WPF Menu disappears when nested too deeply

I'm trying to create a nested menu hierarchy from the main menu bar. For example, the main menu bar would contain a "Goto" menuitem that could be navigated as "Goto" -> "Chapter" -> "Chapter Name" -> "Subchapter name".

The hierachy below "Chapter" needs to be added dynamically. When I do this, the menu disappears when it tries to display the subitems of a chapter name.

If you run the code sample below, you can click on "Goto" and "Chapter", but as soon as you hover over or click on one of the chapter names, the menus disappear without allowing one of the subsection names to be clicked.

Can anyone tell me why?

using System;
using System.Windows;
using System.Windows.Controls;

namespace MenuTest
{
    public class MenuTest : Window
    {
        // Main method
        [STAThread]
        public static void Main()
        {
            Application App = new Application();
            App.Run(new MenuTest());
        }

        // Properties:
        Menu MainMenu;
        MenuItem GotoMenu;
        MenuItem ChapterMenu;

        // Constructor
        public MenuTest()
        {
            // Add a menu with a "Goto" submenu that opens up a pop-up with a "Chapter" selection:
            // ----------------------------------------------------------------------------------
            MainMenu = new Menu();
            DockPanel.SetDock(MainMenu, Dock.Top);
            this.Content = MainMenu;

            GotoMenu = new MenuItem();
            GotoMenu.Header = "Goto";
            GotoMenu.SubmenuOpened += OnSubMenuOpen;
            MainMenu.Items.Add(GotoMenu);

            ChapterMenu = new MenuItem();
            ChapterMenu.Header = "Chapter";
            GotoMenu.Items.Add(ChapterMenu);
        }

        // Add a chapter and subchapter hiearachy to the Chapter menu:
        void OnSubMenuOpen(object Sender, RoutedEventArgs e)
        {
            ChapterMenu.Items.Clear();
            string[] ChapterNames = { "Chapter 1", "Chapter 2", "Chapter 3" };
            foreach (string ChapterName in ChapterNames)
            {
                MenuItem ThisChapterMenu = new MenuItem();
                ThisChapterMenu.Header = ChapterName;
                ChapterMenu.Items.Add(ThisChapterMenu);

                string[] SubChapterNames = { "SubSection 1", "SubSection 2", "SubSection 3" };
                foreach (string SubChapterName in SubChapterNames)
                {
                    MenuItem SectionMenu = new MenuItem();
                    SectionMenu.Header = String.Format("{0} - {1}", ChapterName, SubChapterName);
                    ThisChapterMenu.Items.Add(SectionMenu);
                }
  开发者_如何学运维          }
        }
    }
}

OnSubMenuOpen is being called for each level of menu below GotoMenu, thus clearing ChapterMenu every expand.

On the first line of OnSubMenuOpen, check where the call originated from:

 void OnSubMenuOpen(object Sender, RoutedEventArgs e)
 {
    if (e.Source != GotoMenu) return;