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 04:37:55 -0400
> 
> >> > Indeed, that's my conclusion too.  So until syntax-ppss (at least) is 
> >> > made
> >> > an order of magnitude faster, the right thing to do is to use the forced
> >> > narrowing method.
> >> 
> >> By tying this to "long lines" is wrong, since it has nothing to do with
> >> long lines, only with large buffers.
> >
> > I thought you told me once that syntax-propertize needs to consider
> > complete lines in some (frequent) situations?
> 
> Yes, but we're talking about `syntax-ppss` here.  Admittedly,
> `syntax-ppss` uses `syntax-propertize` internally, but I think the two
> need to be considered separately (and `syntax-propertize` already tries
> to bound its work via `syntax-wholeline-max`).

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.