split-window-preferred-function

split-window-preferred-function is a variable defined in `window.el'.
Its value is

split-window-sensibly

• This variable may be risky if used as a file-local variable.

Documentation:
Function called by `display-buffer' routines to split a window.
This function is called with a window as single argument and is
supposed to split that window and return the new window. If the
window can (or shall) not be split, it is supposed to return nil.
The default is to call the function `split-window-sensibly' which
tries to split the window in a way which seems most suitable.
You can customize the options `split-height-threshold' and/or
`split-width-threshold' in order to have `split-window-sensibly'
prefer either vertical or horizontal splitting.

If you set this to any other function, bear in mind that the
`display-buffer' routines may call this function two times. The
argument of the first call is the largest window on its frame.
If that call fails to return a live window, the function is
called again with the least recently used window as argument. If
that call fails too, `display-buffer' will use an existing window
to display its buffer.

The window selected at the time `display-buffer' was invoked is
still selected when this function is called. Hence you can
compare the window argument with the value of `selected-window'
if you intend to split the selected window instead or if you do
not want to split the selected window.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.