bug#57804: An infinite loop in a `fontify-region' function causes Emacs to hang indefinitely

[dick]

Oh what a horse's ass am I.

I see what you're saying.  A simple evaluation of (while t) is C-g'able.

jit-lock callbacks enjoy special status in that they're invoked
out-of-band in redisplay C code.  That's probably why they're not
interruptible from the interpreter loop.  But as you can tell from my
earlier outburst, I'm constantly talking out of school.