When the uncommitted changes are viewed in either the File Status or the Log / History the divider between the files and the diff view is immovable. It is stuck just right of centre. One work around is to select the uncommitted changes and the previous commit and view the diff without the stage.

Switching to tree view will temporarily restore the divider to mobility. However it will reset itself right of centre again after selecting another commit and back (Log / History) or restarting SourceTree (File Status) and return to immobility.