bug#19060: [FIX INCLUDED] Off-by-one-line scrolling bug in window_scroll_pixel_based

[Eli Zaretskii]

> Date: Sat, 15 Nov 2014 15:32:41 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: kelly@prtime.org, 19060@debbugs.gnu.org
> 
>  > Since the issue is rounding vs truncating, perhaps try playing with
>  > the portion of the last displayed line, trying both less and greater
>  > than half the canonical line height.
> 
> Got it.  The last line must show more than a half of the frame's
> character height.  And the patch fixes it AFAICT.  So please apply it
> with as much comment as you can provide.

Done on the emacs-24 branch.