Mainframe scrollbars should get events even if document prevent-defaults the mouse events.

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 1270 matching lines @@
         const bool wasLastScrollBar = mev.scrollbar() == m_lastScrollbarUnderMouse.get();
         HitTestRequest request(HitTestRequest::ReadOnly | HitTestRequest::Active | HitTestRequest::ConfusingAndOftenMisusedDisallowShadowContent);
         mev = m_frame->document()->prepareMouseEvent(request, documentPoint, mouseEvent);
         if (wasLastScrollBar && mev.scrollbar() != m_lastScrollbarUnderMouse.get())
             m_lastScrollbarUnderMouse = nullptr;
     }
 
     if (swallowEvent) {
         // scrollbars should get events anyway, even disabled controls might be scrollable
         Scrollbar* scrollbar = mev.scrollbar();
+        if (!scrollbar) {
+            FrameView* view = m_frame->view();
+            scrollbar = view ? view->scrollbarAtPoint(mouseEvent.position()) : 0;
+        }
 
         updateLastScrollbarUnderMouse(scrollbar, true);
 
         if (scrollbar)
             passMousePressEventToScrollbar(mev, scrollbar);
     } else {
         if (shouldRefetchEventTarget(mev)) {
             HitTestRequest request(HitTestRequest::ReadOnly | HitTestRequest::Active | HitTestRequest::ConfusingAndOftenMisusedDisallowShadowContent);
             mev = m_frame->document()->prepareMouseEvent(request, documentPoint, mouseEvent);
         }
@@ skipping 2430 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace WebCore