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.

@leocg This is a well known, old problem. The following 'locked' thread has more details.
https://forums.opera.com/topic/25654/opera-stuck-in-full-screen-no-way-out

I created a new problem, and gave my work-around because the original thread is locked and I cannot add.

As for when it happens, I am not sure. I touched something, maybe F11, and it went into full screen. I assumed that Esc would get me out, it didn't. I had to 'Alt-Tab' and go to a different window and google in Firefox, and came across that forum entry above, with no solution. I killed Opera, relaunched, still stuck, and no 'Exit full screen' etc. option anywhere. I went into settings because it was possible and there I see the 'Exit... ' in context menu.

Just implement 'Esc' or add 'Exit Full Screen' in the context menu, please.

@leocg I do not know which page it was. I looked through history but couldn't decide. But here is what I found (and most likely cause).

Opera version 75.0.3969.149
Windows 10
Adblock Plus

1. Go to twitter.com (logged in)
2. F11 to go to full screen.
3. ESC does not work.
4. Right context menu does show Exit Full screen as an option, but not everywhere.

i suspect the page in question had manipulated the context menu via javascript. So it was not showing the option. And Esc definitely did not do anything.

if I can reproduce it (find the page where exit menu entry was not available) I will update.

@leocg This is a well known, old problem. The following 'locked' thread has more details.
https://forums.opera.com/topic/25654/opera-stuck-in-full-screen-no-way-out

I created a new problem, and gave my work-around because the original thread is locked and I cannot add.

As for when it happens, I am not sure. I touched something, maybe F11, and it went into full screen. I assumed that Esc would get me out, it didn't. I had to 'Alt-Tab' and go to a different window and google in Firefox, and came across that forum entry above, with no solution. I killed Opera, relaunched, still stuck, and no 'Exit full screen' etc. option anywhere. I went into settings because it was possible and there I see the 'Exit... ' in context menu.

Just implement 'Esc' or add 'Exit Full Screen' in the context menu, please.

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.