Липсва вертикална лента за превъртане в групирания WPF ListBox

Здравейте, извинете, ако това е очевиден въпрос; Аз съм сравнително нов в WPF. Опитвам се да създам „групиран“ списък, подобен на изображението по-долу. С помощта на Google почти успях да го накарам да работи, освен че по някаква причина виждам вертикалната лента за превъртане. Някой орган има ли някакви идеи? Това ме побърква! Благодаря

Кодът, който използвам, е както следва:

public class SampleData
{
    public string Name { get; set; }
    public string Group { get; set; }

    public SampleData(string name, string group)
    {
        this.Name = name;
        this.Group = group;
    }
}


public partial class MainWindow : Window
{
    private ObservableCollection<SampleData> _items = new ObservableCollection<SampleData>();

    public ObservableCollection<SampleData> Items
    {
        get { return _items; }
    }

    public MainWindow()
    {
        InitializeComponent();

        _items.Add(new SampleData("Item1", "Group1"));
        _items.Add(new SampleData("Item2", "Group1"));
        _items.Add(new SampleData("Item3", "Group1"));
        _items.Add(new SampleData("Item4", "Group1"));
        _items.Add(new SampleData("Item5", "Group1"));
        _items.Add(new SampleData("Item6", "Group1"));

        _items.Add(new SampleData("Item7", "Group2"));
        _items.Add(new SampleData("Item8", "Group2"));
        _items.Add(new SampleData("Item9", "Group2"));
        _items.Add(new SampleData("Item10", "Group2"));
        _items.Add(new SampleData("Item11", "Group2"));
        _items.Add(new SampleData("Item12", "Group2"));
        _items.Add(new SampleData("Item13", "Group2"));
        _items.Add(new SampleData("Item14", "Group2"));
    }
}

и в xaml,

  <CollectionViewSource x:Key="groupedSampleData" Source="{Binding ElementName=main, Path=Items }">
    <CollectionViewSource.GroupDescriptions>
      <PropertyGroupDescription PropertyName="Group" />
    </CollectionViewSource.GroupDescriptions>
  </CollectionViewSource>

  <Style x:Key="LBStyle" TargetType="{x:Type ListBox}">
    <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled"/>
    <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
    <Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
    <Setter Property="ScrollViewer.PanningMode" Value="VerticalOnly"/>
  </Style>


<ListBox Grid.Row="0" Style="{StaticResource LBStyle}" ItemsSource="{Binding Source={StaticResource groupedSampleData}}" VerticalAlignment="Center" HorizontalAlignment="Stretch" Margin="0" >
  <ListBox.Template>
    <ControlTemplate TargetType="{x:Type ListBox}">
      <ScrollViewer CanContentScroll="True" VerticalScrollBarVisibility="Auto" PanningMode="VerticalOnly">
        <StackPanel>
          <ItemsPresenter/>
        </StackPanel>
      </ScrollViewer>
    </ControlTemplate>
  </ListBox.Template>

  <ItemsControl.ItemTemplate>
    <DataTemplate>
      <StackPanel>
        <Border Width="96" Height="96" Margin="4" Background="#DDD"/>
        <TextBlock Margin="4,0,4,4" Text="{Binding Path=Name}" HorizontalAlignment="Center"/>
      </StackPanel>
    </DataTemplate>
  </ItemsControl.ItemTemplate>

  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <WrapPanel />
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>

  <ItemsControl.GroupStyle>
    <GroupStyle>
      <GroupStyle.Panel>
        <ItemsPanelTemplate>
          <VirtualizingStackPanel/>
        </ItemsPanelTemplate>
      </GroupStyle.Panel>
      <GroupStyle.HeaderTemplate>
        <DataTemplate>
          <TextBlock Margin="4,16,4,4" FontWeight="Bold" FontSize="15" Text="{Binding Path=Name}"/>
        </DataTemplate>
      </GroupStyle.HeaderTemplate>
    </GroupStyle>
  </ItemsControl.GroupStyle>
</ListBox>