This would be a Unity problem, wouldn't it? Steam does not handle the window decorations for other apps. The fact that it occurs when Steam is running doesn't mean it's a Steam problem, just that Steam is exposing a problem in Unity.
Ah, that could make sense. The screenshots don't show an occluding window leftover, but it sounds similar.
I assumed it was a Steam problem, as it can be fixed by exiting Steam then restarting. Seeing that link, I do believe that this is the same issue. I'd similar problems with XFCE and am also running with -silent on startup.
Nothing extracted yet.
Steam seems to be the cause of a problem here: http://askubuntu.com/questions/322361/cant-use-window-controls-on-maximized-windows/329678#329678 which caused you to be unable to use window controls on all maximised windows.
Although this can be fixed by restarting Steam, it is still undesirable behaviour.
See this screenshot for an example of the problem:
![over controls]()
Using Ubuntu 13.04 64 bit.