Re: bug-reference-prog-mode slows down CC Mode's scrolling by ~7%

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 1 Sep 2021 18:20:25 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > Then I think the easiest fix is to get bug-reference-prog-mode return
> > a suitable value, or wrap it in a function that would.  Every function
> > that puts itself on jit-lock-functions without actually fontifying
> > anything should return BOB..EOB as the region it fontified, right?
> 
> I'm not sure that's the right fix.  If font-lock mode were disabled, say,
> bug-reference-fontify would be the only function on jit-lock-functions.

But that never happens, does it?

Or if we want to be holier than the Pope, bug-reference-fontify could
check if it's the only function on the list, and then change its
return value accordingly.

> The mechanism with `jit-lock-bounds' in jit-lock.el is a good idea, but
> somehow it's not quite optimal at the moment.

It was designed for functions that actually fontify something.  We
need to either extend it for functions that don't, or change those
functions to pretend they do, but in a way that doesn't disrupt
jit-lock-bounds.

> For example, if the first jit-lock-functions element returns
> (... (new-BEG .  new-END)) we might be able to supply new-BEG, new-END as
> the arguments to the second element.  Or something like that.

I think it's too late to change the protocol, since it was in use for
too long.