This may be related to RT-33028. This has been happening for a few builds now, and is currently happening in b120.

I leave the app I work on running & sometimes lately it has been black. When I wave my mouse around the screen eventually repaints.

This manifestation isn't reproducible (the way the symptoms were the last time I reported this in RT-34410).

At first I thought this only happened when unlocking my machine, but not consistently. Just now, however, my application's window was mostly, but not completely, covered by other windows. I worked in some other applications & then I saw the sliver of my window that was not covered (maybe 5 - 10% of the total area) turned black.

When I clicked on the window to bring it to the front, the whole window was black except for one or two buttons.