Convert main-thread overscroll parameters into viewport space.

third_party/WebKit/Source/core/page/ChromeClient.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple, Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 88 matching lines @@
     // LocalFrame wants to create the new Page. Also, the newly created window
     // should not be shown to the user until the ChromeClient of the newly
     // created Page has its show method called.
     // The FrameLoadRequest parameter is only for ChromeClient to check if the
     // request could be fulfilled. The ChromeClient should not load the request.
     virtual Page* createWindow(LocalFrame*, const FrameLoadRequest&, const WindowFeatures&, NavigationPolicy, ShouldSetOpener) = 0;
     virtual void show(NavigationPolicy = NavigationPolicyIgnore) = 0;
 
     void setWindowFeatures(const WindowFeatures&);
 
-    virtual void didOverscroll(const FloatSize&, const FloatSize&, const FloatPoint&, const FloatSize&) = 0;
+    // All the parameters should be in viewport space. That is, if an event
+    // scrolls by 10 px, but due to a 2X page scale we apply a 5px scroll to the
+    // root frame, all of which is handled as overscroll, we should return 10px
+    // as the overscrollDelta.
+    virtual void didOverscroll(
+        const FloatSize& overscrollDelta,
+        const FloatSize& accumulatedOverscroll,
+        const FloatPoint& positionInViewport,
+        const FloatSize& velocityInViewport) = 0;
 
     virtual void setToolbarsVisible(bool) = 0;
     virtual bool toolbarsVisible() = 0;
 
     virtual void setStatusbarVisible(bool) = 0;
     virtual bool statusbarVisible() = 0;
 
     virtual void setScrollbarsVisible(bool) = 0;
     virtual bool scrollbarsVisible() = 0;
 
@@ skipping 158 matching lines @@
 
     LayoutPoint m_lastToolTipPoint;
     String m_lastToolTipText;
 
     FRIEND_TEST_ALL_PREFIXES(ChromeClientTest, SetToolTipFlood);
 };
 
 } // namespace blink
 
 #endif // ChromeClient_h