I'm using ubuntu 18.04 LTS and gnome-flashback.

After I switch users back to my session, a mouse left-click (eg, on window minimize, maximize or close) does nothing. If I move the mouse, the pointer changes correctly, ie, arrow or text cursor or hand. Also, if I type ctrl-F2 in order to kill a window and its app, the dialog for the cmd line doesn't appear.
I have to ssh in, then kill gnome-session-binary, which leaves me at the login/switch-user screen. After I log in again, the mouse and keybd work OK again.

This has happened twice today, but never before (in the ~21 years I've used linux).

Any hints gratefully accepted.