Fix scroll origin, overflow rects, and coordinate flipping for flexbox.

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 110 matching lines @@
     ScrollAnimatorBase& scrollAnimator() const;
 
     // This getter will return null if the ScrollAnimatorBase hasn't been created yet.
     ScrollAnimatorBase* existingScrollAnimator() const { return m_scrollAnimator; }
 
     ProgrammaticScrollAnimator& programmaticScrollAnimator() const;
     ProgrammaticScrollAnimator* existingProgrammaticScrollAnimator() const { return m_programmaticScrollAnimator; }
 
     virtual CompositorAnimationTimeline* compositorAnimationTimeline() const { return nullptr; }
 
+    // See Source/core/layout/README.md for an explanation of scroll origin.
     const IntPoint& scrollOrigin() const { return m_scrollOrigin; }
     bool scrollOriginChanged() const { return m_scrollOriginChanged; }
 
 
     // This is used to determine whether the incoming fractional scroll offset should
     // be truncated to integer. Current rule is that if preferCompositingToLCDTextEnabled()
     // is disabled (which is true on low-dpi device by default) we should do the truncation.
     // The justification is that non-composited elements using fractional scroll offsets
     // is causing too much nasty bugs but does not add too benefit on low-dpi devices.
     virtual bool shouldUseIntegerScrollOffset() const { return !RuntimeEnabledFeatures::fractionalScrollOffsetsEnabled(); }
@@ skipping 195 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace blink
 
 #endif // ScrollableArea_h