Select a counter (one that highlighted when selected); do a command on it then undo.
After the UnDo, the Counter remains highlighted as selected. However, no piece command will operate. To resume normal operation, click away from the counter and re-select.
I have filed 14226 for this - vassalengine.org/tracker/sho … i?id=14226
(Note that this isn’t a “new” bug – it has been around since at least 3.2.17)
A test-build where I have hopefully solved this BUG-FROM-BEFORE-TIME-BEGAN should shortly appear here: github.com/vassalengine/vassal/ … /665509478