Because I'm nothing if not stubbornly persistent, I uninstalled Avast using aswClear, scrubbed my system of all known traces, rebooted yadda yadda and reinstalled -- only to find: no change. Main interface titlebar icons remain unresponsive. FWIW one new crumb of info: whatever the cause it's tied to my current user profile, because (A) the other Admin profile does not have the problem, and (B) changing this profile to Admin does not solve it. So it's not you, it's me. Humbug.