Hide NSWindow New tab button

Question

In macOS 10.12 there is a new tab bar that is added to NSWindows for NSDocument apps. You can prevent the toolbar from appearing (see How do I disable the Show Tab Bar menu option in Sierra apps?). But how to remove the "+" button for adding new Windows?

Answer 1

According to the AppKit release notes, returning false for responding newWindowForTab(_:) action message in a NSDocumentController subclass disables "+" button in the tab bar.

override func responds(to aSelector: Selector!) -> Bool {

    if #available(OSX 10.12, *) {
        if aSelector == #selector(NSResponder.newWindowForTab(_:)) {
            return false
        }
    }

    return super.responds(to: aSelector)
}

See "New Button" section in the AppKit Release Notes for macOS 10.12.