WPF GroupStyle GroupStyle.HeaderTemplate

//xaml
<Window x:Class="WpfApp148.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:WpfApp148"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ListBox ItemsSource="{Binding}" HorizontalContentAlignment="Stretch">
            <ListBox.GroupStyle>
                <GroupStyle>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <Border BorderBrush="Red" BorderThickness="2" 
                                    Background="White" Margin="2">
                                <TextBlock Text="{Binding Name,StringFormat=Priority:{0}}"
                                           FontSize="20" FontWeight="Bold"
                                           Margin="4"/>
                            </Border>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                </GroupStyle>
            </ListBox.GroupStyle>
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <UniformGrid Rows="1" Columns="3">
                        <TextBlock Text="{Binding ProcessName,StringFormat='Name:{0}'}"/>
                        <TextBlock Text="{Binding Id,StringFormat='Id:{0}'}"/>
                        <TextBlock Text="{Binding PriorityClass,StringFormat='Priority:{0}'}"/>
                    </UniformGrid>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>


//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.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;

namespace WpfApp148
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent(); 
            var procs = Process.GetProcesses().Where(CanAccess).OrderByDescending(x=>x.PriorityClass);
            var view = CollectionViewSource.GetDefaultView(procs);
            view.GroupDescriptions.Add(new PropertyGroupDescription("PriorityClass"));
            DataContext = procs;
        }

        public static bool CanAccess(Process proc)
        {
            try
            {
                var handler = proc.Handle;
                return true;
            }
            catch
            {
                return false;
            }
        }
    }
}

 <ListBox ItemsSource="{Binding}" HorizontalContentAlignment="Stretch">
     <ListBox.GroupStyle>
         <GroupStyle>
             <GroupStyle.HeaderTemplate>
                 <DataTemplate>
                     <Border BorderBrush="Red" BorderThickness="2" 
                             Background="White" Margin="2">
                         <TextBlock Text="{Binding Name,StringFormat=Priority:{0}}"
                                    FontSize="20" FontWeight="Bold"
                                    Margin="4"/>
                     </Border>
                 </DataTemplate>
             </GroupStyle.HeaderTemplate>
         </GroupStyle>
     </ListBox.GroupStyle>
</ListBox>