Yes, all browsers work fine now, including Opera. It was a one-time glitch that I got out of by killing and restarting Opera. I was just wondering what I could possibly have done to cause it.
I think distraction-free writing is the answer. Ctrl-Shift-F. The classic editor doesn't seem to have the button for it. The image on the linked WordPress page portrays the block editor, which I don't use. But I think Ctrl-Shift-F gives me what I saw before - in Opera, and also in Firefox and probably Chrome too.
So it wasn't an Opera issue after all. In which case I am doubly grateful for your efforts. I'm saving a note on it for future reference. Thank you and take care.