I think it would be nice to have a horizontal scroll bar for the 
HierarchicalListMorph, which is a subclass of ScrollPane. ScrollPane 
does not currently have horz scrollbars, but TwoWayScrollPane does. 
TwoWayScrollPane, in turn, needs a little sprucing up to match 
ScrollPane (in terms of look and respecting preferences). I tried 
subclassing HierarchicalListMorph under TwoWayScrollPane just two see 
what happens, and despite some minor problems that look easy to fix (at 
first glance, anyway), it seems to be almost there.

So my question is this: what would you recommend -- adding code to 
ScrollPane to support an hScrollBar, or beefing up TwoWayScrollPane and 
then making HierarchicalListMorph its child?