You don't understand the struggle it took to make this simple post. Opera's login button doesn't return to the thread.
That being said, I've uncovered this issue with Opera. I was able to trigger it by attempting to position two Opera windows side-by-side on my Mac M1 Monterey.
Safe to say it hasn't been fixed. The only way to get rid of the floating thumbnail is to force quit Opera, to which the OS does present some resistance.