Don't clip invalidation of composited scrolling contents

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. 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
@@ skipping 856 matching lines @@
 }
 
 void RenderBox::applyCachedClipAndScrollOffsetForPaintInvalidation(LayoutRect& paintRect) const
 {
     ASSERT(hasLayer());
     ASSERT(hasOverflowClip());
 
     flipForWritingMode(paintRect);
     paintRect.move(-scrolledContentOffset()); // For overflow:auto/scroll/hidden.
 
-    // Do not clip scroll layer contents to reduce the number of paint invalidations while scrolling.
+    // Do not clip scroll layer contents because the compositor expects the whole layer
+    // to be always invalidated in-time.
     if (usesCompositedScrolling()) {
         flipForWritingMode(paintRect);
         return;
     }
 
     // height() is inaccurate if we're in the middle of a layout of this RenderBox, so use the
     // layer's size instead. Even if the layer's size is wrong, the layer itself will issue paint invalidations
     // anyway if its size does change.
     LayoutRect clipRect(LayoutPoint(), layer()->size());
     paintRect = intersection(paintRect, clipRect);
@@ skipping 3632 matching lines @@
     ASSERT(style()->hasBackground() || style()->hasBoxDecorations());
 
     if (m_rareData && m_rareData->m_previousBorderBoxSize.width() != -1)
         return m_rareData->m_previousBorderBoxSize;
 
     // We didn't save the old border box size because it was the same as the size of oldBounds.
     return previousBoundsSize;
 }
 
 } // namespace blink