Extracted the headerview out of the MainWindow.axaml

XSDVisualiser.Desktop/App.axaml

@@ -1,6 +1,7 @@
 <Application xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:fluent="using:Avalonia.Themes.Fluent"
+             xmlns:conv="clr-namespace:XSDVisualiser.Desktop.Converters"
              x:Class="XSDVisualiser.Desktop.App"
              RequestedThemeVariant="Light">
   <Application.Resources>
@@ -13,6 +14,12 @@
     <SolidColorBrush x:Key="MutedTextBrush" Color="#64748B"/>
     <SolidColorBrush x:Key="SeparatorBrush" Color="#E2E8F0"/>
     <SolidColorBrush x:Key="BadgeBackgroundBrush" Color="#F1F5F9"/>
+
+    <!-- Global converters -->
+    <conv:CardinalityToLabelConverter x:Key="CardinalityToLabel"/>
+    <conv:OptionalToBorderBrushConverter x:Key="OptionalToBrush"/>
+    <conv:OptionalToThicknessConverter x:Key="OptionalToThickness"/>
+    <conv:CollectionHasItemsConverter x:Key="HasItems"/>
   </Application.Resources>
 
   <Application.Styles>

XSDVisualiser.Desktop/MainWindow.axaml

@@ -2,6 +2,7 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:m="clr-namespace:XSDVisualiser.Models;assembly=XSDVisualiser.Core"
         xmlns:conv="clr-namespace:XSDVisualiser.Desktop.Converters"
+        xmlns:views="clr-namespace:XSDVisualiser.Desktop.Views"
         x:Class="XSDVisualiser.Desktop.MainWindow"
         x:CompileBindings="False"
         Title="XSD Visualiser" Width="1200" Height="800">
@@ -14,26 +15,7 @@
 
     <Grid RowDefinitions="Auto,*" Margin="12">
         <!-- Header -->
-        <DockPanel LastChildFill="False">
+        <views:HeaderView x:Name="Header" />
-            <TextBlock Text="XSD Visualiser" FontSize="20" FontWeight="SemiBold" Margin="0,0,12,0" DockPanel.Dock="Left"/>
-            
-            <!-- Searchable dropdown for root elements -->
-            <AutoCompleteBox Width="320" Margin="0,0,12,0" Watermark="Search and choose a root element"
-                              ItemsSource="{Binding RootElements}"
-                              FilterMode="Contains"
-                              MinimumPrefixLength="0">
-                <AutoCompleteBox.ItemTemplate>
-                    <DataTemplate>
-                        <TextBlock Text="{Binding .}"/>
-                    </DataTemplate>
-                </AutoCompleteBox.ItemTemplate>
-                <AutoCompleteBox.SelectedItem>
-                    <Binding Path="SelectedRootElement" Mode="TwoWay"/>
-                </AutoCompleteBox.SelectedItem>
-            </AutoCompleteBox>
-            
-            <Button Content="Open XSD and parse" x:Name="OpenBtn" Width="220"/>
-        </DockPanel>
 
         <!-- Body: left tree | splitter | right details -->
         <Grid Grid.Row="1" ColumnDefinitions="2*,Auto,*" RowDefinitions="*">

XSDVisualiser.Desktop/MainWindow.axaml.cs

@@ -6,6 +6,7 @@ using Avalonia.Threading;
 using XSDVisualiser.Core;
 using XSDVisualiser.Models;
 using XSDVisualiser.Desktop.ViewModels;
+using XSDVisualiser.Desktop.Views;
 
 namespace XSDVisualiser.Desktop
 {
@@ -16,7 +17,10 @@ namespace XSDVisualiser.Desktop
         public MainWindow()
         {
             InitializeComponent();
-            _openBtn = this.FindControl<Button>("OpenBtn");
+            var header = this.FindControl<HeaderView>("Header");
+            var btn = header?.FindControl<Button>("OpenBtn");
+            if (btn == null) return;
+            _openBtn = btn;
             _openBtn.Click += OpenBtn_Click;
         }
 
@@ -29,7 +33,7 @@ namespace XSDVisualiser.Desktop
             };
             ofd.Filters!.Add(new FileDialogFilter { Name = "XSD schema", Extensions = { "xsd" } });
             var files = await ofd.ShowAsync(this);
-            if (files != null && files.Length > 0)
+            if (files is { Length: > 0 })
             {
                 await ParseAndShowAsync(files[0]);
             }

