Open a window programmatically

AppDelegate.swift

//
//  AppDelegate.swift
//

import Cocoa

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    private var preferencesWindow = NSWindowController()

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        // Insert code here to initialize your application
    }

    func applicationWillTerminate(_ aNotification: Notification) {
        // Insert code here to tear down your application
    }

    @IBAction func openPreferencesWindow(_ sender: NSMenuItem) {

        if let _ = preferencesWindow.window {
            preferencesWindow.showWindow(self)
            return
        }

        let content = PreferencesViewController()
        let window = NSWindow(contentRect: .zero, styleMask: [.titled, .closable], backing: .buffered, defer: true)
        window.contentViewController = content
        window.windowController = preferencesWindow
        window.title = "Preferences"
        preferencesWindow.window = window
        window.makeKeyAndOrderFront(self)
        window.layoutIfNeeded()
    }

}