Squarish window shadows

[Mikuro]

Every now and then I get a little system-wide visual glitch that makes the top corners of aqua windows have square shadows. See attached picture. Metal windows are unaffected. (Edit: Metal windows actually are affected in some apps, like Safari, but not in the Finder, QuickTime Player, or ffmpegX.)

This tends to happen after I play games. This time, I think it was Hot Spot X that triggered it.

Does anyone else get this, and is there any way to fix it? It's not a big deal, of course, but still, it's bugging me.