WPF MessageBox bug on touch device?

The Xmal goes like this:

<Window x:Class="TouchBug.MainWindow"
        xmlns=
        xmlns:x=
        Title="MainWindow" Height="220" Width="309">
    <Grid>
        <TextBlock Height="23" HorizontalAlignment="Left" Margin="84,36,0,0" Name="textBlock1"
                   Text="2013-01-23 00:00:00" VerticalAlignment="Top" Width="172" />
        <Canvas Height="53" HorizontalAlignment="Left" Margin="29,92,0,0" Name="canvas1"
                VerticalAlignment="Top" Width="93" Background="Red" MouseDown="canvas1_MouseDown" />
        <Canvas Height="53" HorizontalAlignment="Left" Margin="163,92,0,0" Name="canvas2"
                VerticalAlignment="Top" Width="93" Background="Blue" MouseDown="canvas2_MouseDown" />
        <Label Content="Time:" Height="28" HorizontalAlignment="Left" Margin="29,31,0,0" Name="label1"
               VerticalAlignment="Top" Width="49" />
    </Grid>
</Window>

And the codes behind are:

    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void canvas1_MouseDown(object sender, MouseButtonEventArgs e)
        {
            textBlock1.Text = DateTime.Now.ToString();
        }

        private void canvas2_MouseDown(object sender, MouseButtonEventArgs e)
        {
            MessageBox.Show("I'm a message box!", "Message", MessageBoxButton.OK);
            e.Handled = true;
        }
    }

According to the codes, while we click the red canvas, the text after "Time:" will change to the current time, while we click the blue canvas, a MessageBox will be shown. As we expected, any time we click the two canvases with the mouse, the corresponding MouseDown event is fired.

However, something is wrong while I run this application on a touch screen. firstly, I touch the blue canvas and get a MessageBox, then I touch the "OK" button to close the MessageBox. Secondly, I touch the red canvas, the MessageBox appears again. Not only the red canvas, touching any place on the Window the MessageBox comes out.

I don't know whether this bug is relatived with the touch device or not. Any advice is appreciated. Thanks in advance.

By the way, a similar WinForm application runs well on touch device.