Bmbartlett, you're absolutely right. I dumped out to the lock screen, started typing and got it to lock up, and then got the spinning rainbow pinwheel; I unplugged the USB adapter and it un-froze, the Core Animation rotate effect happened, and I was into my computer without a problem.
Good looking out. Still broken, but at least we have a better idea of the problem.
I don't have Parallels or any other VM, if that's pertinent to whoever fixes this.
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