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

[Dmitry Gutov]

On 14.08.2022 19:16, Gregory Heytings wrote:
> > The alternative, of course, is to pay the price of syntax-ppss on 
> > larger spans and wait the corresponding amount of time the first time 
> > the user scrolls to EOB.
>
> Or to speed up syntax-ppss in a way or another.

I suggest you try tweaking parse-partial-sexp's implementation.

> > But as Gregory shows, when you get to _really_ large files (like 1 GB 
> > JSON file in his example), pressing M-> will still make you wait (I 
> > have to wait around 20 seconds).
>
> Or when you have a slower CPU, with moderately large files.

I doubt there are any CPUs in wide use that are even 5x slower than mine.

And even 200 MB is pretty large, and not something we've been reasonable 
able to edit before.

> > But being able to choose this approach (in the absence of better 
> > upcoming alternatives) is a good thing.
>
> The problem is (again) that what you would like does not provide any 
> protection against modes which use widen inside their fontification 
> routines.

Do you have examples of such modes?

Calling 'widen' inside font-lock-keywords is a bug, and the bugs should 
be fixed at the source.