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();
 }
 
+FloatPoint ScrollAnimator::desiredTargetPosition() const
+{
+    return m_animationCurve ? FloatPoint(m_animationCurve->targetValue()) : currentPosition();
+}
+
+float ScrollAnimator::computeDeltaToConsume(ScrollbarOrientation orientation, float pixelDelta) const
+{
+    FloatPoint pos = desiredTargetPosition();
+    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);
 
-    float pixelDelta = step * delta;
-    FloatPoint desiredDelta = (orientation == VerticalScrollbar ? FloatPoint(0, pixelDelta) : FloatPoint(pixelDelta, 0));
+    float usedPixelDelta = computeDeltaToConsume(orientation, step * delta);
+    FloatPoint pixelDelta = (orientation == VerticalScrollbar ? FloatPoint(0, usedPixelDelta) : FloatPoint(usedPixelDelta, 0));
 
-    FloatPoint targetPos = m_animationCurve ? FloatPoint(m_animationCurve->targetValue()) : currentPosition();
-    targetPos.moveBy(desiredDelta);
-    targetPos = FloatPoint(m_scrollableArea->clampScrollPosition(targetPos));
+    FloatPoint targetPos = desiredTargetPosition();
+    targetPos.moveBy(pixelDelta);
 
     if (m_animationCurve) {
         if (!(targetPos - m_animationCurve->targetValue()).isZero())
             m_animationCurve->updateTarget(m_timeFunction() - m_startTime, targetPos);
         return ScrollResultOneDimensional(/* didScroll */ true, /* unusedScrollDelta */ 0);
     }
 
     if ((targetPos - currentPosition()).isZero()) {
         // Report unused delta only if there is no animation and we are not
         // starting one. This ensures we latch for the duration of the
         // animation rather than animating multiple scrollers at the same time.
-        return ScrollResultOneDimensional(/* didScroll */ false, pixelDelta);
+        return ScrollResultOneDimensional(/* didScroll */ false, delta);
     }
 
     m_animationCurve = adoptPtr(Platform::current()->compositorSupport()->createScrollOffsetAnimationCurve(
         targetPos,
         WebCompositorAnimationCurve::TimingFunctionTypeEaseInOut,
         WebScrollOffsetAnimationCurve::ScrollDurationConstant));
 
     m_animationCurve->setInitialValue(currentPosition());
     m_startTime = m_timeFunction();
 
@@ skipping 49 matching lines @@
     TRACE_EVENT0("blink", "ScrollAnimator::notifyPositionChanged");
     notifyPositionChanged();
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink