I don't know whether this is a bug or some other undesired side-effect, but I recently noticed that when I set an animated background image on Opera's speed dial page, it blocks the screensaver and the monitor's energy saving mode while it is running. When I close Opera, it suddenly works again.
Did anyone else observe this yet?

BTW: I'm currently using Opera 34 on Windows 8.1 (x64), nVidia graphics card with latest drivers (I even tried to disable hardware acceleration in Opera to no avail).