The ::show-menu signal is emitted before the menu is shown.
It can be used to populate the menu on demand, using
gtk_menu_tool_button_set_menu().
Note that even if you populate the menu dynamically in this way,
you must set an empty menu on the #GtkMenuToolButton beforehand,
since the arrow is made insensitive if the menu is not set.
The ::show-menu signal is emitted before the menu is shown.
It can be used to populate the menu on demand, using gtk_menu_tool_button_set_menu().
Note that even if you populate the menu dynamically in this way, you must set an empty menu on the #GtkMenuToolButton beforehand, since the arrow is made insensitive if the menu is not set.