Update RootFrameViewport::userScroll to distribute scrolls between viewports.

third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.cpp

 /*
  * Copyright (c) 2010, Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
     : m_scrollableArea(scrollableArea)
     , m_currentPosX(0)
     , m_currentPosY(0)
 {
 }
 
 ScrollAnimatorBase::~ScrollAnimatorBase()
 {
 }
 
+float ScrollAnimatorBase::computeDeltaToConsume(ScrollbarOrientation orientation, float pixelDelta) const
+{
+    float currentPos = (orientation == HorizontalScrollbar) ? m_currentPosX : m_currentPosY;
+    float newPos = clampScrollPosition(orientation, currentPos + pixelDelta);
+    return (currentPos == newPos) ? 0.0f : (newPos - currentPos);
+}
+
 ScrollResultOneDimensional ScrollAnimatorBase::userScroll(ScrollbarOrientation orientation, ScrollGranularity, float step, float delta)
 {
     float& currentPos = (orientation == HorizontalScrollbar) ? m_currentPosX : m_currentPosY;
-    float newPos = clampScrollPosition(orientation, currentPos + step * delta);
+    float usedPixelDelta = computeDeltaToConsume(orientation, step * delta);
+    float newPos = currentPos + usedPixelDelta;
     if (currentPos == newPos)
         return ScrollResultOneDimensional(false, delta);
 
-    float usedDelta = (newPos - currentPos) / step;
     currentPos = newPos;
 
     notifyPositionChanged();
 
-    return ScrollResultOneDimensional(true, delta - usedDelta);
+    return ScrollResultOneDimensional(true, delta - (usedPixelDelta / step));
 }
 
 void ScrollAnimatorBase::scrollToOffsetWithoutAnimation(const FloatPoint& offset)
 {
     m_currentPosX = offset.x();
     m_currentPosY = offset.y();
     notifyPositionChanged();
 }
 
 void ScrollAnimatorBase::setCurrentPosition(const FloatPoint& position)
 {
     m_currentPosX = position.x();
     m_currentPosY = position.y();
 }
 
 FloatPoint ScrollAnimatorBase::currentPosition() const
 {
     return FloatPoint(m_currentPosX, m_currentPosY);
 }
 
 void ScrollAnimatorBase::notifyPositionChanged()
 {
     m_scrollableArea->scrollPositionChanged(DoublePoint(m_currentPosX, m_currentPosY), UserScroll);
 }
 
-float ScrollAnimatorBase::clampScrollPosition(ScrollbarOrientation orientation, float pos)
+float ScrollAnimatorBase::clampScrollPosition(ScrollbarOrientation orientation, float pos) const
 {
     float maxScrollPos = m_scrollableArea->maximumScrollPosition(orientation);
     float minScrollPos = m_scrollableArea->minimumScrollPosition(orientation);
     return std::max(std::min(pos, maxScrollPos), minScrollPos);
 }
 
 DEFINE_TRACE(ScrollAnimatorBase)
 {
     visitor->trace(m_scrollableArea);
 }
 
 } // namespace blink