Right aligned popup menu with GTK#

I have a button on the far right of an application. When clicked I want to show a menu underneath the button but right aligned with it. (think google chrome menu).

This is what I have so far:

protected virtual void HandleMenuClicked (object sender, System.EventArgs e)
{
    Menu menu = new Menu();
    menu.Add(new MenuItem("About"));
    menu.ShowAll();
    menu.Popup(null,null,MenuPosition,0,0);
}

private void MenuPosition(Menu menu, out int x, out int y, out bool pushIn)
{
    _menuButton.ParentWindow.GetOrigin(out x, out y);
    int menuWidth =开发者_JAVA技巧 75; //need to look up actual width.
    x += _menuButton.Allocation.X - menuWidth;
    y += _menuButton.Allocation.Y + _menuButton.Allocation.Height;
   pushIn = true;
}

The problem with this is that I have hard coded the width of the menu. If a longer menu entry was added, the font changed or theme changed then the alignment will be wrong.

I tried looking up the width of the menu as follows:

int menuWidth = menu.Allocation.Width;

This does not work as the menu has not been drawn yet and so the allocation width is 1.

I believe you can use SizeRequest at that point.