extend api only allows to turn titlebar on or off to maintain window integrity, and decoration margins dont include border margins.

src/Avalonia.Controls/Platform/ExtendClientAreaChromeHints.cs

@@ -6,11 +6,10 @@ namespace Avalonia.Platform
     public enum ExtendClientAreaChromeHints
     {
         NoChrome,
-        Default = SystemTitleBar | SystemChromeButtons,        
+        Default = SystemTitleBar,        
         SystemTitleBar = 0x01,
-        SystemChromeButtons = 0x02,
-        ManagedChromeButtons = 0x04,
-        PreferSystemChromeButtons = 0x08,
+        ManagedChromeButtons = 0x02,
+        PreferSystemChromeButtons = 0x04,
         AdaptiveChromeWithTitleBar = SystemTitleBar | PreferSystemChromeButtons,
         AdaptiveChromeWithoutTitleBar = PreferSystemChromeButtons,
     }

src/Avalonia.Controls/Window.cs

@@ -540,11 +540,7 @@ namespace Avalonia.Controls
         {
             IsExtendedIntoWindowDecorations = isExtended;
 
-            WindowDecorationMargins = new Thickness(
-                PlatformImpl.ExtendedMargins.Left, 
-                PlatformImpl.ExtendedMargins.Top - PlatformImpl.OffScreenMargin.Top, 
-                PlatformImpl.ExtendedMargins.Right, 
-                PlatformImpl.ExtendedMargins.Bottom);
+            WindowDecorationMargins = PlatformImpl.ExtendedMargins;
 
             OffScreenMargin = PlatformImpl.OffScreenMargin;
         }

src/Avalonia.Themes.Default/CaptionButtons.xaml

@@ -5,7 +5,7 @@
     <Setter Property="MaxHeight" Value="30" />
     <Setter Property="Template">
       <ControlTemplate>
-        <StackPanel Spacing="2" Margin="0 0 7 0" Background="#7F00FF00" VerticalAlignment="Stretch" TextBlock.FontSize="10" Orientation="Horizontal">
+        <StackPanel Spacing="2" Margin="0 0 7 0" VerticalAlignment="Stretch" TextBlock.FontSize="10" Orientation="Horizontal">
           <StackPanel.Styles>
             <Style Selector="Panel">
               <Setter Property="Width" Value="45" />

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -699,18 +699,14 @@ namespace Avalonia.Win32
 
             margins.cyTopHeight = _extendChromeHints.HasFlag(ExtendClientAreaChromeHints.SystemTitleBar) ? borderCaptionThickness.top : 1;
 
-            _extendedMargins = new Thickness(0, borderCaptionThickness.top / Scaling, 0, 0);
-
-            if (WindowState == WindowState.Maximized && !_extendChromeHints.HasFlag(ExtendClientAreaChromeHints.SystemChromeButtons))
-            {
-                _offScreenMargin = new Thickness();                
-            }
-            else if (WindowState == WindowState.Maximized)
+            if (WindowState == WindowState.Maximized)
             {
+                _extendedMargins = new Thickness(0, (borderCaptionThickness.top - borderThickness.top) / Scaling, 0, 0);
                 _offScreenMargin = new Thickness(borderThickness.left / Scaling, borderThickness.top / Scaling, borderThickness.right / Scaling, borderThickness.bottom / Scaling);
             }
             else
-            {                
+            {
+                _extendedMargins = new Thickness(0, (borderCaptionThickness.top) / Scaling, 0, 0);
                 _offScreenMargin = new Thickness();
             }
 
@@ -739,29 +735,6 @@ namespace Avalonia.Win32
                 var margins = UpdateExtendMargins();
 
                 DwmExtendFrameIntoClientArea(_hwnd, ref margins);
-
-                if(!_extendChromeHints.HasFlag(ExtendClientAreaChromeHints.SystemChromeButtons) ||
-                    (_extendChromeHints.HasFlag(ExtendClientAreaChromeHints.PreferSystemChromeButtons) &&
-                    !_extendChromeHints.HasFlag(ExtendClientAreaChromeHints.SystemTitleBar)))
-                {
-                    var style = GetStyle();
-
-                    style &= ~(WindowStyles.WS_MINIMIZEBOX | WindowStyles.WS_MAXIMIZEBOX | WindowStyles.WS_SYSMENU);
-
-                    SetStyle(style);
-
-                    DisableCloseButton(_hwnd);
-                }
-                else
-                {
-                    var style = GetStyle();
-
-                    style |= (WindowStyles.WS_MINIMIZEBOX | WindowStyles.WS_MAXIMIZEBOX | WindowStyles.WS_SYSMENU);
-
-                    SetStyle(style);
-
-                    EnableCloseButton(_hwnd);
-                }
             }
             else
             {
@@ -770,14 +743,6 @@ namespace Avalonia.Win32
 
                 _offScreenMargin = new Thickness();
                 _extendedMargins = new Thickness();
-
-                var style = GetStyle();
-
-                style |= (WindowStyles.WS_MINIMIZEBOX | WindowStyles.WS_MAXIMIZEBOX | WindowStyles.WS_SYSMENU);
-
-                SetStyle(style);
-
-                EnableCloseButton(_hwnd);
             }
 
             ExtendClientAreaToDecorationsChanged?.Invoke(true);