>> + (defcustom split-window-preferred-function 'split-selected-window
 >> +   "Function called by `display-buffer' to split a window.
 >> + The window that shall be split is selected when this function is
 >> + called.  The function is supposed to either return the new window
 >> + or nil if the window can (or shall) not be split."
 >> +   :type 'function
 >> +   :version "23.1"
 >> +   :group 'windows)
 >
 > If setting this option to `split-window-horizontally' and
 > `split-window-vertically' is possible and it works correctly
 > with these value, then I suggest adding them as constants,
 > e.g. "Always split horizontally" and "Always split vertically".

Done.  But note that a function like `split-window-vertically' is suited
for interactive use and thus may throw an error when the window is too
small.  I can't allow an error in `display-buffer' so I have to wrap
that call in a `condition-case'.

 > Also I suggest keeping the common function name prefix "split-window-",
 > i.e. instead of `split-selected-window' to name it something like
 > "split-window-sensibly" or "split-window-with-thresholds".

I've chosen `split-window-sensibly'.  It also calls
`split-window-vertically' and `split-window-horizontally' now.

Please try the new patch - we're in pretest and I don't want to take too
many chances.

martin