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

[Gregory Heytings]

> Here's a full test case.

Thanks!

> emacs -Q and then:
>
> (progn
>  (pop-to-buffer "foo.txt")
>  (flyspell-mode)
>  (insert "\n\n"
>           (propertize (make-string 20000 ?x) 'display "foo")
>           "\n\nbar\nzot\n")
>  (goto-char (point-min)))
>
> Then type `d'.  Everything after "foo" disappears.

It doesn't, here :(  Strange...