Fix scrollbar thumb drag cancellation in RTL mode

Source/platform/scroll/Scrollbar.cpp

 /*
  * Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 374 matching lines @@
         // By default, we assume that gestures don't deselect the scrollbar.
         return true;
     }
 }
 
 void Scrollbar::mouseMoved(const PlatformMouseEvent& evt)
 {
     if (m_pressedPart == ThumbPart) {
         if (theme()->shouldSnapBackToDragOrigin(this, evt)) {
             if (m_scrollableArea)
-                m_scrollableArea->scrollToOffsetWithoutAnimation(m_orientation, m_dragOrigin);
+                m_scrollableArea->scrollToOffsetWithoutAnimation(m_orientation, m_dragOrigin + m_scrollableArea->minimumScrollPosition(m_orientation));
         } else {
             moveThumb(m_orientation == HorizontalScrollbar ?
                       convertFromContainingWindow(evt.position()).x() :
                       convertFromContainingWindow(evt.position()).y(), theme()->shouldDragDocumentInsteadOfThumb(this, evt));
         }
         return;
     }
 
     if (m_pressedPart != NoPart)
         m_pressedPos = orientation() == HorizontalScrollbar ? convertFromContainingWindow(evt.position()).x() : convertFromContainingWindow(evt.position()).y();
@@ skipping 191 matching lines @@
     if (!m_scrollableArea)
         return 0;
 
     if (m_orientation == HorizontalScrollbar)
         return m_scrollableArea->scrollPosition().x() - m_scrollableArea->minimumScrollPosition().x();
 
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
 } // namespace WebCore