silverlight가 ScrollViewer에 동적으로 컨트롤을 추가할 때 ScrollViewer 스크롤 막대는 항상 아래로

<UserControl x:Class="SilverlightApplication7.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">


    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.RowDefinitions>
            <RowDefinition Height="50"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
        </Grid.RowDefinitions>
        <Button Name="TestButton" Width="200" Height="24" Content="Add" Grid.Row="0"></Button>
        <ScrollViewer Name="TestScrollViewer" Grid.Row="1">
            <StackPanel Name="TestStackPanel" Orientation="Vertical">  
            </StackPanel>
        </ScrollViewer>
    </Grid>
</UserControl>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;


namespace SilverlightApplication7
{
    public partial class MainPage : UserControl
    {
        private int counter = 1;
        public MainPage()
        {
            InitializeComponent();
            TestButton.Click += new RoutedEventHandler(TestButton_Click);
            TestStackPanel.SizeChanged += new SizeChangedEventHandler(TestStackPanel_SizeChanged);
        }


        void TestStackPanel_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            // StackPanel       ScrollViewer     ，ScrollViewer     
            if (TestStackPanel.ActualHeight > TestScrollViewer.ActualHeight)
            {
                TestScrollViewer.ScrollToVerticalOffset(TestStackPanel.ActualHeight);
            }
        }


        void TestButton_Click(object sender, RoutedEventArgs e)
        {
            //   Button
            Button button = new Button();
            button.Width = 200;
            button.Height = 100;
            button.Content = counter++;
            TestStackPanel.Children.Add(button);
        }
    }
}