class ModifiersPanel
{
    public Frame InsertedFrame { get; set; }
    public static readonly DependencyProperty InsertFrameProperty = DependencyProperty.RegisterAttached("InsertedFrame", typeof(Frame), typeof(ModifiersPanel), new FrameworkPropertyMetadata(OnInsertedFrameChanged));
    private static void OnInsertedFrameChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine(">> OnInsertedFrameChanged"); //не выводится
        d as ModifiersPanel.InsertedFrame = e.NewValue;
    }


    public Frame ExpandingFrame { get; set; }
    public static readonly DependencyProperty ExpandingFrameProperty = DependencyProperty.RegisterAttached("ExpandingFrame", typeof(Frame), typeof(ModifiersPanel), new FrameworkPropertyMetadata(OnExpandingFrameChanged));
    private static void OnExpandingFrameChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine(">> OnExpandingFrameChanged"); //не выводится
        d as ModifiersPanel.ExpandingFrame = e.NewValue;
    }


}