Run attached code:


import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.CheckBox;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

/**
 *
 * @author alexandr_kirov
 */
public class MenuOnShown extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) {
        VBox vb = new VBox();
        MenuBar mb = new MenuBar();

        Menu menu = new Menu("Menu");
        mb.getMenus().add(menu);

        menu.getItems().clear();
        for (int i = 0; i < 5; i++) {
            menu.getItems().add(new MenuItem("MENU_ITEM" + i));
        }

        final CheckBox cb = new CheckBox("showing");
        menu.showingProperty().addListener(new ChangeListener<Boolean>() {

            public void changed(ObservableValue<? extends Boolean> ov, Boolean t, Boolean t1) {
                System.out.println("shown?");
                cb.selectedProperty().setValue(t1);
            }
        });

        vb.getChildren().addAll(cb, mb);

        Scene scene = new Scene(vb, 300, 300);
        stage.setScene(scene);
        stage.show();
    }
}

On launch, focus is on checkBox.
Press "Tab"
Press "Space"
you will see dropDown.

But this
        menu.showingProperty().addListener(new ChangeListener<Boolean>() {
is not called.

Showing property is not reflect the currecnt state in such case.

But if you use mouse, and click on menu, than listener will be called.