垂直滚动条在popup-element内的列表框中不可见

Question

垂直滚动条在W 8.1中可见，但在我的模拟器上的WP 8.1中不可见。

我错过了什么？

我还尝试将VerticalScrollBarVisibility设置为Visible

<Popup x:Name="LayerPopupWindow" IsLightDismissEnabled="True" >
<ListBox x:Name="MyList" Margin="3" Width="auto" Background="#DDFFFFFF"
Height="{Binding Path=ActualHeight,ElementName=MyMapView}" ScrollViewer.VerticalScrollBarVisibility="Auto" ScrollViewer.VerticalScrollMode="Enabled">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Height="auto" >
<CheckBox IsChecked="{Binding IsVisible, Mode=TwoWay}" BorderBrush="Black" MinWidth="30"/>
<TextBlock Text="{Binding ID, Mode=OneWay}" VerticalAlignment="Center" >
<ToolTipService.ToolTip>
<StackPanel MaxWidth="400">
<TextBlock FontWeight="Bold" Text="{Binding CopyrightText}" TextWrapping="Wrap" />
<TextBlock Text="{Binding Description}" TextWrapping="Wrap" />
</StackPanel>
</ToolTipService.ToolTip>
</TextBlock>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Popup>