bug#45898: 27.1; wedged in redisplay again

[Stefan Monnier]

> One surprising finding is that sometimes syntactic fontifications
> (triggered via jit-lock.el as part of rendering the buffer) seem to
> run amok: jit-lock asks the mode's fontification to fontify a
> 1500-character block, but that results in parse-partial-sexp being
> called to parse the entire humongous file, BOB to EOB.

IIUC the file is a single long-line.  Font-lock works line-by-line (in
theory major modes can override that by providing their own value for
`font-lock-extend-region-functions`, but regexp-based fontification is
hard to do with our regexp enging if not working line-by-line), so even
if jit-lock only requests fontification of a 1500-char block, font-lock
rounds it up to a whole number of lines (and then proceeds to call
`parse-partial-sexp), which would explain what you're seeing.


        Stefan