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

[Stefan Monnier]

>> Your current code makes it impossible for a major mode to make Emacs slow
>> by widening in a too-long-line.
> Which is a good thing, isn't it?

No: we don't want to prevent people from shooting themselves in the foot.
Basically if it says "impossible" there's a good chance it's not a good thing.

> Or do you think that it's okay for Emacs to become unresponsive just
> because it is busy highlighting characters in the buffer?

Widening in a too long line will not necessarily lead to an unresponsive
Emacs, so by preventing it you're also preventing useful cases.

>> I'd prefer if we made it easy (i.e. the default) for Emacs to work well in
>> that case, without making it impossible for the major mode to mess
>> things up.
> Sure, I would also welcome a better solution.  Until it materializes, the
> only reasonable way is to use a less optimal solution.

It's not difficult to make it possible to re-widen after your narrowing.

> I just sent one such example to Dmitry.  And I pointed to another possible
> solution, namely to scan the whole buffer while opening it (instead of
> scanning it lazily, which is IIUC what currently happens).  From a user
> viewpoint, it's understandable that opening a big file takes some time.

We used to scan eagerly in the background with `jit-lock-stealth`, but that was
not very popular (eats up your battery for fairly little benefit).
We also have "lazier" highlighting via `jit-lock-defer`, but that hasn't
been adapted to `syntax-ppss`.  It might be with investigating.


        Stefan