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

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  Eli Zaretskii <eliz@gnu.org>,
>   56682@debbugs.gnu.org
> Date: Fri, 12 Aug 2022 22:01:54 -0400
> 
> > I can only repeat that the only ways to improve font locking in a non-local
> > way (or at least, the only ways that were identified in the discussions so
> > far) are either to improve the speed of syntax-ppss by an order of
> > magnitude, in which case it would perhaps become feasible to allow
> > syntax-ppss to have access to the whole buffer, or to equip syntax-ppss with
> > heuristics to make it work "as well as possible" even when it only has
> > access to a portion of the buffer.
> 
> But the locked narrowing is applied based on the presence of long-lines
> rather than based on the total size of the buffer.

That's true, but the two are closely related: a buffer with very long
lines tends also to be a large buffer.

> I think we'll want to decouple the two because they aren't quite the
> same (e.g. in many cases the time taken by `syntax-ppss` doesn't depend
> on the length of lines, whereas the time taken by `font-lock-keywords`
> rues does not depend on the total buffer size).

The solutions for stuff that depends on buffer size will indeed be
different from solutions for stuff that depends on line size.
However, font-lock currently depends on both, and narrowing solves
both, albeit in non-specific ways.