User-provided callback function to create a popup for a
GtkMenuButton on demand.
This function is called when the popup of @menu_button is shown,
but none has been provided via [method@Gtk.MenuButton.set_popover]
or [method@Gtk.MenuButton.set_menu_model].
User-provided callback function to create a popup for a GtkMenuButton on demand.
This function is called when the popup of @menu_button is shown, but none has been provided via [method@Gtk.MenuButton.set_popover] or [method@Gtk.MenuButton.set_menu_model].