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

[Eli Zaretskii]

> Date: Tue, 2 Aug 2022 00:50:10 +0300
> Cc: 56682@debbugs.gnu.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> 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).

So a suitably-concocted replace command will still invalidate a lot of
that cache, right?