If that doesn't do the trick, test in a standalone installation. If Opera works fine there, you know it's something with your profile. If Opera doesn't work fine there either, goto the URL opera://settings/system and disable hardware acceleration. When you kill Opera and start it back up, goto the URL again and make sure hardware acceleration is still off. If not, kill Opera and modify its shortcut to pass --disable-gpu to launcher.exe. If that doesn't work, I'd look for issues outside of Opera.

Now, if you find out it's your Opera profile that's causing the issue, I would goto the URL opera://about, take note of the "profile" path, kill Opera, and delete the 3 history files in Opera's profile directory. You'll lose your history of course, but that might fix it as those files recently got corrupted for some users.

@arthurcloud What about the exit menu, does it work? And the keyboard shortcut?

Doesn't work too.
Google says it's Ctrl+Shift+X, is that right?
If yes, then doesn't work too

@arthurcloud The other buttons, minimize, maximize/restore work?

Yes, they work correctly. Only the X button sometimes doesn't work. Like, if I opened the browser just now, I can close it with that X button, but if I try to close it after some time, like few hours, then it doesn't close. So I either have to close it with Task Manager, or CMD Taskkill command