スクロールバーが一番下にあるときのみ要素の追加時に自動スクロールするBehavior

前提条件：
・Xaml.Behaviors.Wpf がインストール済
・言語バージョン C#9.0
・Null安全のオプションがON

class AutoScrollBehavior : Behavior<ListBox>
{
//-----------------依存関係プロパティ--------------------------------
    public static readonly DependencyProperty EndMarginProperty =
        DependencyProperty.Register(
       "EndMargin",
       typeof(double),
       typeof(AutoScrollBehavior), 
       new FrameworkPropertyMetadata
       {
           DefaultValue = 0.0,
           BindsTwoWayByDefault = true
       }); 

    public double EndMargin
    {
        get { return (double)GetValue(EndMarginProperty); }
        set { SetValue(EndMarginProperty, value); }
    }
//-------------------------------------------------------------------

    ScrollViewer? _listBoxScroll;
    bool _shouldAutoScroll = true;

    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.Loaded += OnLoaded;

    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        _listBoxScroll = CaptureScrollViewer();
        if (_listBoxScroll is not null)
        {
            _listBoxScroll.ScrollChanged += OnScrollChanged;
        }
    }

    private void OnScrollChanged(object sender, ScrollChangedEventArgs e)
    {
        if (e.VerticalChange != 0.0)
        {
            _shouldAutoScroll = e.ViewportHeight + e.VerticalOffset + EndMargin >= e.ExtentHeight;
            return;
        }


        if (_shouldAutoScroll)
        {
            _listBoxScroll = _listBoxScroll ?? CaptureScrollViewer();
            _listBoxScroll?.ScrollToEnd();
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        if (_listBoxScroll is not null)
        {
            _listBoxScroll.ScrollChanged -= OnScrollChanged;
        }

        this.AssociatedObject.Loaded -= OnLoaded;

    }

    private ScrollViewer? CaptureScrollViewer()
    {
        var border = VisualTreeHelper.GetChild(AssociatedObject, 0) as Border;
        return border?.Child as ScrollViewer;
    }

}