Sorry for being so late to reply. Installing opera as standalone will require I run it frequently along with the normal installation. Why ? because in the normal installation the problem doesn't show up at all times, but seems to happen once every x sessions. So when the problem shows up, it keeps manifesting for that whole session. Sometimes it will not be there next sessions, and sometimes it will. Last time it showed up (upon which I wrote my last comment), I don't remember if I restarted the browser that same day (hence new session) or not, but the next day I didn't notice it. Few days later (few day eariler than now) It showed up again, and I again haven't noticed it after that.
I do not use opera sync at all. Also, when I wrote in my last post that the problem showed up again when I was in deviantart, I didn't mean that it suddenly happend in that site where as it didn't happen earlier. It's just that I didn't go fullscreen before I visited deviantart. Also, it's no specific URL when theat bug shows up, when it's there, it's there for every URL, within and beyond any specific domain.
@yanta wrote above that the problem occurs regardless of standalone installation and hardware acceleration. And while I assume it will not be any different for me, next time the problem shows up, I will install standalone opera and give feedback.