There seems to be a workaround until this issue is (hopefully) fixed officially.

See second post:
https://forums.opera.com/topic/60909/duplicated-can-t-move-browser-in-max-window?lang=de

There used to be a little space at the end of the tab row, that you could drag a maximized window.
After the last update it is gone. This is so frustrating.!

Yes, I'm also very sad and angy about that.
I created an account here on opera.com specifically for the purposeto tell my deep wish to get back this must-have functionality.
Otherwise I will change my browser, because it really disturbs my daily work.