bug#12403: 24.2; scroll-margin breaks emacs24

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 10 Sep 2012 16:50:22 +0200
> From: Yuri D'Elia <wavexx@thregr.org>
> 
> emacs24 as shipped by debian breaks "scroll-margin".
> 
>    http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=681991
> 
> It seems that setting scroll-margin on *any* value breaks the redisplay
> while scrolling.
> 
> Try with the following (under a graphical session):
> 
> emacs -q
> 
> open the tutorial, scroll with the arrow keys until you reach the bottom.
> Everything looks ok.
> 
> kill emacs. Start again, this time:
> 
> C-x set-variable scroll-margin 5
> 
> open the tutorial, scroll down with the arrow keys until you reach the
> bottom.  Text on the page looks *badly* garbled (this is not just
> character left-overs from the previous screenfull, a lot of lines look
> repeated at some point in the same line).

Confirmed :-(.  However, this bug was present even in the first
pretest of Emacs 24, 24.0.90.

I will take a look.