Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 112 matching lines @@
         m_scrollOrigin = origin;
         m_scrollOriginChanged = true;
     }
 }
 
 GraphicsLayer* ScrollableArea::layerForContainer() const
 {
     return layerForScrolling() ? layerForScrolling()->parent() : 0;
 }
 
-ScrollResultOneDimensional ScrollableArea::userScroll(ScrollDirectionPhysical direction, ScrollGranularity granularity, float delta)
+ScrollbarOrientation ScrollableArea::scrollbarOrientationFromDirection(ScrollDirectionPhysical direction) const
 {
-    ScrollbarOrientation orientation;
-    if (direction == ScrollUp || direction == ScrollDown)
-        orientation = VerticalScrollbar;
-    else
-        orientation = HorizontalScrollbar;
+    return (direction == ScrollUp  || direction == ScrollDown) ? VerticalScrollbar : HorizontalScrollbar;
+}
 
-    if (!userInputScrollable(orientation))
-        return ScrollResultOneDimensional(false, delta);
-
-    cancelProgrammaticScrollAnimation();
-
+float ScrollableArea::scrollStep(ScrollGranularity granularity, ScrollbarOrientation orientation) const
+{
     float step = 0;
     switch (granularity) {
     case ScrollByLine:
         step = lineStep(orientation);
         break;
     case ScrollByPage:
         step = pageStep(orientation);
         break;
     case ScrollByDocument:
         step = documentStep(orientation);
         break;
     case ScrollByPixel:
     case ScrollByPrecisePixel:
         step = pixelStep(orientation);

[jbroman, 2015/12/09 15:29:35]

nit: This line is unreachable; please remove duplicate copy. (While you're here,
you could consider simply using "return" instead of having an intermediate
variable, too, but I won't push on that.)

[jbroman, 2015/12/09 15:31:22]

If this line isn't duplicated in your code, please ignore. In Rietveld this
renders twice for me, but I now suspect that to be a bug in the UI.

[ymalik, 2015/12/09 15:59:42]

Yes. Not duplicated. Moved to using return. Thanks!

         break;
     }
+    return step;
+}
+
+ScrollResultOneDimensional ScrollableArea::userScroll(ScrollDirectionPhysical direction, ScrollGranularity granularity, float delta)
+{
+    ScrollbarOrientation orientation = scrollbarOrientationFromDirection(direction);
+    if (!userInputScrollable(orientation))
+        return ScrollResultOneDimensional(false, delta);
+
+    cancelProgrammaticScrollAnimation();
+
+    float step = scrollStep(granularity, orientation);
 
     if (direction == ScrollUp || direction == ScrollLeft)
         delta = -delta;
 
     return scrollAnimator()->userScroll(orientation, granularity, step, delta);
 }
 
 void ScrollableArea::setScrollPosition(const DoublePoint& position, ScrollType scrollType, ScrollBehavior behavior)
 {
     if (behavior == ScrollBehaviorAuto)
@@ skipping 411 matching lines @@
 
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink