When the graph view scrolls to the sandbox revision
(this happens when the user hits Ctrl+Home or with the
the latest version immediately after showing the graph)
only the vertical position is changed. The view scrolls
vertically even if there is only a horizontal scrollbar
resulting in an invisible upper part of the graph. Due
to the missing vertical scrollbar the only way to show
the hidden part again is hitting Shift+Home to scroll
to the root of the graph.
The attached patch adds horziontal scrolling (when
necessary) and fixes the problem of the hidden graph by
scrolling only in directions for which a scrollbar exists.