I've run the "hold down D at boot" hardware diagnostic, but it completes quickly so clearly it doesn't test much. I also ran memtester, and it found no problems.
So is this, "Oh yeah, 10.14.6 just does that shit sometimes" or is it, "you need to taxi this thousand pound piece of shit into the shop to have its skull scooped out with a rusty spoon"?
Also it there some process I can ssh in and kill that will drop the Mac back to "log out and re-initialize the graphics subsystem", slightly less extreme than rebooting?
It's an iMac Pro 2017 8 core, in case that is relevant.