bug#56682: locked narrowing

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: dgutov@yandex.ru,  56682@debbugs.gnu.org,  gregory@heytings.org
> Date: Wed, 17 Aug 2022 09:50:59 -0400
> 
> >> FWIW, I agree.  I don't think this information is cleanly&reliably
> >> exposed to ELisp
> >
> > It isn't currently.  I was suggesting to do so.
> 
> Last time I looked at it, I couldn't quite understand how the cache
> works there, so I wasn't sure how to make sure it returns valid
> information even if called outside of redisplay.

Looking at what maybe_produce_line_number does doesn't help?