fix(macos): release closed child window engines (#1639)

ui/flutter/test/core/window/window_capability_lifecycle_test.dart

@@ -0,0 +1,65 @@
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:gopeed/core/capabilities/app_capabilities.dart';
+import 'package:gopeed/core/capabilities/capability_rpc.dart';
+import 'package:gopeed/core/window/app_window_appearance.dart';
+import 'package:gopeed/core/window/window_capability_transport.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  const windows = MethodChannel('mixin.one/desktop_multi_window');
+  const channels = MethodChannel('mixin.one/desktop_multi_window/channels');
+  const codec = StandardMethodCodec();
+  final messenger = TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
+
+  tearDown(() async {
+    await AppWindowCapabilityHost.instance.stop();
+    messenger.setMockMethodCallHandler(windows, null);
+    messenger.setMockMethodCallHandler(channels, null);
+  });
+
+  testWidgets('closed engines lose appearance subscriptions without a Dart unsubscribe', (tester) async {
+    final deliveries = <String>[];
+    messenger.setMockMethodCallHandler(channels, (call) async {
+      if (call.method == 'invokeMethod') {
+        final args = Map<String, dynamic>.from(call.arguments as Map);
+        if (args['method'] == AppWindowRpcProtocol.event) deliveries.add(args['channel'] as String);
+      }
+      return null;
+    });
+    messenger.setMockMethodCallHandler(windows, (call) async {
+      if (call.method == 'getAllWindows') {
+        return [
+          {'windowId': 'live-child', 'windowArgument': ''},
+        ];
+      }
+      return null;
+    });
+    final host = AppWindowCapabilityHost.instance;
+    await host.start(CapabilityRegistry(createAppCapabilityCodecs()));
+    for (final id in ['closed-child', 'live-child']) {
+      await messenger.handlePlatformMessage(
+        channels.name,
+        codec.encodeMethodCall(
+          MethodCall('methodCall', {
+            'channel': AppWindowRpcProtocol.channelName,
+            'method': AppWindowRpcProtocol.subscribe,
+            'arguments': {'windowId': id},
+          }),
+        ),
+        (_) {},
+      );
+    }
+    await tester.pump();
+    deliveries.clear();
+    await messenger.handlePlatformMessage(
+      windows.name,
+      codec.encodeMethodCall(const MethodCall('onWindowsChanged')),
+      (_) {},
+    );
+    await tester.pump();
+    host.updateAppearance(const AppWindowAppearance(themeMode: 'dark', themeColor: 'green', locale: 'zh'));
+    await tester.pump();
+    expect(deliveries, ['mixin.one/window_controller/live-child']);
+  });
+}