imenu-use-popup-menu

imenu-use-popup-menu is a variable defined in `imenu.el'.
Its value is

on-mouse

Documentation:
Use a popup menu rather than a minibuffer prompt.
If nil, always use a minibuffer prompt.
If t, always use a popup menu,
If `on-mouse' use a popup menu when `imenu' was invoked with the mouse.

You can customize this variable.