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

[Gregory Heytings]

> > I already did, and told you that these multi-second delays were still 
> > present in larger JSON files (and therefore also in smaller files with 
> > slower CPUs).
>
> Delays or "a delay"? Just the first time you scroll to EOB? Or did you 
> do the whole dance with editing near BOB, then goto EOB, and repeat?

Delays.  When scrolling to EOB and when using isearch, for example.

> > > Then the goal failed because we don't disable bidi hpa? It has a much 
> > > more noticeable effect on editing that font-lock.
> >
> > That's not correct, no.  The BPA algorithm makes C-n and C-p slower at 
> > the beginning of large files only in a very specific case: when the 
> > beginning of the buffer contains opening brackets whose matching 
> > closing bracket is at the end of the buffer.
>
> To be fair, that's pretty much all JSON files.

Again, the aim of this effort has _nothing_ to do with JSON files.

> The branch:
>
> 1) Allows everyone interested to evaluate the performance of 
> unrestricted font-lock even in large files (single-line or not) and see 
> how big on a problem the delays caused by syntax-ppss actually are in 
> their experience. It's an important question.
>
> 2) Figure out the file sizes where syntax-ppss's performance really does 
> become a problem. That can give us data for better defaults later.

Buffers with long lines are rare enough.  It wouldn't make sense (and 
would be a nightmare in terms of long term maintenance) to add 25 
defcustoms to allow anyone to fine-tune what Emacs does in such buffers.

> 3) Play around with two easy solutions that we discussed previously: 
> narrowing during font-lock (one of the values for font-lock-large-files 
> pretty much matches the current behavior on master), or fontifying only 
> the first X characters (e.g. 1000000) of the buffer, and skipping on the 
> rest.
>
> 4) It should be plain to see that implementing additional approaches 
> should be easy enough. For instance, a hybrid like "fontify the first 1 
> MB correctly, and the rest - on best-effort basis". Although the value 
> '(narrow . 1000000) should provide behavior a very similar behavior 
> already. Maybe ever a better one: the boundaries are stable. Maybe sure 
> to try it together with (setq bidi-inhibit-hpa t): the result looks very 
> fast to me.

These 3) and 4) assume that the major (and minor) modes are well-behaving, 
and will obey the restrictions.  Which isn't true, and was the reason why 
locked narrowing was introduced.