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

[Dmitry Gutov]

On 16.08.2022 16:09, Eli Zaretskii wrote:
> > From: Stefan Monnier<monnier@iro.umontreal.ca>
> > Cc: Eli Zaretskii<eliz@gnu.org>,56682@debbugs.gnu.org,dgutov@yandex.ru
> > Date: Tue, 16 Aug 2022 09:07:59 -0400
> >
> > > The way I see this is that Emacs is doing efforts to solve editing slowdowns
> > > as best as possible, but that there is a "last mile" that cannot be solved
> > > without the collaboration from major and minor modes.
> > The locked narrowing prevents that "last mile", sadly.
> Nonsense, it doesn't prevent any such things.
>
> Let's not exaggerate when we try to make a point, okay?

Why nonsense?

I've made the same point a bunch of times: arbitrary narrowing without 
regard for context stops any potential major-mode specific code from 
looking for a "safe position" which might be outside of that narrowing.

The new variable (which will let us increase the font-lock narrowing 
radius) should get us 95% there, though.