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

[Eli Zaretskii]

> Date: Fri, 5 Aug 2022 15:59:57 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> On 01.08.2022 14:58, Eli Zaretskii wrote:
> 
> > As I wrote elsewhere, I'm okay with extending 'widen' so that it could
> > "unlock" the locked narrowing, which could then be used in major modes
> > that convince us their performance is adequate (or clearly announce in
> > their docs that they don't care about files with long lines ;-).
> 
> And to address the idea of "unlocking" the narrowing: I think I have 
> demonstrated that the remaining slowdown can be caused purely by the 
> length of the buffer and how long 'parse-partial-sexp' takes to parse 
> it.

No, you haven't demonstrated that.

> And of course the more, let's say, *complex* modes like CC Mode will opt 
> for unlocking narrowing right away because its font-lock logic has to 
> jump around to previously-saved positions in its syntax cache, which 
> will inevitably spam errors here and there when those positions are not 
> accessible.

CC Mode is extremely unlikely to happen in files with such long lines,
so what it does is largely irrelevant to this discussion.