Dash contents missing

[Colin Law]

On one of my machines, running 12.04, Unity-2d (I don't know whether
that is significant), sometimes the Dash does not show correctly.
Just the window control buttons are drawn, and the background.  I
can't see anything of note in the logs.  Once it gets into this mode I
have to logout and back in to recover.  I can't find anything similar
in launchpad.  Has anyone any suggestions what I might kill and
restart (or whatever) to recover without logging out.  I am waiting
till I have diagnosed a bit more before submitting a bug.  I have not
yet worked out what it is that puts it in this mode, if it is indeed
something I am doing.

Colin