Apple mail has found itself in a strange state: an email I tried to send this morning (which apparently failed) keeps appearing for perhaps half a second, then disappearing again. This happens even when I restart Mail or even restart the computer. I have tried interacting with it during it's brief appearance (close button, send button, ...) but it doesn't appear to be reacting to my clicks.
Any suggestions on what may be happening or what I can do about it?
EDIT: I have just upgraded from Yosemite to Capitan. Perhaps that's a contributing factor to the behaviour.
EDIT: I have realised that the phantom window DOES receive keystrokes: if I'm typing into another email when the phantom appears, 3 or 4 keystrokes end up in the phantom, then the subsequent keystrokes appear in the new email after the phantom disappears. It seems the phantom grabs the focus if mail is active - also evidenced by document view popups getting cleared each time the phantom appears.