managed titlebar declared in xaml.

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -12,91 +12,34 @@ namespace Avalonia.Controls.Chrome
     public class TitleBar : TemplatedControl
     {
         private CompositeDisposable? _disposables;
-        private readonly Window? _hostWindow;
         private CaptionButtons? _captionButtons;
 
-        public TitleBar(Window hostWindow)
-        {
-            _hostWindow = hostWindow;
-        }
-
         public TitleBar()
         {
 
         }
 
-        public void Attach()
-        {
-            if (_disposables == null)
-            {
-                if (_hostWindow != null)
-                {
-                    var layer = ChromeOverlayLayer.GetOverlayLayer(_hostWindow);
-
-                    layer?.Children.Add(this);
-                    
-                    _disposables = new CompositeDisposable
-                    {
-                        _hostWindow.GetObservable(Window.WindowDecorationMarginProperty)
-                            .Subscribe(x => UpdateSize()),
-
-                        _hostWindow.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
-                            .Subscribe(x => UpdateSize()),
-
-                        _hostWindow.GetObservable(Window.OffScreenMarginProperty)
-                            .Subscribe(x => UpdateSize()),
-
-                        _hostWindow.GetObservable(Window.WindowStateProperty)
-                            .Subscribe(x =>
-                            {
-                                PseudoClasses.Set(":minimized", x == WindowState.Minimized);
-                                PseudoClasses.Set(":normal", x == WindowState.Normal);
-                                PseudoClasses.Set(":maximized", x == WindowState.Maximized);
-                                PseudoClasses.Set(":fullscreen", x == WindowState.FullScreen);
-                            })
-                    };
-
-                    _captionButtons?.Attach(_hostWindow);
-                }
-
-                UpdateSize();
-            }
-        }
-
-        private void UpdateSize()
+        private void UpdateSize(Window window)
         {
-            if (_hostWindow != null)
+            if (window != null)
             {
                 Margin = new Thickness(
-                    _hostWindow.OffScreenMargin.Left,
-                    _hostWindow.OffScreenMargin.Top,
-                    _hostWindow.OffScreenMargin.Right,
-                    _hostWindow.OffScreenMargin.Bottom);
+                    window.OffScreenMargin.Left,
+                    window.OffScreenMargin.Top,
+                    window.OffScreenMargin.Right,
+                    window.OffScreenMargin.Bottom);
 
-                if (_hostWindow.WindowState != WindowState.FullScreen)
+                if (window.WindowState != WindowState.FullScreen)
                 {
-                    Height = _hostWindow.WindowDecorationMargin.Top;
+                    Height = window.WindowDecorationMargin.Top;
 
                     if (_captionButtons != null)
                     {
                         _captionButtons.Height = Height;
                     }
                 }
-            }
-        }
-
-        public void Detach()
-        {
-            if (_disposables != null)
-            {
-                var layer = ChromeOverlayLayer.GetOverlayLayer(_hostWindow);
-
-                layer?.Children.Remove(this);
 
-                _disposables.Dispose();
-                _disposables = null;
-
-                _captionButtons?.Detach();
+                IsVisible = window.PlatformImpl.NeedsManagedDecorations;
             }
         }
 
@@ -106,12 +49,39 @@ namespace Avalonia.Controls.Chrome
 
             _captionButtons = e.NameScope.Get<CaptionButtons>("PART_CaptionButtons");
 
-            if (_hostWindow != null)
+            if (VisualRoot is Window window)
             {
-                _captionButtons.Attach(_hostWindow);
-            }
+                _disposables = new CompositeDisposable
+                {
+                    window.GetObservable(Window.WindowDecorationMarginProperty)
+                        .Subscribe(x => UpdateSize(window)),
+
+                    window.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
+                        .Subscribe(x => UpdateSize(window)),
 
-            UpdateSize();
+                    window.GetObservable(Window.OffScreenMarginProperty)
+                        .Subscribe(x => UpdateSize(window)),
+                    
+                    window.GetObservable(Window.ExtendClientAreaChromeHintsProperty)
+                        .Subscribe(x => UpdateSize(window)),
+
+                    window.GetObservable(Window.WindowStateProperty)
+                        .Subscribe(x =>
+                        {
+                            PseudoClasses.Set(":minimized", x == WindowState.Minimized);
+                            PseudoClasses.Set(":normal", x == WindowState.Normal);
+                            PseudoClasses.Set(":maximized", x == WindowState.Maximized);
+                            PseudoClasses.Set(":fullscreen", x == WindowState.FullScreen);
+                        }),
+                    
+                    window.GetObservable(Window.IsExtendedIntoWindowDecorationsProperty)
+                        .Subscribe(x => UpdateSize(window))
+                };
+
+                _captionButtons.Attach(window);   
+                
+                UpdateSize(window);
+            }
         }
     }
 }

src/Avalonia.Controls/Primitives/ChromeOverlayLayer.cs

@@ -24,6 +24,11 @@ namespace Avalonia.Controls.Primitives
             return null;
         }
 
