fix nits.

src/Avalonia.Controls/Chrome/CaptionButtons.cs

@@ -38,7 +38,7 @@ namespace Avalonia.Controls.Chrome
             {
                 var layer = ChromeOverlayLayer.GetOverlayLayer(_hostWindow);
 
-                layer.Children.Remove(this);
+                layer?.Children.Remove(this);
 
                 _disposables.Dispose();
                 _disposables = null;
@@ -49,10 +49,10 @@ namespace Avalonia.Controls.Chrome
         {
             base.OnApplyTemplate(e);
 
-            var closeButton = e.NameScope.Find<Panel>("PART_CloseButton");
-            var restoreButton = e.NameScope.Find<Panel>("PART_RestoreButton");
-            var minimiseButton = e.NameScope.Find<Panel>("PART_MinimiseButton");
-            var fullScreenButton = e.NameScope.Find<Panel>("PART_FullScreenButton");
+            var closeButton = e.NameScope.Get<Panel>("PART_CloseButton");
+            var restoreButton = e.NameScope.Get<Panel>("PART_RestoreButton");
+            var minimiseButton = e.NameScope.Get<Panel>("PART_MinimiseButton");
+            var fullScreenButton = e.NameScope.Get<Panel>("PART_FullScreenButton");
 
             closeButton.PointerReleased += (sender, e) => _hostWindow.Close();
             restoreButton.PointerReleased += (sender, e) => _hostWindow.WindowState = _hostWindow.WindowState == WindowState.Maximized ? WindowState.Normal : WindowState.Maximized;

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -26,36 +26,36 @@ namespace Avalonia.Controls.Chrome
             {
                 var layer = ChromeOverlayLayer.GetOverlayLayer(_hostWindow);
 
-                layer.Children.Add(this);
+                layer?.Children.Add(this);
 
                 _disposables = new CompositeDisposable
                 {
                     _hostWindow.GetObservable(Window.WindowDecorationMarginsProperty)
-                    .Subscribe(x => InvalidateSize()),
+                        .Subscribe(x => UpdateSize()),
 
                     _hostWindow.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
-                    .Subscribe(x => InvalidateSize()),
+                        .Subscribe(x => UpdateSize()),
 
                     _hostWindow.GetObservable(Window.OffScreenMarginProperty)
-                    .Subscribe(x => InvalidateSize()),
+                        .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);
-                    })
+                        .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);
 
-                InvalidateSize();
+                UpdateSize();
             }
         }
 
-        void InvalidateSize()
+        void UpdateSize()
         {
             Margin = new Thickness(
                 _hostWindow.OffScreenMargin.Left,
@@ -80,7 +80,7 @@ namespace Avalonia.Controls.Chrome
             {
                 var layer = ChromeOverlayLayer.GetOverlayLayer(_hostWindow);
 
-                layer.Children.Remove(this);
+                layer?.Children.Remove(this);
 
                 _disposables.Dispose();
                 _disposables = null;
@@ -93,11 +93,11 @@ namespace Avalonia.Controls.Chrome
         {
             base.OnApplyTemplate(e);
 
-            _captionButtons = e.NameScope.Find<CaptionButtons>("PART_CaptionButtons");
+            _captionButtons = e.NameScope.Get<CaptionButtons>("PART_CaptionButtons");
 
             _captionButtons.Attach(_hostWindow);
 
-            InvalidateSize();
+            UpdateSize();
         }
     }
 }

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -68,6 +68,34 @@ namespace Avalonia.Platform
         /// </summary>
         Func<bool> Closing { get; set; }
 
+        /// <summary>
+        /// Gets a value to indicate if the platform was able to extend client area to non-client area.
+        /// </summary>
+        bool IsClientAreaExtendedToDecorations { get; }
+
+        /// <summary>
+        /// Gets or Sets an action that is called whenever one of the extend client area properties changed.
+        /// </summary>
+        Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
+
+        /// <summary>
+        /// Gets a flag that indicates if Managed decorations i.e. caption buttons are required.
+        /// This property is used when <see cref="IsClientAreaExtendedToDecorations"/> is set.
+        /// </summary>
+        bool NeedsManagedDecorations { get; }
+
+        /// <summary>
+        /// Gets a thickness that describes the amount each side of the non-client area extends into the client area.
+        /// It includes the titlebar.
+        /// </summary>
+        Thickness ExtendedMargins { get; }
+
+        /// <summary>
+        /// Gets a thickness that describes the margin around the window that is offscreen.
+        /// This may happen when a window is maximized and <see cref="IsClientAreaExtendedToDecorations"/> is set.
+        /// </summary>
+        Thickness OffScreenMargin { get; }
+
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler.
         /// </summary>
@@ -99,12 +127,7 @@ namespace Avalonia.Platform
         /// Sets if the ClientArea is extended into the non-client area.
         /// </summary>
         /// <param name="extendIntoClientAreaHint">true to enable, false to disable</param>
-        void SetExtendClientAreaToDecorationsHint(bool extendIntoClientAreaHint);
-
-        /// <summary>
-        /// Gets a value to indicate if the platform was able to extend client area to non-client area.
-        /// </summary>
-        bool IsClientAreaExtendedToDecorations { get; }
+        void SetExtendClientAreaToDecorationsHint(bool extendIntoClientAreaHint);        
 
         /// <summary>
         /// Sets hints that configure how the client area extends. 
@@ -116,29 +139,6 @@ namespace Avalonia.Platform
         /// Sets how big the non-client titlebar area should be.
         /// </summary>
         /// <param name="titleBarHeight">-1 for platform default, otherwise the height in DIPs.</param>
-        void SetExtendClientAreaTitleBarHeightHint(double titleBarHeight);
-
-        /// <summary>
-        /// Gets or Sets an action that is called whenever one of the extend client area properties changed.
-        /// </summary>
-        Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
-
-        /// <summary>
-        /// Gets a flag that indicates if Managed decorations i.e. caption buttons are required.
-        /// This property is used when <see cref="IsClientAreaExtendedToDecorations"/> is set.
-        /// </summary>
-        bool NeedsManagedDecorations { get; }
-        
-        /// <summary>
-        /// Gets a thickness that describes the amount each side of the non-client area extends into the client area.
-        /// It includes the titlebar.
-        /// </summary>
-        Thickness ExtendedMargins { get; }
-
-        /// <summary>
-        /// Gets a thickness that describes the margin around the window that is offscreen.
-        /// This may happen when a window is maximized and <see cref="IsClientAreaExtendedToDecorations"/> is set.
-        /// </summary>
-        Thickness OffScreenMargin { get; } 
+        void SetExtendClientAreaTitleBarHeightHint(double titleBarHeight);       
     }
 }