This is sort of embarassing.
At the same time of the emacs upgrade to 26, I also installed an alternate desktop notification engine (dunst, which I need because at work I use i3wm).
On receiveing a notification, dunst pops up a transient window and the default config binding for dismissing it is: C-SPC. I also happen to have a fair number of (nagios, moslty) notifications at any time. So dunst was grabbing the shortcut intended for emacs (apparently, it steals the shortcut regardless of what has focus at the time)
I did not realize this until I noticed that:
- C-h-l never showed repeat C-SPC occurences
- Something was flashing on the desktop at the extreme right ov my FOV (that's were notifications pop up) just after I was hitting C-SPC
Reconfiguring that binding removed my woes.