dependencies:
  window_manager: ^0.1.8

dependencies:
  window_manager:
    git:
      url: https://github.com/leanflutter/window_manager.git
      ref: main

import 'package:flutter/material.dart';
import 'package:window_manager/window_manager.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // Must add this line.
  await windowManager.ensureInitialized();

  // Use it only after calling `hiddenWindowAtLaunch`
  windowManager.waitUntilReadyToShow().then((_) async{
    // Hide window title bar
    await windowManager.setTitleBarStyle('hidden');
    await windowManager.setSize(Size(800, 600));
    await windowManager.center();
    await windowManager.show();
    await windowManager.setSkipTaskbar(false);
  });

  runApp(MyApp());
}

import 'package:flutter/cupertino.dart';
import 'package:window_manager/window_manager.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with WindowListener {
  @override
  void initState() {
    windowManager.addListener(this);
    super.initState();
  }

  @override
  void dispose() {
    windowManager.removeListener(this);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // ...
  }

  @override
  void onWindowEvent(String eventName) {
    print('[WindowManager] onWindowEvent: $eventName');
  }

  @override
  void onWindowClose() {
    // do something
  }

  @override
  void onWindowFocus() {
    // do something
  }

  @override
  void onWindowBlur() {
    // do something
  }

  @override
  void onWindowMaximize() {
    // do something
  }

  @override
  void onWindowUnmaximize() {
    // do something
  }

  @override
  void onWindowMinimize() {
    // do something
  }

  @override
  void onWindowRestore() {
    // do something
  }

  @override
  void onWindowResize() {
    // do something
  }

  @override
  void onWindowMove() {
    // do something
  }

  @override
  void onWindowEnterFullScreen() {
    // do something
  }

  @override
  void onWindowLeaveFullScreen() {
    // do something
  }
}

import Cocoa
import FlutterMacOS

@NSApplicationMain
class AppDelegate: FlutterAppDelegate {
  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
-    return true
+    return false
  }
}

import 'package:flutter/cupertino.dart';
import 'package:window_manager/window_manager.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with WindowListener {
  @override
  void initState() {
    windowManager.addListener(this);
    super.initState();
  }

  void _init() async {
    // Add this line to override the default close handler
    await windowManager.setPreventClose(true);
    setState(() {});
  }

  @override
  void dispose() {
    windowManager.removeListener(this);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // ...
  }

  @override
  void onWindowClose() async {
    bool _isPreventClose = await windowManager.isPreventClose();
    if (_isPreventClose) {
      showDialog(
        context: context,
        builder: (_) {
          return AlertDialog(
            title: Text('Are you sure you want to close this window?'),
            actions: [
              TextButton(
                child: Text('No'),
                onPressed: () {
                  Navigator.of(context).pop();
                },
              ),
              TextButton(
                child: Text('Yes'),
                onPressed: () {
                  Navigator.of(context).pop();
                  exit(0);
                },
              ),
            ],
          );
        },
      );
    }
  }
}

import Cocoa
import FlutterMacOS
+import window_manager

class MainFlutterWindow: NSWindow {
    override func awakeFromNib() {
        let flutterViewController = FlutterViewController.init()
        let windowFrame = self.frame
        self.contentViewController = flutterViewController
        self.setFrame(windowFrame, display: true)

        RegisterGeneratedPlugins(registry: flutterViewController)

        super.awakeFromNib()
    }

+    override public func order(_ place: NSWindow.OrderingMode, relativeTo otherWin: Int) {
+        super.order(place, relativeTo: otherWin)
+        hiddenWindowAtLaunch()
+    }
}

bool Win32Window::CreateAndShow(const std::wstring& title,
                                const Point& origin,
                                const Size& size) {
  ...                              
  HWND window = CreateWindow(
-      window_class, title.c_str(), WS_OVERLAPPEDWINDOW | WS_VISIBLE,
+      window_class, title.c_str(),
+      WS_OVERLAPPEDWINDOW, // do not add WS_VISIBLE since the window will be shown later
      Scale(origin.x, scale_factor), Scale(origin.y, scale_factor),
      Scale(size.width, scale_factor), Scale(size.height, scale_factor),
      nullptr, nullptr, GetModuleHandle(nullptr), this);

import 'package:flutter/cupertino.dart';
import 'package:window_manager/window_manager.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with WindowListener {
  @override
  void initState() {
    windowManager.addListener(this);
    super.initState();
  }

  @override
  void dispose() {
    windowManager.removeListener(this);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // ...
  }

  @override
  void onWindowFocus() {
    // Make sure to call once.
    setState(() {});
    // do something
  }
}