Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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

 /*
  * Copyright (c) 2011, 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 41 matching lines @@
     : ScrollAnimatorBase(scrollableArea)
     , m_timeFunction(timeFunction)
 {
 }
 
 ScrollAnimator::~ScrollAnimator()
 {
     cancelAnimations();
 }
 
+float ScrollAnimator::usedScrollDelta(ScrollbarOrientation orientation, float pixelDelta) const

[bokan, 2015/12/07 18:43:17]

I'd make this "computeTargetPosition" and use it in this and other scroll
animators' ::userScroll method so that we're not duplicating this code (if
someone changes the computation in userScroll they'd likely forget to update
this). You can then compare this to the animator's current position in
ScrollableArea::userScroll to get the delta.

[bokan, 2015/12/07 19:52:54]

ok, delta is fine, but can we call it 'computeDeltaToConsume' or something more
descriptive? imo, usedScrollDelta is a bit confusing.

[ymalik, 2015/12/07 20:52:33]

Renamed.

+{
+    FloatPoint pos = m_animationCurve
+        ? FloatPoint(m_animationCurve->targetValue()) : currentPosition();
+    float currentPos = (orientation == HorizontalScrollbar) ? pos.x() : pos.y();
+    float newPos = clampScrollPosition(orientation, currentPos + pixelDelta);
+    return (currentPos == newPos) ? 0.0f : (newPos - currentPos);
+}
+
 ScrollResultOneDimensional ScrollAnimator::userScroll(ScrollbarOrientation orientation, ScrollGranularity granularity, float step, float delta)
 {
     if (!m_scrollableArea->scrollAnimatorEnabled())
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
     TRACE_EVENT0("blink", "ScrollAnimator::scroll");
 
     if (granularity == ScrollByPrecisePixel)
         return ScrollAnimatorBase::userScroll(orientation, granularity, step, delta);
 
@@ skipping 77 matching lines @@
     TRACE_EVENT0("blink", "ScrollAnimator::notifyPositionChanged");
     notifyPositionChanged();
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink