Opera gets stuck in full screen because there is no context menu to exit, and F11 does not work, nor does Escape.
Opera is not following Windows standards, and possible bug (or incompetent design decision) prevents context menu from offering exit out of full screen.
Work-around until Opera fixes the bug:
Try to get to the 'Settings' page. This context menu seems to be available while in settings page.
In the settings page, right click (context menu) on the left side of the page will show the 'exit full screen' option.
Hope this helps and saves a reduces the swear words being sent to Opera design folks.