动态生成的下拉菜单出现在屏幕左上角

我在鼠标中插入了一些代码,然后单击工具栏菜单项的事件以在运行时生成菜单,但菜单显示在屏幕的左上角,而不是菜单项下。不管代码是鼠标向下还是点击,菜单总是在错误的地方。我做错什么了?

以下是代码示例:

private void windowToolStripMenuItem_MouseDown(object sender, MouseEventArgs e)
{
    windowToolStripMenuItem.BuildOpenWindowsDropDown(Program.windowManager, (Form f) => (f.SomeProperty == SomeValue));
}

扩展方法:

static class ExtensionMethods
{
    public static void BuildOpenWindowsDropDown(this ToolStripDropDownItem toModify, WindowManager windowManager, Predicate<Form> constraint)
    {
        toModify.DropDownItems.Clear();
        List<Form> windows = windowManager.FindOpenWindows(constraint);
        if (windows != null)
        {
            windows.ForEach((Form f) =>
            {
                ToolStripItem tsi = toModify.DropDownItems.Add(f.Text);
                tsi.Tag = f;
                EventHandler clickHandler = new EventHandler(
                    (object sender, EventArgs e) =>
                    {
                        Form fToShow = (Form)((ToolStripItem)sender).Tag;
                        fToShow.Show();
                    });
                tsi.Click += clickHandler;
            });
        }
    }
}

以及WindowManager类中的代码段:

    public List<Form> FindOpenWindows(Predicate<Form> constraint)
    {
        var foundTs = from form in windows
                          where constraint(form)
                                && form.Created
                          select form;

        return foundTs.ToList();
    }