Use float for scroll offset.

third_party/WebKit/Source/core/events/MouseRelatedEvent.cpp

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2005, 2006, 2008 Apple Inc. 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 56 matching lines @@
                           cancelable,
                           abstractView,
                           detail,
                           modifiers,
                           platformTimeStamp,
                           sourceCapabilities),
       m_screenLocation(screenLocation),
       m_movementDelta(movementDelta),
       m_positionType(positionType) {
   LayoutPoint adjustedPageLocation;
-  LayoutPoint scrollPosition;
+  LayoutSize scrollOffset;
 
   LocalFrame* frame = view() && view()->isLocalDOMWindow()
                           ? toLocalDOMWindow(view())->frame()
                           : nullptr;
   if (frame && hasPosition()) {
     if (FrameView* frameView = frame->view()) {
-      scrollPosition = frameView->scrollPosition();
+      scrollOffset = LayoutSize(frameView->scrollOffsetInt());
       adjustedPageLocation = frameView->rootFrameToContents(rootFrameLocation);
       float scaleFactor = 1 / frame->pageZoomFactor();
       if (scaleFactor != 1.0f) {
         adjustedPageLocation.scale(scaleFactor, scaleFactor);
-        scrollPosition.scale(scaleFactor, scaleFactor);
+        scrollOffset.scale(scaleFactor, scaleFactor);
       }
     }
   }
 
-  m_clientLocation = adjustedPageLocation - toLayoutSize(scrollPosition);
+  m_clientLocation = adjustedPageLocation - scrollOffset;
   m_pageLocation = adjustedPageLocation;
 
   // Set up initial values for coordinates.
   // Correct values are computed lazily, see computeRelativePosition.
   m_layerLocation = m_pageLocation;
   m_offsetLocation = m_pageLocation;
 
   computePageLocation();
   m_hasCachedRelativePosition = false;
 }
@@ skipping 153 matching lines @@
   // See Microsoft documentation and
   // <http://www.quirksmode.org/dom/w3c_events.html>.
   return m_clientLocation.y().toInt();
 }
 
 DEFINE_TRACE(MouseRelatedEvent) {
   UIEventWithKeyState::trace(visitor);
 }
 
 }  // namespace blink