Gtk.MenuToolButton::show-menu

function callback(menu_tool_button, ):void;

The ::show-menu signal is emitted before the menu is shown.

It can be used to populate the menu on demand, using Gtk.MenuToolButton.prototype.set_menu.

Note that even if you populate the menu dynamically in this way, you must set an empty menu on the Gtk.MenuToolButton beforehand, since the arrow is made insensitive if the menu is not set.