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

[Eli Zaretskii]

> Date: Mon, 01 Aug 2022 13:14:20 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: dgutov@yandex.ru, 56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> 
> > You can maybe have that for C-p that follows M->, but wouldn't the wait 
> > return, with a vengeance, if you insert a single character (because then 
> > the buffer needs to be re-scanned)?  If so, we've gained nothing, 
> > really.
> 
> Fortunately no: the buffer doesn't need to be rescanned, syntax-ppss 
> caches its result, to avoid having to rescan the whole buffer again and 
> again.

But the buffer has changed, so the cache is not necessarily valid,
right?