popup menu-bar in lucid

[Colin Baxter]

Hello,

1. Compile emacs-28 with gtk3 via ./configure <RET>
2. emacs -Q  <RET>
3. Evaluate (menu-bar-mode -1) in scratch.
4. Menu-bar is removed.
5. <F10> brings popup menu-bar.
6. Navigate popup menu with the mouse or arrow keys.

Now repeat above steps for emacs-28 compiled with lucid. The popup menu
can be navigated with the mouse, but not using arrow keys.

This seems to have happened with emacs-28. It does not occur with
emacs-27, where arrow keys can be used.

Colin Baxter.