Little black window on back up

Probably this has been discussed before, but I am not able to find this topic on the forum.

I think that there is a little bug: when I close/open any project it appears on my screen a little black window with a blue line in the middle, which should be a window that says that the program is performing a back up. Probably it is a bug of El Capitan.

Have a nice day.

Yes, this appears to be an El Capitan display bug. It’s been spotted in a couple of other windows as well.