Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
index a6dc9bffc3c47ebd627718a9fbf18deb93ab410c..3a795584d4ca0355b0694de10fcc9ff93bd3c419 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
@@ -59,6 +59,15 @@ 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())