I'll note
https://bugs.launchpad.net/ubuntu/+source/sddm/+bug/1865837 "Lubuntu 20.04 qa-test install; flashing at sddm greeter when mouse at top panel (pre-login)"
which reads very similar, maybe even same cause (similar pattern at least in flashing), but occurs different positions on the screen.
This was not the duplicate I'm hunting for...
I'll note
https:/ /bugs.launchpad .net/ubuntu/ +source/ sddm/+bug/ 1865837
"Lubuntu 20.04 qa-test install; flashing at sddm greeter when mouse at top panel (pre-login)"
which reads very similar, maybe even same cause (similar pattern at least in flashing), but occurs different positions on the screen.
This was not the duplicate I'm hunting for...