Fix top level logical tree

samples/BehaviorsTestApplication/ViewModels/MainWindowViewModel.cs

@@ -21,6 +21,8 @@ public partial class MainWindowViewModel : ViewModelBase
 
     public IObservable<int> Values { get; }
 
+    public ICommand InitializeCommand { get; set; }
+
     public ICommand MoveLeftCommand { get; set; }
 
     public ICommand MoveRightCommand { get; set; }
@@ -31,6 +33,8 @@ public MainWindowViewModel()
     {
         Count = 0;
         Position = 100.0;
+        InitializeCommand = ReactiveCommand.Create(Initialize);
+        MoveLeftCommand = ReactiveCommand.Create(() => Position -= 5.0);
         MoveLeftCommand = ReactiveCommand.Create(() => Position -= 5.0);
         MoveRightCommand = ReactiveCommand.Create(() => Position += 5.0);
         ResetMoveCommand = ReactiveCommand.Create(() => Position = 100.0);
@@ -89,6 +93,11 @@ public MainWindowViewModel()
         Values = Observable.Interval(TimeSpan.FromSeconds(1)).Select(_ => _value++);
     }
 
+    private void Initialize()
+    {
+        Console.WriteLine("InitializeCommand");
+    }
+
     public void IncrementCount() => Count++;
 
     public void DecrementCount(object? sender, object parameter) => Count--;

samples/BehaviorsTestApplication/Views/MainWindow.axaml

@@ -1,7 +1,15 @@
 <Window x:Class="BehaviorsTestApplication.Views.MainWindow"
         xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:BehaviorsTestApplication.ViewModels"
         xmlns:views="using:BehaviorsTestApplication.Views"
-        Title="XamlBehaviors Test Application" Width="1000" Height="700">
+        Title="XamlBehaviors Test Application" Width="1000" Height="700"
+        x:DataType="vm:MainWindowViewModel"
+        x:CompileBindings="False">
+  <Interaction.Behaviors>
+    <EventTriggerBehavior EventName="Opened">
+      <InvokeCommandAction Command="{Binding InitializeCommand, Mode=OneTime}" />
+    </EventTriggerBehavior>
+  </Interaction.Behaviors>
   <views:MainView />
 </Window>

src/Avalonia.Xaml.Interactivity/StyledElementBehavior.cs

@@ -61,6 +61,12 @@ public void Attach(AvaloniaObject? associatedObject)
         Debug.Assert(associatedObject is not null, "Cannot attach the behavior to a null object.");
         AssociatedObject = associatedObject ?? throw new ArgumentNullException(nameof(associatedObject));
         _dataContextDisposable = SynchronizeDataContext(associatedObject);
+
+        // NOTE: Special case handling for TopLevel as it does not trigger attached to logical or visual tree events.
+        if (AssociatedObject is TopLevel)
+        {
+            AttachBehaviorToLogicalTree();
+        }
 
         OnAttached();
     }
@@ -189,17 +195,32 @@ protected virtual void OnUnloaded()
 
     internal virtual void AttachBehaviorToLogicalTree()
     {
-        if (AssociatedObject is not StyledElement styledElement || styledElement.Parent is null)
+        StyledElement? parent = null;
+        AvaloniaObject? templatedParent = null;
+
+        if (AssociatedObject is TopLevel topLevel)
         {
-            return;
+            parent = topLevel;
+            templatedParent = topLevel.TemplatedParent;
+        }
+
+        if (parent is null)
+        {
+            if (AssociatedObject is not StyledElement styledElement || styledElement.Parent is null)
+            {
+                return;
+            }
+
+            parent = styledElement;
+            templatedParent = styledElement.TemplatedParent;
         }
 
         // Required for $parent binding in XAML
         ((ISetLogicalParent)this).SetParent(null);
-        ((ISetLogicalParent)this).SetParent(styledElement);
+        ((ISetLogicalParent)this).SetParent(parent);
 
         // Required for TemplateBinding in XAML
-        if (styledElement.TemplatedParent is { } templatedParent)
+        if (templatedParent is not null)
         {
             TemplatedParentHelper.SetTemplatedParent(this, templatedParent);
         }
@@ -209,7 +230,7 @@ internal virtual void DetachBehaviorFromLogicalTree()
     {
         ((ISetLogicalParent)this).SetParent(null);
 
-        if (AssociatedObject is StyledElement { TemplatedParent: not null })
+        if (AssociatedObject is StyledElement { TemplatedParent: not null } or TopLevel)
         {
             TemplatedParentHelper.SetTemplatedParent(this, null);
         }

src/Avalonia.Xaml.Interactivity/StyledElementTrigger.cs

@@ -1,3 +1,4 @@
+using Avalonia.Controls;
 using Avalonia.Metadata;
 
 namespace Avalonia.Xaml.Interactivity;
@@ -25,12 +26,21 @@ internal override void AttachBehaviorToLogicalTree()
     {
         base.AttachBehaviorToLogicalTree();
 
-        if (AssociatedObject is not StyledElement styledElement || styledElement.Parent is null)
+        StyledElement? parent = null;
+
+        if (AssociatedObject is TopLevel topLevel)
         {
-            return;
+            parent = topLevel;
         }
+        else
+        {
+            if (AssociatedObject is not StyledElement styledElement || styledElement.Parent is null)
+            {
+                return;
+            }
 
-        var parent = this;
+            parent = this;
+        }
 
         foreach (var action in Actions)
         {