+        public void Add(Control c)
+        {
+            base.Children.Add(c);
+        }
+
         public bool HitTest(Point point) => Children.HitTestCustom(point);
     }
 }

src/Avalonia.Controls/Primitives/VisualLayerManager.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using Avalonia.LogicalTree;
+using Avalonia.Metadata;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -12,15 +13,8 @@ namespace Avalonia.Controls.Primitives
         private ILogicalRoot _logicalRoot;
         private readonly List<Control> _layers = new List<Control>();
 
-        public static readonly StyledProperty<Panel> ChromeOverlayLayerProperty =
-            AvaloniaProperty.Register<VisualLayerManager, Panel>(nameof(ChromeOverlayLayer));
-
-        public VisualLayerManager()
-        {
-            var chromeOverlayLayer = new ChromeOverlayLayer();
-            AddLayer(chromeOverlayLayer, ChromeZIndex);
-            ChromeOverlayLayer = chromeOverlayLayer;
-        }
+        public static readonly StyledProperty<ChromeOverlayLayer> ChromeOverlayLayerProperty =
+            AvaloniaProperty.Register<VisualLayerManager, ChromeOverlayLayer>(nameof(ChromeOverlayLayer));
         
         public bool IsPopup { get; set; }
         
@@ -34,11 +28,25 @@ namespace Avalonia.Controls.Primitives
                 return rv;
             }
         }
-
-        public Panel ChromeOverlayLayer
+        
+        public ChromeOverlayLayer ChromeOverlayLayer
         {
-            get => GetValue(ChromeOverlayLayerProperty);
-            private set => SetValue(ChromeOverlayLayerProperty, value);
+            get
+            {
+                var current = GetValue(ChromeOverlayLayerProperty);
+
+                if (current is null)
+                {
+                    var chromeOverlayLayer = new ChromeOverlayLayer();
+                    AddLayer(chromeOverlayLayer, ChromeZIndex);
+
+                    SetValue(ChromeOverlayLayerProperty, chromeOverlayLayer);
+
+                    current = chromeOverlayLayer;
+                }
+
+                return current;
+            }
         }
 
         public OverlayLayer OverlayLayer

src/Avalonia.Controls/Window.cs

@@ -71,12 +71,10 @@ namespace Avalonia.Controls
     /// </summary>
     public class Window : WindowBase, IStyleable, IFocusScope, ILayoutRoot
     {
-        private readonly List<(Window child, bool isDialog)> _children = new List<(Window, bool)>();        
-        private TitleBar _managedTitleBar;
+        private readonly List<(Window child, bool isDialog)> _children = new List<(Window, bool)>();
         private bool _isExtendedIntoWindowDecorations;
         private Thickness _windowDecorationMargin;
         private Thickness _offScreenMargin;
-        private bool _templateApplied;
 
         /// <summary>
         /// Defines the <see cref="SizeToContent"/> property.
@@ -554,29 +552,6 @@ namespace Avalonia.Controls
             IsExtendedIntoWindowDecorations = isExtended;
             WindowDecorationMargin = PlatformImpl.ExtendedMargins;
             OffScreenMargin = PlatformImpl.OffScreenMargin;
-
-            if (PlatformImpl.NeedsManagedDecorations && _templateApplied)
-            {
-                if (_managedTitleBar == null)
-                {
-                    _managedTitleBar = new TitleBar(this);
-                    _managedTitleBar.Attach();
-                }                
-            }
-            else
-            {
-                _managedTitleBar?.Detach();
-                _managedTitleBar = null;
-            }
-        }
-
-        protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
-        {
-            base.OnApplyTemplate(e);
-
-            _templateApplied = true;
-            
-            ExtendClientAreaToDecorationsChanged(PlatformImpl.IsClientAreaExtendedToDecorations);
         }
 
         /// <summary>

src/Avalonia.Themes.Default/Window.xaml

@@ -10,6 +10,9 @@
         <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <Border Background="{TemplateBinding Background}" IsHitTestVisible="False" />
         <VisualLayerManager>
+          <VisualLayerManager.ChromeOverlayLayer>
+            <TitleBar />
+          </VisualLayerManager.ChromeOverlayLayer>
           <ContentPresenter Name="PART_ContentPresenter"
                             ContentTemplate="{TemplateBinding ContentTemplate}"
                             Content="{TemplateBinding Content}"

src/Avalonia.Themes.Fluent/Window.xaml

@@ -10,6 +10,9 @@
         <Border Name="PART_TransparencyFallback" IsHitTestVisible="False" />
         <Border Background="{TemplateBinding Background}" IsHitTestVisible="False" />
         <VisualLayerManager>
+          <VisualLayerManager.ChromeOverlayLayer>
+            <TitleBar />
+          </VisualLayerManager.ChromeOverlayLayer>
           <ContentPresenter Name="PART_ContentPresenter"
                             ContentTemplate="{TemplateBinding ContentTemplate}"
                             Content="{TemplateBinding Content}"