How do I disable the Show Tab Bar menu option in Sierra apps?

You can also do this on IB, on the Window’s attributes inspector

NSWindow attribute inspector

On 10.12, you need to now set the following when the window is created as Tab Bar is now available by default:

[NSWindow setAllowsAutomaticWindowTabbing: NO];

The answer is the same in Swift and SwiftUI

func applicationWillFinishLaunching(_ notification: Notification) {
    NSWindow.allowsAutomaticWindowTabbing = false
}

Note that the call is made on the class NSWindow not on an instance of NSWindow

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