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

[Eli Zaretskii]

> Cc: 57207@debbugs.gnu.org, yantar92@gmail.com
> Date: Thu, 18 Aug 2022 16:23:01 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Lars Ingebrigtsen <larsi@gnus.org>
> > Cc: yantar92@gmail.com,  57207@debbugs.gnu.org
> > Date: Thu, 18 Aug 2022 15:02:15 +0200
> > 
> > If I do the following in this running Emacs:
> > 
> > C-x C-f /tmp/empty-file.txt
> > M-: (insert (propertize (make-string 20000 ?x) 'display "foo"))
> > 
> > Then hit RET a couple of times, and then type any key (like `d'), the
> > mode line starts saying "Narrow".  If I hit it again, it switches off,
> > and then hitting it again switches it on again.
> 
> Can you attach a debugger to this session, and produce a C and Lisp
> backtrace from Fnarrow_to_region and Fwiden when you do the above?

Also, do you see any error messages in *Messages*?