click_event.rs

 let widget = files_tree.upcast::<Widget>();

widget.connect_button_release_event(|_, event| {
    println!("{}", event.get_button());
    Inhibit(false)
});

context_menu.rs

let widget = files_tree.upcast::<Widget>();

widget.connect_button_release_event(|_, event| {
    if event.get_button() != 3 {
        return Inhibit(false)
    }

    let popup_menu = Menu::new();

    let item = MenuItem::new_with_label("test");
    popup_menu.append(&item);

    popup_menu.show_all();
    popup_menu.popup_easy(event.get_button(), event.get_time());

    Inhibit(false)
});