Win7触控拖动图片到StackPanel却没有显示

Question

正在做一个自己的小项目，如下图：

目的是将Canvas中的小红圈拖动到下边的StackPanel里，类似字母排序游戏，但要使用触控技术来实现。我在代码中使用了操作事件，其他部分还好，但在StackPanel.Children.Add(someVar)语句后，StackPanel里并未显示someVar，诡异的是调试时StackPanel的Children数量确实增加了1……求论坛里老师给予解答，谢谢！全部代码如下。

[前端Xaml]

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="650" Width="800">
    <Grid Name="mainFrame">
        <Grid.RowDefinitions>
            <RowDefinition Height="538*" />
            <RowDefinition Height="73*" />
        </Grid.RowDefinitions>
        <Canvas Grid.Row="0" Name="myCanvas"/>
        <StackPanel Grid.Row="1" Name="myStack" Orientation="Horizontal"/>
    </Grid>
</Window>

[后台CS]

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        Rectangle _Rect;
        Label _Label; //用来在运行时监视_Rect的视觉父元素

        public MainWindow()
        {
            InitializeComponent();
            this.Loaded+=new RoutedEventHandler(MainWindow_Loaded);
        }
       
        void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            _Rect = new Rectangle() {
                Fill = new SolidColorBrush(Colors.BlanchedAlmond),
                Width = 50,
                Height = 50
            };
            _Rect.RenderTransform = new MatrixTransform(1, 0, 0, 1, 30, 30);
            _Rect.IsManipulationEnabled = true;
           
            _Rect.ManipulationStarting += (s, arg) => {
                arg.ManipulationContainer = myCanvas;
                arg.Handled = true;
            };               
            _Rect.ManipulationDelta+=
                new EventHandler<ManipulationDeltaEventArgs>(_Rect_ManipulationDelta);
            myCanvas.Children.Add(_Rect);
           
            mainFrame.Children.Remove(myStack);
            Border border = new Border() {
                BorderThickness  = new Thickness(3),
                BorderBrush = new SolidColorBrush(Colors.Chocolate),
                Child = myStack
            };
            mainFrame.Children.Add(border);
            Grid.SetRow(border, 1);

            _Label = new Label() { Content = ""};
            myCanvas.Children.Add(_Label);
            Canvas.SetLeft(_Label, 5);
            Canvas.SetTop(_Label, 5);
            Canvas.SetZIndex(_Label, 0);
        }     

        void _Rect_ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
        {
            double x, y;
            Point currentPosition = e.ManipulationOrigin;
            Rectangle rect = e.Source as Rectangle;
            MatrixTransform xform = rect.RenderTransform as MatrixTransform;
            Matrix matrix = xform.Matrix;
            ManipulationDelta delta = e.DeltaManipulation;

            x = delta.Translation.X;
            y = delta.Translation.Y;
            if (currentPosition.Y + rect.ActualHeight / 2 > myCanvas.ActualHeight)
            {
                myCanvas.Children.Remove(rect);
                myStack.Children.Add(rect);
               
                e.Complete();
            }
            else
            {
                if (currentPosition.X - rect.ActualWidth / 2 < 0 ||
                currentPosition.X + rect.ActualWidth / 2 > myCanvas.ActualWidth)
                {
                    x = 0;
                }
                if (currentPosition.Y - rect.ActualHeight / 2 < 0)
                {
                    y = 0;
                }
                delta = new ManipulationDelta(new Vector(x, y), 0, new Vector(), new Vector());
                matrix.Translate(delta.Translation.X, delta.Translation.Y);
                xform.Matrix = matrix;
            }

            _Label.Content = rect.Parent.ToString();
            e.Handled = true;
        }
    }
}