title: How to add Tabs in a non-document-based app under macOS?
body: <p>I like to build an app in Swift 3 with Xcode 8 and it should enable the Apple tab bar. It is not document-based. I learned <a href="https://developer.apple.com/videos/play/wwdc2016/203/?time=938" rel="noreferrer">here</a>, that the tabs can be enabled if I override the method <code>@IBAction func newWindowForTab(_ sender: Any?)</code> for example in the window controller. To test this, I created a new project in Xcode using storyboard, added a subclass of <code>NSWindowController</code> and assigned it in the storyboard. I then implemented</p>
  
  <pre><code>@IBAction override func newWindowForTab(_ sender: Any?) {}
  </code></pre>
  
  <p>and the tab bar appears when the app is build. Once rebuild, I noticed that the "+"-button only appears if the tab bar was not visible when the app is closed prior to build. Is that a bug? How would I add new tabs?</p>