Comment 14 for bug 1566157

I tested the wip/test-3-18 branch and got the following bug:

At some point (when I was switching windows?) the screen became completely black, and only when the windows repainted themselves they started to become visible. I.e. first the window after the mouse became again visible, then when I pressed Alt-Tab, the other window became visible, then the clock on the panel (because the minute changed), etc.

Unfortunately I was not able to take a screenshot. It is not reproducible, I got it only once so far. I never got this bug before, can it be a regression of that branch?