Now that you have the NerdSplitViewController working, worry about the draggable divider. You will need a way to receive mouse dragged events. You will then change out the constraints in response to the dragging.
As you will recall from Chapter 18, there are a few different ways to receive the mouse events. Because NSViewControllers are in the responder chain, you can simply implement mouseDragged(_:) and so forth in the NerdSplitViewController. Another option is to use an NSPanGestureRecognizer added to a view representing the divider.