How can I fix a weird WindowServer bug which only occurs on one machine? I've got an odd bug on one of my machines.  It only occurs on the one Mac so it's not a bug in the OS per-se, but it's very annoying and I can't work out how to fix it.

Basically panel windows which are not set to Hide on Deactivate (such as Mail's Activity Window) aren't properly hidden when I hide their parent application.  The window remains on-screen but cannot be interacted with.

See a demo here: http://www.youtube.com/watch?v=t9BPCYqpdKI

I suspect that WindowServer has become screwed-up somehow.

How can I restore WindowServer to sanity?  Does anyone have any ideas?

Thanks for any help.
I had to wait a while - but upgrading to Mountan Lion fixed this for me. (just in case anyone else had this issue - unlikely though it seems.)