XSDVisualiser.Desktop/ViewModels/MainWindowViewModel.cs

@@ -7,15 +7,11 @@ using XSDVisualiser.Models;
 
 namespace XSDVisualiser.Desktop.ViewModels
 {
-    public class MainWindowViewModel : INotifyPropertyChanged
+    public class MainWindowViewModel(SchemaModel model) : INotifyPropertyChanged
     {
-        private SchemaModel _model;
+        private SchemaModel _model = model ?? throw new ArgumentNullException(nameof(model));
         private SchemaNode? _selectedRootElement;
-
+        private SchemaNode? _selectedNode;
-        public MainWindowViewModel(SchemaModel model)
-        {
-            _model = model ?? throw new ArgumentNullException(nameof(model));
-        }
 
         public event PropertyChangedEventHandler? PropertyChanged;
 
@@ -29,15 +25,13 @@ namespace XSDVisualiser.Desktop.ViewModels
             get => _model;
             set
             {
-                if (!ReferenceEquals(_model, value))
+                if (ReferenceEquals(_model, value)) return;
-                {
                 _model = value ?? throw new ArgumentNullException(nameof(value));
                 OnPropertyChanged();
                 OnPropertyChanged(nameof(RootElements));
                 OnPropertyChanged(nameof(VisibleRootElements));
             }
         }
-        }
 
         public IEnumerable<SchemaNode> RootElements => _model?.RootElements ?? Enumerable.Empty<SchemaNode>();
 
@@ -46,23 +40,24 @@ namespace XSDVisualiser.Desktop.ViewModels
             get => _selectedRootElement;
             set
             {
-                if (!EqualityComparer<SchemaNode?>.Default.Equals(_selectedRootElement, value))
+                if (EqualityComparer<SchemaNode?>.Default.Equals(_selectedRootElement, value)) return;
-                {
                 _selectedRootElement = value;
                 OnPropertyChanged();
                 OnPropertyChanged(nameof(VisibleRootElements));
             }
         }
+
+        public SchemaNode? SelectedNode
+        {
+            get => _selectedNode;
+            set
+            {
+                if (EqualityComparer<SchemaNode?>.Default.Equals(_selectedNode, value)) return;
+                _selectedNode = value;
+                OnPropertyChanged();
+            }
         }
 
-        public IEnumerable<SchemaNode> VisibleRootElements
+        public IEnumerable<SchemaNode> VisibleRootElements => SelectedRootElement != null ? [SelectedRootElement] : RootElements;
-        {
-            get
-            {
-                if (SelectedRootElement != null)
-                    return new[] { SelectedRootElement };
-                return RootElements;
-            }
-        }
     }
 }

XSDVisualiser.Desktop/Views/HeaderView.axaml

@@ -0,0 +1,24 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="XSDVisualiser.Desktop.Views.HeaderView"
+             x:CompileBindings="False">
+    <DockPanel LastChildFill="False">
+        <TextBlock Text="XSD Visualiser" FontSize="20" FontWeight="SemiBold" Margin="0,0,12,0" DockPanel.Dock="Left"/>
+
+        <AutoCompleteBox Width="320" Margin="0,0,12,0" Watermark="Search and choose a root element"
+                          ItemsSource="{Binding RootElements}"
+                          FilterMode="Contains"
+                          MinimumPrefixLength="0">
+            <AutoCompleteBox.ItemTemplate>
+                <DataTemplate>
+                    <TextBlock Text="{Binding .}"/>
+                </DataTemplate>
+            </AutoCompleteBox.ItemTemplate>
+            <AutoCompleteBox.SelectedItem>
+                <Binding Path="SelectedRootElement" Mode="TwoWay"/>
+            </AutoCompleteBox.SelectedItem>
+        </AutoCompleteBox>
+
+        <Button Content="Open XSD and parse" x:Name="OpenBtn" Width="220"/>
+    </DockPanel>
+</UserControl>

XSDVisualiser.Desktop/Views/HeaderView.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace XSDVisualiser.Desktop.Views;
+
+public partial class HeaderView : UserControl
+{
+    public HeaderView()
+    {
+        InitializeComponent();
+    }
+}

