I don't think I've had this issue before, but I can't say I've tested it before either. Anyway, function key+ volume control exists full screen to show a volume bar that pops in the middle, and disappears after a few seconds. HP computers show a similar kind of volume bar when changing volume, and googling this issue shows HP problems.
I suppose one way to fix it would be to uninstall the software that shows this kind of pop-ups, but it also shows low battery warnings and such, so that might be a bit problematic as well.
Does anyone know a fix for this, or what the software is called? I have googled that as well, but found no information of value.
The game that exists full screen is Trine 2 from Steam. I'll try to restart the computer to see it if does anything for now