Add buttons to Mac window Title Bars, system-wide

The officially-supported way to add a title bar button in OS X 10.10 (Yosemite) and later is by creating an NSTitlebarAccessoryViewController and adding it to your window using -[NSWindow addTitlebarAccessoryViewController].

For example, I have a window that has a title bar accessory view:

To set this up, I started by adding a standalone view to the window controller scene in my storyboard. (You don't have to use a storyboard but I did in this project.)

My accessory view is an NSView with an NSButton subview. The button title uses Font Awesome to display the pushpin.

I connected the accessory view to an outlet (cleverly named accessoryView) in my NSWindowController subclass:

Then, in my window controller's windowDidLoad, I create the NSTitlebarAccessoryViewController, set its properties, and add it to the window:

@IBOutlet var accessoryView: NSView!
var accessoryViewController: NSTitlebarAccessoryViewController?

override func windowDidLoad() {
    super.windowDidLoad()
    createAccessoryViewControllerIfNeeded()
}

fileprivate func createAccessoryViewControllerIfNeeded() {
    guard self.accessoryViewController == nil else { return }
    let accessoryViewController = NSTitlebarAccessoryViewController()
    self.accessoryViewController = accessoryViewController
    accessoryViewController.view = accessoryView
    accessoryViewController.layoutAttribute = .right
    self.window?.addTitlebarAccessoryViewController(accessoryViewController)
}

This is really a two-part question. As for how to get a button up there, I’d suggest using -[NSWindow standardWindowButton:] to get an existing window button and its superview (i. e. the title bar):

NSButton *closeButton = [window standardWindowButton:NSWindowCloseButton]; // Get the existing close button of the window. Check documentation for the other window buttons.
NSView *titleBarView = closeButton.superview; // Get the view that encloses that standard window buttons.
NSButton *myButton = …; // Create custom button to be added to the title bar.
myButton.frame = …; // Set the appropriate frame for your button. Use titleBarView.bounds to determine the bounding rect of the view that encloses the standard window buttons.
[titleBarView addSubview:myButton]; // Add the custom button to the title bar.

The plugging-in is probably easiest to do as a SIMBL plug-in.

This answer addresses the latest Xcode Version 9.3

• Go to a storyboard.
• Drag and drop a toolbar in the storyboard to a Window.

• The toolbar will be visible below a title.

• Locate the Window on the storyboard

• Check "Hide Title" in properties of the Window.

• The toolbar is a part of the title now.