Exclude cases of full repaint on viewport height change

Source/core/rendering/RenderView.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 183 matching lines @@
     // Otherwise, per-descendant repaint is more likely to avoid unnecessary full repaints.
 
     if (shouldUsePrintingLayout())
         return true;
 
     if (!style()->isHorizontalWritingMode() || width() != viewWidth())
         return true;
 
     if (height() != viewHeight()) {
         if (RenderObject* backgroundRenderer = this->backgroundRenderer()) {
-            if (backgroundRenderer->mustRepaintFillLayersOnHeightChange(*backgroundRenderer->style()->backgroundLayers()))
+            // When background-attachment is 'fixed', we treat the viewport (instead of the 'root'
+            // i.e. html or body) as the background positioning area, and we should full repaint
+            // viewport resize if the background image is not composited and needs full repaint on
+            // background positioning area resize.
+            if (!m_compositor || !m_compositor->needsFixedRootBackgroundLayer(layer())) {
+                if (backgroundRenderer->style()->hasFixedBackgroundImage()
+                    && mustRepaintFillLayersOnHeightChange(*backgroundRenderer->style()->backgroundLayers()))
                 return true;
+            }
         }
     }
 
     return false;
 }
 
 void RenderView::layout()
 {
     if (!document().paginated())
         setPageLogicalHeight(0);
@@ skipping 777 matching lines @@
     return viewWidth(IncludeScrollbars) / scale;
 }
 
 double RenderView::layoutViewportHeight() const
 {
     float scale = m_frameView ? m_frameView->frame().pageZoomFactor() : 1;
     return viewHeight(IncludeScrollbars) / scale;
 }
 
 } // namespace WebCore