Variable: mouse-buffer-menu-mode-mult

Group the buffers by the major mode groups on ?
This number which determines (in a hairy way) whether
will split the buffer menu by the major modes (see
`mouse-buffer-menu-mode-groups') or just by menu length.
Set to 1 (or even 0!) if you want to group by major mode always, and to
a large number if you prefer a mixed multitude. The default is 4.