NSBox makes your task easier in NerdTabViewController by simplifying adding the content view to the hierarchy and sizing it appropriately. Test your understanding of the view hierarchy and Auto Layout constraints by replacing the NSBox in NerdTabViewController with a plain NSView.
Instead of setting the box’s contentView, you will add the selected child view controller’s view as a subview. Use Auto Layout to ensure that the view resizes with its container view. Do not forget to remove the previously selected view controller’s view!