running XQuartz: display background is black after screen is unlocked or awakened I use XQuartz 99.9% of the time I'm on my Mac. When I lock my screen (using Lock Screen in the apple menu) -- or the screen saver kicks in -- and the screen remains locked/darkened for more than a few minutes -- not sure what the minimum amount of time is for this -- the screen background is often black when I unlock the screen or wake up the display. I am still able to see and use everything on the desktop -- mac icons, mac apps, X window apps, etc. -- but the screen background is black -- really hard on the eyes! If I exit XQuartz, my screen returns to normal but this is not a solution since I then lose all my X applications. If I lock the screen and immediately unlock it, I don't see the problem. 

I'm running macOS High Sierra (10.13.6) and XQuartz 2.7.11. I have had my Mac (it's a work mac) since May, 2017. The Mac came with Sierra and I installed High Sierra early last fall but did not start having this problem until January. It mysteriously went away for a few months but has now returned.

Any ideas or suggestions? I'd be happy with a reasonable workaround that doesn't involve killing XQuartz.
Get your XQuartz showing in the menu bar. Click on XQuartz menu item and select Preferences. When the preference pane appears, click on the Output tab. One of the options is the Full-screen mode.  Make sure the box is NOT checked. See the images below.