Decorations Margin shouldnt include the border margin other wise users have to do subtractions.

src/Avalonia.Controls/Chrome/CaptionButtons.cs

@@ -55,7 +55,7 @@ namespace Avalonia.Controls.Chrome
         void InvalidateSize ()
         {
             Margin = new Thickness(1, _hostWindow.OffScreenMargin.Top, 1, 1);
-            Height = _hostWindow.WindowDecorationMargins.Top - _hostWindow.OffScreenMargin.Top;
+            Height = _hostWindow.WindowDecorationMargins.Top;
         }
 
         public void Detach()

src/Avalonia.Controls/Window.cs

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