Re: split-window-preferred-function

[martin rudalics]

> This change could involve exposing whatever else is needed.

I'm not yet sure how to handle the following two:

                  || (NILP (XWINDOW (window)->parent))

and

          if (!NILP (XWINDOW (window)->prev))
            other = upper = XWINDOW (window)->prev;
          if (!NILP (XWINDOW (window)->next))
            other = XWINDOW (window)->next, upper = window;
          ...

I initially planned to use `window-edges' to check whether two windows
are "arrayed" in some sense.  That's not quite accurate when window
edges match but the involved windows have different parents.  Hence
enlarge_window could affect other windows and the overall behavior of
`display-buffer' might change.

XEmacs handles this by exposing `window-parent', `window-next', ... to
Elisp.  This would, however, contradict the Emacs ideology that Elisp
code should never see a non-leaf window.  In particular, we would have
to rewrite things like `adjust-window-trailing-edge' which currently
chokes on non-leaf windows.

BTW, do we want a `split-width-threshold'?