XSDVisualiser.Desktop/Views/LeftTreeView.axaml

@@ -0,0 +1,60 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:m="clr-namespace:XSDVisualiser.Models;assembly=XSDVisualiser.Core"
+             x:Class="XSDVisualiser.Desktop.Views.LeftTreeView"
+             x:CompileBindings="False">
+  <Border Background="{DynamicResource PanelBackgroundBrush}"
+          BorderBrush="{DynamicResource PanelBorderBrush}"
+          BorderThickness="1" CornerRadius="6" Margin="0,8,8,0">
+    <ScrollViewer>
+      <TreeView x:Name="SchemaTree" ItemsSource="{Binding VisibleRootElements}" SelectedItem="{Binding SelectedNode, Mode=TwoWay}">
+        <TreeView.DataTemplates>
+          <TreeDataTemplate DataType="{x:Type m:SchemaNode}" ItemsSource="{Binding Children}">
+            <StackPanel>
+              <!-- Connector line from parent to this node (hidden for roots where ContentModel is null) -->
+              <Grid Margin="0,8,8,2" IsVisible="{Binding ContentModel, Converter={x:Static ObjectConverters.IsNotNull}}"
+                    ColumnDefinitions="16,*">
+                <!-- leading small elbow -->
+                <Rectangle Grid.Column="0" Width="16" Height="2" Fill="{DynamicResource SeparatorBrush}" VerticalAlignment="Center"/>
+                <Grid Grid.Column="1">
+                  <Rectangle Height="2" Fill="{DynamicResource SeparatorBrush}" VerticalAlignment="Center"/>
+                  <TextBlock Text="{Binding Cardinality, Converter={StaticResource CardinalityToLabel}}"
+                             Background="{DynamicResource BadgeBackgroundBrush}" Padding="6,0"
+                             Foreground="{DynamicResource SubtleTextBrush}"
+                             HorizontalAlignment="Center"/>
+                </Grid>
+              </Grid>
+
+              <!-- Node content -->
+              <Border CornerRadius="4"
+                      BorderBrush="{Binding Cardinality, Converter={StaticResource OptionalToBrush}}"
+                      BorderThickness="{Binding Cardinality, Converter={StaticResource OptionalToThickness}}"
+                      Padding="8" Margin="0,0,8,6" Background="{DynamicResource PanelBackgroundBrush}">
+                <StackPanel Orientation="Vertical" Spacing="2">
+                  <!-- Name on its own line, prominent -->
+                  <TextBlock Text="{Binding Name}" FontWeight="SemiBold"/>
+
+                  <!-- Path/Namespace on its own line -->
+                  <TextBlock Text="{Binding Namespace, StringFormat=Namespace: {0}}"
+                             Foreground="{DynamicResource SubtleTextBrush}"/>
+
+                  <!-- Type info on its own line (TypeName with fallback to BuiltInType) -->
+                  <StackPanel Orientation="Horizontal" Spacing="6">
+                    <TextBlock Text="Type:" FontWeight="Medium"/>
+                    <TextBlock Text="{Binding TypeName}"/>
+                    <TextBlock Text="{Binding BuiltInType, StringFormat=({0})}"
+                               Foreground="{DynamicResource MutedTextBrush}"/>
+                  </StackPanel>
+
+                  <!-- Optional: show content model as a subtle hint -->
+                  <TextBlock Text="{Binding ContentModel, StringFormat=Model: {0}}"
+                             Foreground="{DynamicResource SubtleTextBrush}" FontWeight="SemiBold"/>
+                </StackPanel>
+              </Border>
+            </StackPanel>
+          </TreeDataTemplate>
+        </TreeView.DataTemplates>
+      </TreeView>
+    </ScrollViewer>
+  </Border>
+</UserControl>

XSDVisualiser.Desktop/Views/LeftTreeView.axaml.cs

@@ -0,0 +1,12 @@
+using Avalonia.Controls;
+
+namespace XSDVisualiser.Desktop.Views
+{
+    public partial class LeftTreeView : UserControl
+    {
+        public LeftTreeView()
+        {
+            InitializeComponent();
+        }
+    }
+}