Same problem here, thought was windows at first, but now it's clear it's opera. Man this is annoying. It looks like a broken internet explorer at this point. I don't think they want to fix it, I think they want to drive us to use something else.
I think it happens every time there's a visual change on the screen. Like you open e'mail, and then alt tab to another screen, but then the email loads the ads, it switches back to the email without you wanting it to.