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 78 matching lines @@
 
 ScrollBehavior RootFrameViewport::scrollBehaviorStyle() const
 {
     return layoutViewport().scrollBehaviorStyle();
 }
 
 ScrollResult RootFrameViewport::handleWheel(const PlatformWheelEvent& event)
 {
     updateScrollAnimator();
 
-    ScrollResult viewScrollResult;
-    if (layoutViewport().isScrollable())
-        viewScrollResult = layoutViewport().handleWheel(event);
+    ScrollResult viewScrollResult = layoutViewport().handleWheel(event);
 
     // The visual viewport will only accept pixel scrolls.
     if (!event.canScroll() || event.granularity() == ScrollByPageWheelEvent)
         return viewScrollResult;
 
-    // Move the location by the negative of the remaining scroll delta.
+    // TODO(sataya.m) : The delta in PlatformWheelEvent is negative when scrolling the
+    // wheel towards the user, so negate it to get the scroll delta that should be applied
+    // to the page. unusedScrollDelta computed in the ScrollResult is also negative. Say
+    // there is WheelEvent({0, -10} and page scroll by 2px and unusedScrollDelta computed
+    // is {0, -8}. Due to which we have to negate the unusedScrollDelta to obtain the expected
+    // animation.Please address http://crbug.com/504389.
     DoublePoint oldOffset = visualViewport().scrollPositionDouble();
     DoublePoint locationDelta;
     if (viewScrollResult.didScroll()) {
         locationDelta = -DoublePoint(viewScrollResult.unusedScrollDeltaX, viewScrollResult.unusedScrollDeltaY);
     } else {
         if (event.railsMode() != PlatformEvent::RailsModeVertical)
             locationDelta.setX(-event.deltaX());
         if (event.railsMode() != PlatformEvent::RailsModeHorizontal)
             locationDelta.setY(-event.deltaY());
     }
 
     DoublePoint targetPosition = visualViewport().clampScrollPosition(
         visualViewport().scrollPositionDouble() + toDoubleSize(locationDelta));
     visualViewport().setScrollPosition(targetPosition, UserScroll);
 
     DoublePoint usedLocationDelta(visualViewport().scrollPositionDouble() - oldOffset);
-    if (!viewScrollResult.didScroll() && usedLocationDelta == DoublePoint::zero())
-        return ScrollResult();
 
-    DoubleSize unusedLocationDelta(locationDelta - usedLocationDelta);
-    bool didScrollX = viewScrollResult.didScrollX || unusedLocationDelta.width();
-    bool didScrollY = viewScrollResult.didScrollY || unusedLocationDelta.height();
-    return ScrollResult(didScrollX, didScrollY, -unusedLocationDelta.width(), -unusedLocationDelta.height());
+    bool didScrollX = viewScrollResult.didScrollX || usedLocationDelta.x();
+    bool didScrollY = viewScrollResult.didScrollY || usedLocationDelta.y();
+    return ScrollResult(didScrollX, didScrollY, -viewScrollResult.unusedScrollDeltaX - usedLocationDelta.x(), -viewScrollResult.unusedScrollDeltaY - usedLocationDelta.y());
 }
 
 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
     // the pinch viewport with the scrollbar-excluded frameView content rect. However, we don't
     // use visibleContentRect directly since it floors the scroll position. Instead, we use
     // FrameView::scrollPositionDouble and construct a LayoutRect from that (the FrameView size
     // is always integer sized.
 
@@ skipping 182 matching lines @@
 }
 
 void RootFrameViewport::updateCompositorScrollAnimations()
 {
     ScrollableArea::updateCompositorScrollAnimations();
     layoutViewport().updateCompositorScrollAnimations();
 }
 
 
 } // namespace blink