mouse-buffer-menu-mode-mult

mouse-buffer-menu-mode-mult is a variable defined in `mouse.el'.
Its value is

4

Documentation:
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.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 20.3 of Emacs.