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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  56682@debbugs.gnu.org,  Eli Zaretskii
>  <eliz@gnu.org>
> Date: Tue, 02 Aug 2022 17:19:53 -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?