WPF CollectionViewSource GroupDescriptions GroupStyle ItemsPanelTemplate

<Window x:Class="WpfApp83.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp83"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ListBox ItemsSource="{Binding}" HorizontalContentAlignment="Stretch">
            <ListBox.GroupStyle>
                <GroupStyle>
                    <GroupStyle.Panel>
                        <ItemsPanelTemplate>
                            <StackPanel Orientation="Horizontal"/>
                        </ItemsPanelTemplate>
                    </GroupStyle.Panel>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <Border BorderBrush="Red" BorderThickness="2"
                                    Background="White" Margin="2">
                                <TextBlock Text="{Binding Name,StringFormat=Priority:{0}}"
                                           FontSize="16" FontWeight="ExtraBold"
                                           Margin="4"/>
                            </Border>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                </GroupStyle>
            </ListBox.GroupStyle> 
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <UniformGrid Rows="1" Columns="2" >
                        <TextBlock Text="{Binding ProcessName}"/>
                        <TextBlock Text="{Binding Id,StringFormat=ID:\{0\}}"/>
                        <!--<TextBlock Text="{Binding PriorityClass}"/>-->
                    </UniformGrid>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>


//xaml.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;

namespace WpfApp83
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public static bool CanAccess(Process proc)
        {
            try
            {
                var h = proc.Handle;
                return true;
            }
            catch (Exception)
            {
                 return false;
            }
        }
        public MainWindow()
        {
            InitializeComponent();
            var procs = Process.GetProcesses().Where(CanAccess);
            var view = CollectionViewSource.GetDefaultView(procs);
            view.GroupDescriptions.Add(new PropertyGroupDescription("PriorityClass"));
            DataContext = procs;
        }
    }
}