bug#17969: 24.3.92; C-p does not move the cursor for some scale text

[Eli Zaretskii]

> Date: Tue, 08 Jul 2014 11:17:25 +0900
> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> 
> 1. $ emacs -Q -D &
> 2. Type the following expression and hit C-j in the *scrath* buffer.
> 
>   (progn
>     (text-scale-set 6)
>     (insert (make-string 9 ?\t) "aaaa\n"))
> 
>    In my environment, the screen looks like the attached screenshot.
> 
> 3. Hit C-p twice.
> 
> Result:
> 
> The first C-p moves the cursor upward, but for the second one (the
> cursor stays at the same position).
> 
> Unfortunately, some adjustment seems to be necessary for the
> reproducing recipe depending on the environment.  The above one is for
> the GTK+ build with the xft font backend on OS X 10.9.  For the Mac
> port, I had to use the following one, for example:
> 
>   (progn
>     (text-scale-set 5)
>     (insert (make-string 10 ?\t) "aaaaaa\n"))

I cannot reproduce this with either of the 2 recipes.  So please step
through vertical-motion and see what's going on there.

Thanks.