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

ui/flutter/macos/Runner/MainFlutterWindow.swift

@@ -4,6 +4,8 @@ import desktop_multi_window
 import window_manager
 
 class MainFlutterWindow: NSWindow {
+  private var childWindowCloseObservers: [ObjectIdentifier: NSObjectProtocol] = [:]
+
   override func awakeFromNib() {
     let flutterViewController = FlutterViewController()
     let windowFrame = self.frame
@@ -11,13 +13,45 @@ class MainFlutterWindow: NSWindow {
     self.setFrame(windowFrame, display: true)
 
     RegisterGeneratedPlugins(registry: flutterViewController)
-    FlutterMultiWindowPlugin.setOnWindowCreatedCallback { controller in
+    FlutterMultiWindowPlugin.setOnWindowCreatedCallback { [weak self] controller in
       RegisterGeneratedPlugins(registry: controller)
+      self?.releaseChildEngineOnClose(controller)
     }
 
     super.awakeFromNib()
   }
 
+  // desktop_multi_window removes closed windows from its registry, but plugin
+  // channels can still retain the window/controller/engine cycle. NSWindow.close
+  // alone therefore leaves each child Dart isolate and its heap running.
+  private func releaseChildEngineOnClose(_ controller: FlutterViewController) {
+    guard let window = controller.view.window else { return }
+    let key = ObjectIdentifier(window)
+    childWindowCloseObservers[key] = NotificationCenter.default.addObserver(
+      forName: NSWindow.willCloseNotification, object: window, queue: .main
+    ) { [weak self, weak controller, weak window] _ in
+      if let observer = self?.childWindowCloseObservers.removeValue(forKey: key) {
+        NotificationCenter.default.removeObserver(observer)
+      }
+      // Let close notifications and the plugin's registry removal finish before
+      // tearing down the engine that delivered the close request.
+      DispatchQueue.main.async { [weak controller, weak window] in
+        // window_manager's onEvent closure retains its plugin, which retains
+        // the manager and NSWindow even after the engine has stopped.
+        (window?.delegate as? WindowManager)?.onEvent = nil
+        window?.delegate = nil
+        controller?.engine.shutDownEngine()
+        window?.contentViewController = nil
+      }
+    }
+  }
+
+  deinit {
+    for observer in childWindowCloseObservers.values {
+      NotificationCenter.default.removeObserver(observer)
+    }
+  }
+
   override public func order(_ place: NSWindow.OrderingMode, relativeTo otherWin: Int) {
     super.order(place, relativeTo: otherWin)
     hiddenWindowAtLaunch()