bug#14825: 24.3.50; split-window-below miscounts window lines

[Eli Zaretskii]

> Date: Tue, 09 Jul 2013 11:09:17 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: 14825@debbugs.gnu.org
> 
>  > The reason for this seems to be that window.el and window.c do all
>  > calculations in canonical lines, which is incorrect when the font of the
>  > default face changes.  Moreover, the documentation barely hints on the
>  > fact that "lines" actually means "canonical-height lines" in almost all
>  > window-* functions that deal with vertical dimensions.
> 
> Currently, all window and frame sizing functions are based on canonical
> character sizes.  This will change when we do resizing pixelwise.

Resizing is only part of the problem.  There are functions that are
unrelated to resizing, which report dimensions in canonical units.
They should at least document this fact in the doc string, if not fix
it.  (The fix is not too hard, btw; see window-screen-lines for an
example.)

> However, your example seems contrived: You don't change the default face
> but the buffer's default face.  In many cases, split-window is followed
> by displaying another buffer in the new window so this buffer's default
> might not be appropriate for the other buffer anyway.

We have this problem already: one could split the window, and _then_
change the default face in one of the child windows.