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

[Visuwesh]

[வெள்ளி ஆகஸ்ட் 19, 2022] Eli Zaretskii wrote:

> Didn't you say the reproduction recipe doesn't work in "emacs -Q",
> only in some session of yours?  If so, maybe what you have in that
> session explains why you see it and I don't?

I can reproduce it in emacs -Q using the recipe Lars posted upthread.

    (progn
      (pop-to-buffer "foo.txt")
      (flyspell-mode)
      (insert (propertize (make-string 20000 ?x) 'display "foo")
          "\n\n"))

If I type 'a' in EOB, I see Narrow appear in the mode-line if I move
using C-p, it disappears.  If I insert a character again, it reappears.
I'm on Debian, and the git HEAD is

    % git log |head -6
    commit f117b5df4dc69a38a5568c5926c1e417a35314eb
    Author: Lars Ingebrigtsen <larsi@gnus.org>
    Date:   Fri Aug 19 15:22:29 2022 +0200

        Add new functions eol and bol