bug#74281: 30.0.91; font-lock mode hangs on scrolling large Scheme file

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Apologies, I mistyped during travelling, the file I was referring to was the
> original file of the bug report. `tex.scm` is the file from the Guix source
> tree: /gnu/packages/tex.scm

Hmm... on my tests, I can reproduce in `emacs-30` the very slow
font-locking you described when we're at the end of the buffer, but not
on `master`.  Just to confirm that you're running the "right" code, can
you do `M-: font-lock--syntax-table-affects-ppss RET` from within that
`tex.scm` buffer?  It should return nil if my fix was applied.  If it
returns t then you're presumably still using the old code (or there's
something else amiss).  If it returns something else than nil or t then
there's something else amiss.

FWIW, even with the fix, Emacs is not quite able to keep up with `M-v`
with an auto-repeat of 40/s when done from the end of the buffer, but:
- When I release the key it catches up fairly quickly (hence the "not
  quite").  So I wouldn't describe it as "hangs".
- This is on my 16 years old 2.5GHz Core 2 Duo (Thinkpad T61).
- It's an Emacs compiled with `-Og` and with `--enable-checking`.


        Stefan