Use SelectionModel for DevTools tree.

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNode.cs

@@ -1,11 +1,11 @@
 using System;
+using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Reactive;
 using System.Reactive.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls;
 using Avalonia.LogicalTree;
-using Avalonia.Styling;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Diagnostics.ViewModels
@@ -82,5 +82,41 @@ namespace Avalonia.Diagnostics.ViewModels
             get;
             private set;
         }
+
+        public IndexPath Index
+        {
+            get
+            {
+                var indices = new List<int>();
+                var child = this;
+                var parent = Parent;
+                
+                while (parent is object)
+                {
+                    indices.Add(IndexOf(parent.Children, child));
+                    child = child.Parent;
+                    parent = parent.Parent;
+                }
+
+                indices.Add(0);
+                indices.Reverse();
+                return new IndexPath(indices);
+            }
+        }
+
+        private static int IndexOf(IReadOnlyList<TreeNode> collection, TreeNode item)
+        {
+            var count = collection.Count;
+
+            for (var i = 0; i < count; ++i)
+            {
+                if (collection[i] == item)
+                {
+                    return i;
+                }
+            }
+
+            throw new AvaloniaInternalException("TreeNode was not present in parent Children collection.");
+        }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreePageViewModel.cs

@@ -6,28 +6,40 @@ namespace Avalonia.Diagnostics.ViewModels
 {
     internal class TreePageViewModel : ViewModelBase, IDisposable
     {
-        private TreeNode _selected;
+        private TreeNode _selectedNode;
         private ControlDetailsViewModel _details;
         private string _propertyFilter;
 
         public TreePageViewModel(TreeNode[] nodes)
         {
             Nodes = nodes;
-        }
+            Selection = new SelectionModel
+            { 
+                SingleSelect = true,
+                Source = Nodes 
+            };
+
+            Selection.SelectionChanged += (s, e) =>
+            {
+                SelectedNode = (TreeNode)Selection.SelectedItem;
+            };
+       }
 
         public TreeNode[] Nodes { get; protected set; }
 
+        public SelectionModel Selection { get; }
+
         public TreeNode SelectedNode
         {
-            get => _selected;
-            set
+            get => _selectedNode;
+            private set
             {
                 if (Details != null)
                 {
                     _propertyFilter = Details.PropertyFilter;
                 }
 
-                if (RaiseAndSetIfChanged(ref _selected, value))
+                if (RaiseAndSetIfChanged(ref _selectedNode, value))
                 {
                     Details = value != null ?
                         new ControlDetailsViewModel(value.Visual, _propertyFilter) :
@@ -83,7 +95,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
             if (node != null)
             {
-                SelectedNode = node;
+                Selection.SelectedIndex = node.Index;
                 ExpandNode(node.Parent);
             }
         }

src/Avalonia.Diagnostics/Diagnostics/Views/TreePageView.xaml

@@ -6,7 +6,7 @@
     <TreeView Name="tree"
               BorderThickness="0"
               Items="{Binding Nodes}"
-              SelectedItem="{Binding SelectedNode, Mode=TwoWay}">
+              Selection="{Binding Selection}">
       <TreeView.DataTemplates>
         <TreeDataTemplate DataType="vm:TreeNode"
                           ItemsSource="{Binding Children}">