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

[Lars Ingebrigtsen]

This reproduces for me from "emacs -Q":

(progn
  (run-at-time 1 1 (lambda ()))
  (pop-to-buffer "foo.txt")
  (text-mode)
  (flyspell-mode)
  (insert (propertize (make-string 20000 ?x) 'display "foo")
          "\n\n"))
 
If you hit "d", the mode line says "Narrow", and hitting RET will remove
the narrowing, and repeating that repeats the toggle.