Re-creating Gtk Menu in event handler with Vala fails

There is a plugin that I'd like to get running, which fails because of this issue; I've discussed it, and manage to isolate it, in these two posts:

* https://askubuntu.com/questions/1089516/gnome3-appindicator-vala-app-cannot-show-submenu-it-auto-closes-immediately/
* https://stackoverflow.com/questions/53805975/re-creating-gtk-menu-in-event-handler-with-vala

The `test.vala` file from the second post is attached, it can be built with:

valac -X -D'GETTEXT_PACKAGE="my-indicator"' -D NEWMETHOD --pkg=gtk+-3.0 --pkg appindicator3-0.1 test.vala

... the `-D NEWMETHOD` simply replaces the deprecated Gtk.ImageMenuItem with Gtk.MenuItem, so as to get rid of deprecation warnings, but the behavior is exactly the same.

And the problem is: if a submenu creation code runs in an activate.connect handler, the created menu items never show. The animated .gif showing the issue (also in the above posts) is here:

https://i.stack.imgur.com/wA6kT.gif

Since this (creating submenu items in activate.connect handler) apparently worked at a point in the past, I'd consider the behavior I currently experience a bug.