added Nicolas P's suggestion to write from inside the mouse button procedures by setting a "something changed" flag to decide whether to update the Output window.
[however, I now question the plan to display info on all subsets of
the selected subnet, i.e. for close-knittedness, since e.g. for a
subnet of only 10 nodes, that will be 1K values, 64K for 16 nodes,
etc.  Crazy.  I don't want to even calculate this automatically, since
it may take a long time. e.g. for as few as 30 nodes, I have 1G
subsets to calculate, and for each one I run through each of the
nodes in it and look at their links, if I do it naively.