Something

using Gtk;
using System;

public class ToggleButtonApp  {

     int dialog_x = 0;
     int dialog_y = 0;     
     ToggleButton btn;
     Menu contextMenu;
     TreeView categoryListView;
     TreeIter listIter;
     ListStore listStore;
     Gtk.Window dialogWindow;
     Gtk.Window parentWindow;

     public static int Main (string[] args)
     {
          new ToggleButtonApp();
          return 0;
     }
          
     public ToggleButtonApp()
     {
          Application.Init ();
          parentWindow = new Window ("ToggleButton Tester");
          parentWindow.SetDefaultSize (150, 25);
          parentWindow.DeleteEvent += new DeleteEventHandler (Window_Delete);
          //parentWindow
          
          btn = new ToggleButton ("Unselected");
          btn.Active = false;
          btn.Toggled += new EventHandler (btn_toggled);
          parentWindow.Add (btn);
          
          contextMenu = new Menu ();
          CheckMenuItem menuItem = new CheckMenuItem ("Item 1");
          contextMenu.Append (menuItem); 
          
          CheckMenuItem menuItem1 = new CheckMenuItem ("Item 1");
          contextMenu.Append (menuItem1); 
          //contextMenu.AttachToWidget 
          //(btn, new MenuDetachFunc (OnDetachContextMenu));       
          //contextMenu.ShowAll ();
          btn.ShowAll ();
          
          SetupPopUpWindow ();
          
          parentWindow.ShowAll ();
          Application.Run ();
     }

     void OnSelectionChanged(object o, EventArgs args)
     {
       Console.WriteLine("Hello!");
     }     

     void SetupPopUpWindow ()
     {
       dialogWindow = new Window ("Categories");
       dialogWindow.Decorated = false;

       categoryListView = new TreeView ();
       
       categoryListView.HeadersVisible = false;
       categoryListView.HeadersClickable = false;
       
       categoryListView.Selection.Changed 
         += new EventHandler(OnSelectionChanged);

       categoryListView.AppendColumn("Categories",
        new Gtk.CellRendererText (), "text", 0);
       
       listStore = new ListStore (typeof(string));
       
       categoryListView.Model = listStore;
       
       listIter = new TreeIter ();
       
       for(int i=0; i < 6; i++) {
        string temp = "Hello";
        Console.WriteLine ("{0}:\t{1}", i.ToString (), temp);
        
        object [] columns = new object [1];
        columns [0] = temp + i.ToString (); 
        
        listIter = listStore.AppendValues (columns);
       }

       Frame frame = new Frame ();
       frame.ShadowType = ShadowType.In;
       
       Alignment align = new Alignment (0,0,0,0);
       uint padding = 1;
       align.SetPadding (padding, padding, padding, padding);
       
       Frame frame2 = new Frame ();
       frame2.ShadowType = ShadowType.In;
       
       frame2.Add (categoryListView);       
       align.Add (frame2);
       frame.Add (align);     
       dialogWindow.Add (frame);      

       dialogWindow.Gravity = Gdk.Gravity.North; 
       dialogWindow.GetPosition (out dialog_x, 
                                 out dialog_y);
       dialogWindow.ShowAll ();
       dialogWindow.GdkWindow.TypeHint = Gdk.WindowTypeHint.Utility;
       
       dialogWindow.Hide ();
       
       
     }

    void OnDetachContextMenu (Gtk.Widget widget, Gtk.Menu menu)
    {
      if(btn.Active)
      {
        contextMenu.Show ();
      }
      else
      {
        contextMenu.Hide ();
      }
    }

    void WindowPosition(Window window)
    {
      int height =0;
      int width = 0;
      window.GetSize(out width , out height);
      Console.WriteLine("Width: {0}, Height: {1}" , width , height);
    }

    void PositionWidgetPopup(bool rightPosition)
    {     
      int btn_x = 0;
      int btn_y = 0;
      
      btn.GdkWindow.GetOrigin (out btn_x, out btn_y);
     
      int btn_w = btn.Allocation.Width;
      int btn_h = btn.Allocation.Height;

      if(rightPosition)
      {
        btn_x = btn_x + btn_w;
      }
      else
      {
        btn_y = btn_y + btn_h;
      }

      dialogWindow.Move(btn_x, btn_y);
    }

    void btn_toggled (object obj, EventArgs args)
    {
      Console.WriteLine ("Button Toggled");

      if (btn.Active)
      {           
        WindowPosition (dialogWindow);
        PositionWidgetPopup (true);
        dialogWindow.Show ();
        btn.Label = "Selected";
      }
      else
      {
        PositionWidgetPopup (true);
        dialogWindow.Hide ();
        btn.Label = "Unselected";
      }
    }

     static void Window_Delete (object obj, DeleteEventArgs args)
     {
          Application.Quit ();
          args.RetVal = true;
     }

		static void DeactivateMenu (object sender, EventArgs args) 
		{
			Gtk.Menu menu = (Gtk.Menu) sender;
			menu.Popdown ();
		}
		
    static void PopupMenu (Gtk.Menu menu, Gdk.EventButton ev)
		{
			menu.Deactivated += DeactivateMenu;
			menu.Popup (null, 
				    null, 
				    new Gtk.MenuPositionFunc (GetMenuPosition), 
				    IntPtr.Zero, 
				    (ev == null) ? 0 : ev.Button, 
				    (ev == null) ? Gtk.Global.CurrentEventTime : ev.Time);
		}
		static void GetMenuPosition (Gtk.Menu menu, 
                 						    out int  x, 
                 						    out int  y, 
                 						    out bool push_in)
		{
			Gtk.Requisition menu_req = menu.SizeRequest ();

			menu.AttachWidget.GdkWindow.GetOrigin (out x, out y);

			if (y + menu_req.Height >= menu.AttachWidget.Screen.Height)
				y -= menu_req.Height;
			else
				y += menu.AttachWidget.Allocation.Height;

			push_in = true;
		}
}