bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Eli Zaretskii]

> Date: Sun, 21 Aug 2022 13:18:39 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: larsi@gnus.org, 57207@debbugs.gnu.org, yantar92@gmail.com
> 
> >> Stefan said (IIUC) that it could be useful for Lisp programs, so it is 
> >> now possible to use it anywhere (but the docstring says that it should 
> >> be done sparingly).
> >
> > Which means that the code you installed on the branch will "undo" such 
> > narrowing done from Lisp as well, doesn't it?  That might not be what 
> > the Lisp program which did that wanted.
> 
> I'm not sure I understand what you mean.  What the modeline should display 
> is the state of the "user" narrowing, not of a narrowing decided by a Lisp 
> function.

We don't currently discern between these two, so some Lisp program
could use this locked narrowing expecting to see it on the modeline.
If that is not supposed to happen and/or is unsupported, we should at
least document it.