Update RootFrameViewport::userScroll to distribute scrolls between viewports.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm

 /*
  * Copyright (C) 2010, 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 724 matching lines @@
                                                .scrollAnimationEnabledForSystem();
     m_haveScrolledSincePageLoad = true;
 
     if (!scrollAnimationEnabledForSystem || !m_scrollableArea->scrollAnimatorEnabled())
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
     if (granularity == ScrollByPixel || granularity == ScrollByPrecisePixel)
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
     float currentPos = orientation == HorizontalScrollbar ? m_currentPosX : m_currentPosY;
-    float newPos = std::max<float>(std::min<float>(currentPos + (step * delta), m_scrollableArea->maximumScrollPosition(orientation)), m_scrollableArea->minimumScrollPosition(orientation));
+    float usedPixelDelta = computeDeltaToConsume(orientation, step * delta);
+    float newPos = currentPos + usedPixelDelta;
     if (currentPos == newPos)
         return ScrollResultOneDimensional(false);
 
     NSPoint newPoint;
     if ([m_scrollAnimationHelper.get() _isAnimating]) {
         NSPoint targetOrigin = [m_scrollAnimationHelper.get() targetOrigin];
         newPoint = orientation == HorizontalScrollbar ? NSMakePoint(newPos, targetOrigin.y) : NSMakePoint(targetOrigin.x, newPos);
     } else
         newPoint = orientation == HorizontalScrollbar ? NSMakePoint(newPos, m_currentPosY) : NSMakePoint(m_currentPosX, newPos);
 
     [m_scrollAnimationHelper.get() scrollToPoint:newPoint];
 
-    float usedDelta = (newPos - currentPos) / step;
-    return ScrollResultOneDimensional(true, delta - usedDelta);
+    return ScrollResultOneDimensional(true, delta - (usedPixelDelta / step));
 }
 
 void ScrollAnimatorMac::scrollToOffsetWithoutAnimation(const FloatPoint& offset)
 {
     [m_scrollAnimationHelper.get() _stopRun];
     immediateScrollTo(offset);
 }
 
 FloatPoint ScrollAnimatorMac::adjustScrollPositionIfNecessary(const FloatPoint& position) const
 {
@@ skipping 431 matching lines @@
         return;
 
     m_visibleScrollerThumbRect = rectInViewCoordinates;
 }
 
 bool ScrollAnimatorMac::canUseCoordinatedScrollbar() {
     return ScrollbarThemeMacCommon::isOverlayAPIAvailable();
 }
 
 } // namespace blink