Silverlight列表框在选择时始终高亮显示第一项

<Canvas x:Name="HeadlineCanvas">
    <ListBox x:Name="HeadlineListBox" Width="260" Height="380"  BorderBrush="Gainsboro" BorderThickness="1" ScrollViewer.VerticalScrollBarVisibility="Auto" ScrollViewer.HorizontalScrollBarVisibility="Hidden">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Vertical">
                    <HyperlinkButton MinHeight="20" MaxHeight="40" Width="240" NavigateUri="{Binding Url}" IsTabStop="False">
                        <TextBlock TextWrapping="Wrap" Text="{Binding Title}"/>
                    </HyperlinkButton>
                </StackPanel>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
</Canvas>

C#代码

public partial class MainPage : UserControl
{
    public MainPage()
    {
        InitializeComponent();

        HeadlineData hd = new HeadlineData();
        hd.Title = "Title 1";
        hd.HeadlineNewsTitle = "Title 1";
        hd.LastModifiedTime = DateTime.Now;
        hd.Url = "http://www.google.com";

        this.HeadlineListBox.Items.Add(hd);

        hd = new HeadlineData();
        hd.Title = "Title 2";
        hd.HeadlineNewsTitle = "Title 2";
        hd.LastModifiedTime = DateTime.Now;
        hd.Url = "http://www.google.com";

        this.HeadlineListBox.Items.Add(hd);
    }
}