bug#61580: line-prefix and display text properties cause odd cursor movement

[Eli Zaretskii <eliz@gnu.org>]

> From: Darius Foo <darius.foo.tw@gmail.com>
> Date: Sat, 18 Feb 2023 00:39:00 +0800
> 
> When moving the cursor up and down on lines which have both line-prefix and display text properties, the
> cursor column isn't preserved, jumping ahead or back by a fixed amount.
> 
> This can be reproduced with the following config
> 
>       (add-text-properties 12 13 '(line-prefix "  "))
>       (add-text-properties 27 28 '(line-prefix "  "))
>       (add-text-properties 57 58 '(line-prefix "    "))
>       (add-text-properties 72 73 '(line-prefix "    "))
> 
>       (add-text-properties 12 13 '(display "•"))
>       (add-text-properties 27 28 '(display "•"))
>       (add-text-properties 57 58 '(display "•"))
>       (add-text-properties 72 73 '(display "•"))
> 
> and this input file (which mimics org-indent).
> 
>     * h1
>     ** h2
>     - a.y.........
>     - x...........
>     *** h3
>     **** h4
>     - ....w.......
>     - z...........
> 
> With the cursor on x, pressing up moves it to y instead of a, and pressing down moves it back to x. The
> amount moved depends on the indentation added by line-prefix: with the cursor on z, pressing up moves it to
> w.

Thanks, should be fixed now on the emacs-29 branch.

(This bug was a leftover from incomplete fix of bug#49695.)