Implement missing properties

src/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -245,6 +245,8 @@ namespace Avalonia.Headless
 
         public Thickness OffScreenMargin => new Thickness();
 
+        public bool IsEnabled => true;
+
         public Action LostFocus { get; set; }
 
         public AcrylicPlatformCompensationLevels AcrylicCompensationLevels => new AcrylicPlatformCompensationLevels(1, 1, 1);

tests/Avalonia.UnitTests/TestRoot.cs

@@ -62,6 +62,8 @@ namespace Avalonia.UnitTests
 
         public bool ShowAccessKeys { get; set; }
 
+        public bool? IsPlatformEnabled { get; }
+
         public IStyleHost StylingParent { get; set; }
 
         IStyleHost IStyleHost.StylingParent => StylingParent;