There's two different "Full screen" messages that pop up.
One is the "Exit Full Screen (F11)" message on any normal website.
The other happens when I'm playing video (such as on Amazon.com or Youtube.com), with the "Press Esc to exit Full Screen mode". I believe they're both HTML5 video (though I'm uncertain about Amazon).
Frankly, I know how to exit full screen, and just want these messages to go away. It was possible in classic Opera, but there doesn't appear to be any such option in ChrOpera. Yes, I've finally been forced to make the transition (even though certain features are lacking that I sorely miss, such as the 'right-click and scroll' to browse through tabs), and for the most part it's fine. I would really like to be able to block these popup messages, though. The only option I've seen so far, is from a post made in 2009 (for Chrome), and it's likely outdated. Not to mention there was a warning that it was detected as a virus by most AV software, which makes me leery of it to begin with.
I've not had any success from searching Google, so I was hoping someone here would know of a way that I'm unaware of.