Fix desktop lifetime non-mainwindow cancellation

src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using Avalonia.Collections;
@@ -184,19 +183,24 @@ private bool DoShutdown(
                 // When an OS shutdown request is received, try to close all non-owned windows. Windows can cancel
                 // shutdown by setting e.Cancel = true in the Closing event. Owned windows will be shutdown by their
                 // owners.
-                foreach (var w in Windows.ToArray())
+                foreach (var w in new List<Window>(_windows))
                 {
                     if (w.Owner is null)
                     {
                         w.CloseCore(WindowCloseReason.ApplicationShutdown, isProgrammatic);
                     }
                 }
 
-                if (!force && Windows.Count > 0)
+                if (!force)
                 {
-                    e.Cancel = true;
-                    shutdownCancelled = true;
-                    return false;
+                    if (ShutdownMode == ShutdownMode.OnMainWindowClose ?
+                            MainWindow?.IsVisible == true :
+                            Windows.Count > 0)
+                    {
+                        e.Cancel = true;
+                        shutdownCancelled = true;
+                        return false;
+                    }
                 }
 
                 var args = new ControlledApplicationLifetimeExitEventArgs(exitCode);

src/Avalonia.Controls/Window.cs

@@ -1054,10 +1054,12 @@ protected sealed override Size ArrangeSetBounds(Size size)
 
         private protected sealed override void HandleClosed()
         {
-            RaiseEvent(new RoutedEventArgs(WindowClosedEvent));
+            _shown = false;
 
             base.HandleClosed();
 
+            RaiseEvent(new RoutedEventArgs(WindowClosedEvent));
+
             Owner = null;
         }
 

tests/Avalonia.Controls.UnitTests/DesktopStyleApplicationLifetimeTests.cs

@@ -126,6 +126,62 @@ public void Should_Exit_After_MainWindow_Closed()
             }
         }
 
+        [Fact]
+        public void OnMainWindowClose_Overrides_Secondary_Window_Cancellation()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            using(var lifetime = new ClassicDesktopStyleApplicationLifetime())
+            {
+                lifetime.ShutdownMode = ShutdownMode.OnMainWindowClose;
+                lifetime.SetupCore(Array.Empty<string>());
+
+                var hasExit = false;
+
+                lifetime.Exit += (_, _) => hasExit = true;
+
+                var mainWindow = new Window();
+                mainWindow.Show();
+
+                lifetime.MainWindow = mainWindow;
+
+                var window = new Window();
+                window.Closing += (_, args) => { args.Cancel = true; };
+                window.Show();
+
+                mainWindow.Close();
+
+                Assert.True(hasExit);
+            }
+        }
+
+        [Fact]
+        public void OnMainWindowClose_Overrides_Secondary_Window_Cancellation_From_TryShutdown()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            using(var lifetime = new ClassicDesktopStyleApplicationLifetime())
+            {
+                lifetime.ShutdownMode = ShutdownMode.OnMainWindowClose;
+                lifetime.SetupCore(Array.Empty<string>());
+
+                var hasExit = false;
+
+                lifetime.Exit += (_, _) => hasExit = true;
+
+                var mainWindow = new Window();
+                mainWindow.Show();
+
+                lifetime.MainWindow = mainWindow;
+
+                var window = new Window();
+                window.Closing += (_, args) => { args.Cancel = true; };
+                window.Show();
+
+                lifetime.TryShutdown();
+
+                Assert.True(hasExit);
+            }
+        }
+
         [Fact]
         public void Should_Exit_After_Last_Window_Closed()
         {

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -114,6 +114,8 @@ public void IsVisible_Should_Be_False_After_Impl_Signals_Close()
                 var window = new Window();
 
                 window.Show();
+                Assert.True(window.IsVisible);
+
                 windowImpl.Object.Closed();
 
                 Assert.False(window.IsVisible);