Moving... dialogue persists

Today (for the first time IIRC) I see a small dialogue describing 
Moving... a message from Incoming mail to Family.Ailie (which I 
initiated). But the dialogue does not close, has not got a close icon, 
and has two buttons (Abort, Pause) neither of which is effective. I 
attach a PNG of it.

The message did get moved, but the dialogue stays until I close MPro.

Bug, or what?

-- 
Bernard