imenu--mouse-menu
imenu--mouse-menu is a compiled Lisp function in `imenu.el
'.
(imenu--mouse-menu INDEX-ALIST EVENT &optional TITLE)
Let the user select from a buffer index from a mouse menu.
INDEX-ALIST is the buffer index and EVENT is a mouse event.
Returns t for rescan and otherwise an element or subelement of INDEX-ALIST.