bug#21468: 24.5; When next-line (visual) crosses overlay with before-string="\n" point goes to column 0

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 13 Sep 2015 13:20:04 -0400
> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Cc: 21468-done@debbugs.gnu.org
> 
> > why can't Magit simply change the buffer text to be what it wants to
> > display? why does it need to use an overlay string?
> 
> In this particular case, the overlays are delimiting a "magit region"
> (consisting of whole logical lines touched by Emacs' region), having
> them as actual text in the buffer would interfere with point movement
> even more.

Sorry, I don't understand: how can buffer contents interfere with
cursor motion?

> Or at least, that's what I saw when I tried just now to do something
> similar with text properties:
> 
> (with-current-buffer (get-buffer-create "*text property test*")
>   (erase-buffer)
>   (insert-file "next-line-fail.el")
>   (let ((pos 65))
>     (goto-char pos)
>     (insert (propertize (concat (propertize "\s" 'display '(space :height (1)))
>                         (propertize "\n" 'line-height t))
>                         'face 'highlight))
>     (pop-to-buffer (current-buffer))))

Please elaborate: what problems do you wee in this example?