Several mouse clicks with the <CTRL> and <SHIFT> modifiers bring up menus.
The MSB (“mouse select buffer”) global minor mode makes this
menu smarter and more customizable. See Buffer Menus.
 Some systems use Mouse-3 for a mode-specific
menu. We took a survey of users, and found they preferred to keep
Mouse-3 for selecting and killing regions. Hence the decision
to use C-Mouse-3 for this menu. To use Mouse-3 instead,
(global-set-key [mouse-3] 'mouse-popup-menubar-stuff).