bug#45820: menu-bar-mode: show drop-down menu instead of tmm-menubar in terminal Emacs

[Tim Lee <progscriptclone@gmail.com>]

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The problem: In terminal Emacs, clicking on an item in the menu bar will
open "tmm-menubar".

Expected behavior: "menu-bar-open" should be run instead (i.e. a
drop-down menu should appear instead of tmm-menubar).

For example, if I left-click on the "Edit" menu item in the menu bar, I
expect the "Edit" drop-down to appear. Currently however, a left-click
on a menu item will open tmm-menubar in a *Completions* window. This is
surprising behavior.

By default, in terminal Emacs, "<menu-bar> <mouse-1>" is bound to
"tmm-menubar-mouse". There doesn't seem to be an easy way to change this
to the drop-down menu instead.

There exist a question about this issue on Emacs Stack Exchange site:
https://emacs.stackexchange.com/questions/50478/replace-clicking-a-menu-bar-option-from-m-to-f10-behavior
The proposed "solution" there is similar to this:

(define-key global-map [menu-bar mouse-1]
            (lambda (event)
              (interactive "e")
              ;; Adapted from the implementation of "menu-bar-open" in
Emacs 25.2.2.
              (let* ((x (car (posn-x-y (event-start event))))
                     (menu (menu-bar-menu-at-x-y x 0)))
                (popup-menu (or (lookup-key-ignore-too-long global-map
(vector 'menu-bar menu))
                                (lookup-key-ignore-too-long
(current-local-map) (vector 'menu-bar menu))
                                (cdar (minor-mode-key-binding (vector
'menu-bar menu)))
                                (mouse-menu-bar-map))
                            (posn-at-x-y x 0 nil t)
                            nil
                            t))))

However, there are serious flaws with this "solution":

  * Clicking on a menu bar item (e.g. "Help"), followed immediately by
    clicking outside the menus and drop-downs (e.g. clicking in the
    window) will not cancel the selected action. Instead, the
    highlighted action will actually run.
  * Clicking on a particular menu bar item (e.g. "Help") and immediately
    clicking on it again will not close the drop-down menu.

(Emacs version: 25.2.2, Terminal: GNOME Terminal 3.28.2, OS: Ubuntu 18.04)



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