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

[Eli Zaretskii]

> Date: Tue, 16 Aug 2022 00:18:35 +0300
> Cc: 56682@debbugs.gnu.org, gregory@heytings.org, monnier@iro.umontreal.ca
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> >> It should also be possible to move the whole invocation of the
> >> fontification-functions up to Lisp
> > 
> > How do you do that, given that these functions should be called from
> > redisplay with the region of buffer text only redisplay knows about?
> 
> Something like in the attached patch.

I fail to see the difference which would justify such a change.

> If we apply narrowing in handle_fontified_prop (or like in the 
> attachment), and also in font-lock-fontify-region, it seems like every 
> significant (*) execution path will go through two repetitions of the 
> same logic.
> 
> That's not ideal.

But not a catastrophe, either.

> >> And nlinum might not appreciate being narrowed. It seems to be working
> >> okay without that in my 20 MB XML file. And in 200 MB one too.
> > 
> > Why is it important what nlinum does, when we have native line numbers
> > nowadays?
> 
> Just an example of a mode which uses jit-lock-functions. With an 
> implication that other, less known modes, might have similar needs and 
> behavior. There aren't too many known examples.

Let's hear them hollering, and then see what are their problems.