clampedScrollOffset = clampScrollOffset(adjustedScrollOffset());
         if (clampedScrollOffset != adjustedScrollOffset())
@@ skipping 948 matching lines @@
 void RenderLayerScrollableArea::setForceNeedsCompositedScrolling(ForceNeedsCompositedScrollingMode mode)
 {
     if (m_forceNeedsCompositedScrolling == mode)
         return;
 
     m_forceNeedsCompositedScrolling = mode;
     layer()->didUpdateNeedsCompositedScrolling();
 }
 
 } // Namespace WebCore