I've got a strange one that has happened in the last 24 hours.

Some apps (so far Mail and Muse, a Pandora app) run fine but won't display their windows.

I had an email and tried to open it. It showed as open but did not display. If I hit forward, it opened just fine, ready to be forwarded.

Muse is playing music, shows as open, but there is no sign of the window.

Any ideas?