This thread exemplifies everything frustrating about the internet; It contains some of the right words but the words are not employed effectively.
The "Exit Fullscreen" message is feature of a Chromium. It can be toggled on or off in the Chrome browser. chrome://settings/contentExceptions#fullscreen
So is the ability to disable a feature, a feature itself?
Opera has (foolishly) decided to exclude the feature to disable this feature..... in their software. So the original post should say "Please enable toggling the built-in Chromium 'Exit Fullscreen' message off." It should be a fifteen minute patch fix, not a multi-year long and multi-thread argument.
P.S. Please do it. What other hotkey prompt is reiterated every time the trigger action is performed? Imagine if you hit CTRL+C and the screen said "You Have Copied Text Please Press CTRL+V To Paste" for 5 seconds in 24pt font. Anyone using excel for more than ten minutes would become suicidal. The internet has changed. User go full-screen dozens of times per day. Pair that with the fact that Chrome is now the most popular browser on earth and that Opera users are typically not day one computer users, I can't imagine a single Opera user over the past 3 years, has received value from seeing that message. Not one.