WPF AutoCompleteBox - 添加水平滚动条

Question

以下 XAML 将水平滚动条添加到 WPF AutoCompleteBox 下拉列表中的各个项目。有没有办法将其添加到整个下拉列表的底部？

<toolkit:AutoCompleteBox x:Name="PrimaryTextBox" DropDownClosed="PrimaryTextBox_DropDownClosed" SelectedItem="{Binding Symbol, Mode=TwoWay}" Populating="PrimaryTextBox_Populating"
                    MinimumPrefixLength="2" Background="Transparent" Foreground="Black" LostFocus="symbolCellLostFocus" GotFocus="DataGridCellGotFocus" BorderBrush="Transparent">
<toolkit:AutoCompleteBox.ItemTemplate>
    <DataTemplate>
        <StackPanel Orientation="Horizontal">
            <Border Margin="-15,0,0,0" Width="{Binding ActualWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGridCell}}}" Background="Transparent" BorderThickness="0">
                <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Disabled">
                    <TextBlock Margin="15,0,0,0" Text="{Binding}" HorizontalAlignment="Left"/>
                </ScrollViewer>
            </Border>
        </StackPanel>
    </DataTemplate>
</toolkit:AutoCompleteBox.ItemTemplate>

注意：我使用的是 Microsoft 的原始 WPFToolkit 3.5.40128.8