Updates: Since doing the PRAM / SMC reset, I'm now seeing graphics glitches when scrolling. Sometimes, but far more often than beforehand. I've had these issues even with Chrome not running. At one point, while coding in IntelliJ, my screen blanked out for two seconds, and now it's back again.

My Chrome browser doesn't seem to have any way to disable WebGL. I do have the "WebGL Draft Extensions" disabled, which seems to be the default.

Very disturbing. I fear that my computer is crapping out as I'm using it. I really, really wanted to wait until a MacPro sequel came out. I might be forced to act sooner than that.