NOT FOR REVIEW - Update comp-scrolling state at a well defined point in the pipeline.

Source/core/page/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 692 matching lines @@
             applyOverflowToViewport(rootRenderer, hMode, vMode);
     }    
 }
 
 void FrameView::updateCompositingLayersAfterStyleChange()
 {
     RenderView* renderView = this->renderView();
     if (!renderView)
         return;
 
+    // PATCH 2
+    // Catch promotions due to style changes (e.g., z-order changes).
+    renderView->compositor()->updateCompositingRequirementsState(CompositingUpdateAfterStyleChange);
+
     // If we expect to update compositing after an incipient layout, don't do so here.
     if (m_doingPreLayoutStyleUpdate || layoutPending() || renderView->needsLayout())
         return;
 
     // This call will make sure the cached hasAcceleratedCompositing is updated from the pref
     renderView->compositor()->cacheAcceleratedCompositingFlags();
     renderView->compositor()->updateCompositingLayers(CompositingUpdateAfterStyleChange);
 }
 
 void FrameView::updateCompositingLayersAfterLayout()
 {
     RenderView* renderView = this->renderView();
     if (!renderView)
         return;
 
     // This call will make sure the cached hasAcceleratedCompositing is updated from the pref
     renderView->compositor()->cacheAcceleratedCompositingFlags();
+    // PATCH 2
+    // Catch promotions due to layout changes (e.g., content now overflows).
+    renderView->compositor()->updateCompositingRequirementsState(CompositingUpdateAfterLayout);
     renderView->compositor()->updateCompositingLayers(CompositingUpdateAfterLayout);
 }
 
 bool FrameView::usesCompositedScrolling() const
 {
     RenderView* renderView = this->renderView();
     if (!renderView)
         return false;
     if (m_frame->settings() && m_frame->settings()->compositedScrollingForFramesEnabled())
         return renderView->compositor()->inForcedCompositingMode();
@@ skipping 2798 matching lines @@
 }
 
 AXObjectCache* FrameView::axObjectCache() const
 {
     if (frame() && frame()->document())
         return frame()->document()->existingAXObjectCache();
     return 0;
 }
     
 } // namespace WebCore