wpf <ItemsControl>下拉框不显示，超出的内容无法显示，请问样式该怎么改？

Question

  <ItemsControl x:Name="downloadfileslist" AlternationCount="100" Height="200" Background="{StaticResource LightBackground}" BorderThickness="0" ItemsSource="{Binding DownloadFiles}">
                                <ItemsControl.ItemTemplate>
                                    <DataTemplate>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Width="250" Foreground="{StaticResource Foreground}" Text="{Binding}" ToolTip="{Binding}" />
                                            <Button Width="14" Height="14"
                                                    Margin="10,0,0,0"
                                                    Background="Transparent" BorderBrush="Transparent" BorderThickness="1"
                                                    Command="{Binding Main.DownloadFilesDeleteCommand, Source={StaticResource Locator}}"
                                                    CommandParameter="{Binding RelativeSource={RelativeSource Mode=TemplatedParent}, Path=(ItemsControl.AlternationIndex)}"
                                                    Cursor="Hand"  ToolTip="删除">
                                                <Path Margin="2" Data="" Fill="{StaticResource Foreground}" Stretch="Uniform" />
                                            </Button>
                                        </StackPanel>
                                    </DataTemplate>
                                </ItemsControl.ItemTemplate>
                            </ItemsControl>