Only exiting the CK at this point and restarting it works. Once this happens the render window is useless for the remainder of the session. This isn't a case of it suddenly not opening windows, because it can happen within just a few minutes of loading an area.
I haven't updated my video drivers in some time because Catalyst 12.8 doesn't offer anything that looks necessary to my setup. I'm currently on 12.6 and up until a few days ago, had zero trouble with this.
It's become an annoyance on all 3 installation folders I have, so it's not just an issue of one setup being corrupt or something. They didn't sneak some kind of silly stealth update down recently did they?