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: Thu, 04 Aug 2022 18:35:36 -0400
> 
> >> 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?
> 
> The use of `syntax-wholeline-max` in
> `font-lock-extend-region-wholelines` supposedly fixed this problem since
> it changed `font-lock` so it doesn't ask `syntax-ppss` to compute the
> whole line/buffer.

It did?

And if it did, how is that better or different from a locked
narrowing?