It's known, but it's likely just how the window draws itself and it happens so quick it's hard to notice (it's pretty trivial, IMO). I honestly don't believe this to be an actual bug (I believe it's how it's intended) and can easily be ignored.
IMO, the OP is likely the same person who has been bringing this 'issue' up multiple times (using a new account, the names seem similar from what I recall), even after his topics were closed and was informed if there's any progress one of the devs would post an update.
I actually know about a related 'issue' to this one, which involves trying to minimize/maximize/restore the MC window while explorer.exe is killed. But again, I think that's how it's intended and how MC draws its window(s).
