polish managed titlebar.

src/Avalonia.Controls/Chrome/CaptionButtons.cs

@@ -20,18 +20,6 @@ namespace Avalonia.Controls.Chrome
 
                 _disposables = new CompositeDisposable
                 {
-                    _hostWindow.GetObservable(Window.WindowDecorationMarginsProperty)
-                    .Subscribe(x =>
-                    {
-                        Height = x.Top;
-                    }),
-
-                    _hostWindow.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
-                    .Subscribe(x => InvalidateSize()),
-
-                    _hostWindow.GetObservable(Window.OffScreenMarginProperty)
-                    .Subscribe(x => InvalidateSize()),
-
                     _hostWindow.GetObservable(Window.WindowStateProperty)
                     .Subscribe(x =>
                     {
@@ -44,12 +32,6 @@ namespace Avalonia.Controls.Chrome
             }
         }
 
-        void InvalidateSize ()
-        {
-            Margin = new Thickness(1, _hostWindow.OffScreenMargin.Top, 1, 1);
-            Height = _hostWindow.WindowDecorationMargins.Top;
-        }
-
         public void Detach()
         {
             if (_disposables != null)

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -18,21 +18,6 @@ namespace Avalonia.Controls.Chrome
             _hostWindow = hostWindow;
         }
 
-        public TitleBar()
-        {
-
-        }
-
-        protected override Size MeasureOverride(Size availableSize)
-        {
-            return base.MeasureOverride(availableSize);
-        }
-
-        protected override Size ArrangeOverride(Size finalSize)
-        {
-            return base.ArrangeOverride(finalSize);
-        }
-
         public void Attach()
         {
             if (_disposables == null)
@@ -44,10 +29,7 @@ namespace Avalonia.Controls.Chrome
                 _disposables = new CompositeDisposable
                 {
                     _hostWindow.GetObservable(Window.WindowDecorationMarginsProperty)
-                    .Subscribe(x =>
-                    {
-                        Height = x.Top;
-                    }),
+                    .Subscribe(x => InvalidateSize()),
 
                     _hostWindow.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
                     .Subscribe(x => InvalidateSize()),
@@ -71,8 +53,16 @@ namespace Avalonia.Controls.Chrome
 
         void InvalidateSize()
         {
-            Margin = new Thickness(1, _hostWindow.OffScreenMargin.Top, 1, 1);
-            Height = _hostWindow.WindowDecorationMargins.Top;
+            Margin = new Thickness(
+                _hostWindow.OffScreenMargin.Left,
+                _hostWindow.OffScreenMargin.Top,
+                _hostWindow.OffScreenMargin.Right,
+                _hostWindow.OffScreenMargin.Bottom);
+
+            if (_hostWindow.WindowState != WindowState.FullScreen)
+            {
+                Height = _hostWindow.WindowDecorationMargins.Top;
+            }
         }
 
         public void Detach()
@@ -94,7 +84,7 @@ namespace Avalonia.Controls.Chrome
         {
             base.OnApplyTemplate(e);
 
-            _captionButtons = e.NameScope.Find<CaptionButtons>("PART_CaptionButtons");            
+            _captionButtons = e.NameScope.Find<CaptionButtons>("PART_CaptionButtons");
 
             _captionButtons.Attach(_hostWindow);
         }

src/Avalonia.Themes.Fluent/TitleBar.xaml

@@ -5,23 +5,27 @@
     </Border>
   </Design.PreviewWith>
   <Style Selector="TitleBar">
-    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>
-    <Setter Property="Background" Value="Red" />
+    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundBrush}"/>    
     <Setter Property="VerticalAlignment" Value="Top" />
     <Setter Property="HorizontalAlignment" Value="Stretch" />
+    <Setter Property="Background" Value="Transparent" />
     <Setter Property="Template">
       <ControlTemplate>
         <Panel HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="Stretch">
-          <Panel x:Name="PART_MouseTracker" Height="15" VerticalAlignment="Top" />
+          <Panel x:Name="PART_MouseTracker" Height="1" VerticalAlignment="Top" />
           <Panel x:Name="PART_Container">
             <Border x:Name="PART_Background" Background="{TemplateBinding Background}" />
-            <CaptionButtons x:Name="PART_CaptionButtons" VerticalAlignment="Top" HorizontalAlignment="Right" Foreground="{TemplateBinding Foreground}" />
+            <CaptionButtons x:Name="PART_CaptionButtons" VerticalAlignment="Stretch" HorizontalAlignment="Right" Foreground="{TemplateBinding Foreground}" MaxHeight="30" />
           </Panel>
         </Panel>
       </ControlTemplate>
     </Setter>
   </Style>
 
+  <Style Selector="TitleBar:fullscreen">
+    <Setter Property="Background" Value="{DynamicResource SystemAccentColor}" />
+  </Style>
+
   <Style Selector="TitleBar /template/ Border#PART_Background">
     <Setter Property="IsHitTestVisible" Value="False" />
   </Style>

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -671,6 +671,8 @@ namespace Avalonia.Win32
             }
 
             TaskBarList.MarkFullscreen(_hwnd, fullscreen);
+            
+            ExtendClientArea();
         }
 
         private MARGINS UpdateExtendMargins()
@@ -724,33 +726,41 @@ namespace Avalonia.Win32
             {
                 _isClientAreaExtended = false;
                 return;
-            }                       
-
-            GetWindowRect(_hwnd, out var rcClient);
+            }
 
-            // Inform the application of the frame change.
-            SetWindowPos(_hwnd,
-                         IntPtr.Zero,
-                         rcClient.left, rcClient.top,
-                         rcClient.Width, rcClient.Height, 
-                         SetWindowPosFlags.SWP_FRAMECHANGED);
-            
-            if(_isClientAreaExtended)
+            if (!_isClientAreaExtended || WindowState == WindowState.FullScreen)
             {
-                var margins = UpdateExtendMargins();
-
-                DwmExtendFrameIntoClientArea(_hwnd, ref margins);
+                _extendedMargins = new Thickness(0, 0, 0, 0);
+                _offScreenMargin = new Thickness();
             }
             else
             {
-                var margins = new MARGINS();
-                DwmExtendFrameIntoClientArea(_hwnd, ref margins);
+                GetWindowRect(_hwnd, out var rcClient);
 
-                _offScreenMargin = new Thickness();
-                _extendedMargins = new Thickness();
+                // Inform the application of the frame change.
+                SetWindowPos(_hwnd,
+                             IntPtr.Zero,
+                             rcClient.left, rcClient.top,
+                             rcClient.Width, rcClient.Height,
+                             SetWindowPosFlags.SWP_FRAMECHANGED);
+
+                if (_isClientAreaExtended)
+                {
+                    var margins = UpdateExtendMargins();
+
+                    DwmExtendFrameIntoClientArea(_hwnd, ref margins);
+                }
+                else
+                {
+                    var margins = new MARGINS();
+                    DwmExtendFrameIntoClientArea(_hwnd, ref margins);
+
+                    _offScreenMargin = new Thickness();
+                    _extendedMargins = new Thickness();
+                }
             }
 
-            ExtendClientAreaToDecorationsChanged?.Invoke(true);
+            ExtendClientAreaToDecorationsChanged?.Invoke(_isClientAreaExtended);
         }
 
         private void ShowWindow(WindowState state)