I saw this happen once in the Showdown demo on my computer. I was never able to directly figure out what happened to corrupt the grid like this, but by deleting the config folder in the project folder something was reset for me that allowed it to work correctly. The biggest thing when deleting the config folder is to make sure that you’ve not got anything in there that has been setup for anything specific. If so, you’ll likely need to set that up again.
Alternatively, you can simply disable the grid while working in the editor by going to Show > Grid and uncheck it.
I got the same issue using the VRShowdown demo settings in my VR project. To be more precise, I used the [SystemSettings] from the ROOT/Config/DefaultEngine.ini and the issue appears after recompiling shaders
So I just tried to figure which settings breaks the grid. And I’m almost sure it’s r.compileshadersfordevelopment=0.
According to this solved topic this settings should only be used when shipping the game.
I’ve tested it in a fresh new project, putting this setting in the DefaultEngine.ini file, and the black lines appears.