bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: gregory@heytings.org,  dgutov@yandex.ru,  56682@debbugs.gnu.org
> Date: Wed, 03 Aug 2022 16:38:57 -0400
> 
> > What about parse-partial-sexp, which calls scan_sexps_forward?  It
> > looks like I've misremembered, and that was the culprit in the
> > scenario we discussed, see
> > https://debbugs.gnu.org/cgi/bugreport.cgi?bug=45898#92.
> 
> The wholelines problem did not kick in because of PPS nor `syntax-ppss`
> but because of font-lock (which then called `syntax-ppss` which then
> called PPS).

If it's font-lock that forces syntax-ppss to examine the whole huge
line, then what is your proposal for avoiding that which doesn't
involve some more-or-less arbitrary restrictions on the part of the
buffer that can be examined by syntax-ppss?