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

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

Hello again Stefan and Happy GNU/Year.

> 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.

I just checked and my compiled build was of an older commit. The 
variable was set `t`. I built from the latest commit and indeed now I 
can scroll flawlessly in the reported file.

> 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`.

That's reasonable for the system. I do not notice any sort of lag with 
`M-v` on my system (Ryzen 7 5700G).

Thank you Eli and Stefan for the prompt fix. Really admire the work you 
guys do.

Regards,

Divya Ranjan,
Mathematics, Philosophy and Libre Software