@johnlowrie Got rid of it in the end, for a while. There was an X to click but it was obscured as it corresponded with a small black part of my background image.
@burnout426 hi! thanks for your help, forgot this existed. i think opera was just doing a clean-out or update or something, because it's gone back to the new version now. thanks anyway!
I figured it out. One has to click on the purple vertical "I" on the right, which then shows extensions. Why Opera is setting this not showing by default on update is really annoying. Trying to figure this out and the various steps I tried to get back the extensions wasted a good part of the afternoon.