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

[Eli Zaretskii]

> 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?