Make Window.OpenWindows readonly

And hack around the fact that it's static in unit tests.

src/Avalonia.Controls/Window.cs

@@ -47,12 +47,12 @@ namespace Avalonia.Controls
     /// </summary>
     public class Window : WindowBase, IStyleable, IFocusScope, ILayoutRoot, INameScope
     {
-        private static IList<Window> s_windows = new List<Window>();
+        private static List<Window> s_windows = new List<Window>();
 
         /// <summary>
         /// Retrieves an enumeration of all Windows in the currently running application.
         /// </summary>
-        public static IList<Window> OpenWindows => s_windows;
+        public static IReadOnlyList<Window> OpenWindows => s_windows;
 
         /// <summary>
         /// Defines the <see cref="SizeToContent"/> property.

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -4,6 +4,7 @@
 // </copyright>
 // -----------------------------------------------------------------------
 
+using System.Collections.Generic;
 using Avalonia.Platform;
 using Avalonia.UnitTests;
 using Moq;
@@ -120,13 +121,12 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
+                ClearOpenWindows();
                 var window = new Window();
 
                 window.Show();
 
                 Assert.Equal(new[] { window }, Window.OpenWindows);
-
-                window.Close();
             }
         }
 
@@ -135,6 +135,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
+                ClearOpenWindows();
                 var window = new Window();
 
                 window.Show();
@@ -152,6 +153,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
+                ClearOpenWindows();
                 var window = new Window();
 
                 window.Show();
@@ -173,6 +175,7 @@ namespace Avalonia.Controls.UnitTests
 
             using (UnitTestApplication.Start(services))
             {
+                ClearOpenWindows();
                 var window = new Window();
 
                 window.Show();
@@ -181,5 +184,12 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Empty(Window.OpenWindows);
             }
         }
+
+        private void ClearOpenWindows()
+        {
+            // HACK: We really need a decent way to have "statics" that can be scoped to
+            // AvaloniaLocator scopes.
+            ((IList<Window>)Window.OpenWindows).Clear();
+        }
     }
 }