public class MyApp extends UI
.....
    Navigator navigator;
    VerticalLayout rootLayout = new VerticalLayout();
    VerticalLayout contentLayout = new VerticalLayout();
    HorizontalLayout headerLayout = new HorizontalLayout();
    MenuBar header = new MenuBar();
...

        header.setWidth("100%");
        headerLayout.setWidth("100%");
        headerLayout.addComponent(header);

        rootLayout.addComponents(headerLayout, contentLayout);

       setContent(rootLayout);
       
       navigator = new Navigator(this,contentLayout);