Variable: imenu-submenus-on-top
imenu-submenus-on-top is a buffer-local variable defined in
imenu.el.gz.
Documentation
Flag specifying whether items with sublists should be kept at top.
For some indexes, such as those describing sections in a document, it makes sense to keep their original order even in the menubar.
Source Code
;; Defined in /usr/src/emacs/lisp/imenu.el.gz
;;;###autoload
(defvar-local imenu-submenus-on-top t
"Flag specifying whether items with sublists should be kept at top.
For some indexes, such as those describing sections in a document, it
makes sense to keep their original order even in the menubar.")