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

[Dmitry Gutov]

On 01.08.2022 16:19, Eli Zaretskii wrote:
> > 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?

syntax-ppss cache is a list of checkpoints spread along the buffer.

After a modification, only the checkpoints below it are invalidated (to 
be recomputed on-demand later).