Report accurate Overscroll on handleWheel.

Source/core/frame/RootFrameViewport.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/frame/RootFrameViewport.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/ScrollAlignment.h"
 #include "platform/geometry/DoubleRect.h"
@@ skipping 104 matching lines @@
         if (event.railsMode() != PlatformEvent::RailsModeHorizontal)
             locationDelta.setY(-event.deltaY());
     }
 
     DoublePoint targetPosition = visualViewport().adjustScrollPositionWithinRange(
         visualViewport().scrollPositionDouble() + toDoubleSize(locationDelta));
     visualViewport().setScrollPosition(targetPosition, UserScroll);
 
     DoublePoint usedLocationDelta(visualViewport().scrollPositionDouble() - oldOffset);
     if (!viewScrollResult.didScroll() && usedLocationDelta == DoublePoint::zero())
-        return ScrollResult();
+        return ScrollResult(false, false, viewScrollResult.unusedScrollDeltaX, viewScrollResult.unusedScrollDeltaY);

[bokan, 2015/06/23 16:20:45]

This is returning the layout viewport's overscroll, which can be consumed by the
visual viewport. You need to subtract out the usedLocationDelta (how much the
visual viewport scrolled)

[MuVen, 2015/06/25 12:55:51]

Done.

 
     DoubleSize unusedLocationDelta(locationDelta - usedLocationDelta);
     bool didScrollX = viewScrollResult.didScrollX || unusedLocationDelta.width();
     bool didScrollY = viewScrollResult.didScrollY || unusedLocationDelta.height();
     return ScrollResult(didScrollX, didScrollY, -unusedLocationDelta.width(), -unusedLocationDelta.height());
 }
 
 LayoutRect RootFrameViewport::scrollIntoView(const LayoutRect& rectInContent, const ScrollAlignment& alignX, const ScrollAlignment& alignY)
 {
     // We want to move the rect into the viewport that excludes the scrollbars so we intersect
@@ skipping 187 matching lines @@
 }
 
 void RootFrameViewport::updateCompositorScrollAnimations()
 {
     ScrollableArea::updateCompositorScrollAnimations();
     layoutViewport().updateCompositorScrollAnimations();
 }
 
 
 } // namespace blink