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

[martin rudalics]

> 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.

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.

martin