bug#12419: Mouse click changes layout

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 24 Sep 2012 10:17:25 +0200
> From: martin rudalics <rudalics@gmx.at>
> Cc: 12419@debbugs.gnu.org
> 
>    Beginning with Emacs 24.1, windows have a normal height (a floating
>    point number) which is the fraction of their ideal height wrt to their
>    parent.  When you do C-x 2 the normal height of both emanating windows
>    is 0.5.  However, when the original window has an odd number of lines,
>    I have to give the lower window the one remaining line in order to be
>    consistent with the traditional splitting behavior.

Where in the code or the infrastructure do we enforce an integral
number of lines in a window?

AFAIK, the only restriction imposed by the display engine is that a
window's first line must be completely visible (unless its height is
larger than the window).  But the last line of a window can be only
partially visible.  That seems to imply that you should be able to
split a window such that each child gets exactly half, in pixels.

What am I missing?