Well, running a repair from Control Panel did nothing. Neither did dropping by Reynholm Industries briefly.
As I write, Avast itself appears to be working correctly in all other regards except that none of the icons in the main interface titlebar -- Alert, Help, Min, Close -- function or respond to mouseover. I can grab the window by the titlebar and move it, but not engage with those items. Still able to close from statusbar context menu.
I'll probably wait it out until the next update and see what's what. Meantime if anyone else experiences this issue and/or has a eureka moment, please let me know.
THX all