It happens to me too, and happened with the previous version too (the one with the slow speed dial). It doesn't happen all the time, so I guess it must be after I do something in the browser, regardless of the resulting active page. It even happens in Speed Dial or with Opera minimized.
I did some attempts, like using Youtube full screen and then closing the tab, but that didn't cause the problem. It just happened again minutes ago while Opera was in https://www.facebook.com/messages. I'll try to do some further testing to see if I can find exactly how to reproduce it.
Yes, at first I thought it was YouTube that I would leave open on Pause so I stopped that. Then I thought it was Facebook so I closed that. Then I picked just a random page like Google Search results to leave open to be sure it wasn't the page code specifically causing it. Seems it doesn't matter.
Steve, I guess once Opera is in "avoid screensaver" mode, it doesn't matter the active page. You will have to close it and reopen it to try to find what site produces this.