@thebe14 Update: if I resize my browser to not be full screen (still top to bottom, but a narrow colum on the left size not covered by Opera), the flashing still happens but it cover the whole width of the screen, a narrow band on top on my screen. So probably not caused by Opera, and not any extension in Opera. Turning off the Bitwarden extension and then not happening for a while was a lucky coicidence, apparently.