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

[Gregory Heytings]

> > > No, I want a simple variable that just gives the size of the narrowed 
> > > region, with nil meaning don't narrow at all.
> >
> > FWIW, I strongly object to the addition of such a variable (unless it 
> > is clearly stated that it is added temporarily and should not be used 
> > except for testing purposes).
> >
> > Adding such a variable only two weeks after locked narrowing has been 
> > introduced means that modes will have little incentive to adapt to that 
> > stronger constraint, if they can "fix" whatever problems that 
> > constraint might cause by setting that variable to nil in their 
> > initialization hooks.
>
> Modes can do that already by changing long-line-threshold to nil, don't 
> they?

They can indeed, but that variable is documented as "for debugging 
purposes" only.  Moreover setting that variable to nil implies that they 
deactivate all long line optimizations, which is a higher price to pay.

The way I see this is that Emacs is doing efforts to solve editing 
slowdowns as best as possible, but that there is a "last mile" that cannot 
be solved without the collaboration from major and minor modes.