NSScrollView with AutoLayout constrains made window unresponsive to resizing and dragging - macos

Question

In Apple's official Exhibition app demo, there's such usage in viewDidLoad() in the ImageViewerController.swift
/*
Turn on translatesAutoresizingMaskIntoConstraints, Interface Builder
turns this off for all views in a nib. The `NSScrollView` and `NSClipView`
expect to be able to control their documentView via modifying its `frame`.
*/
imageView.translatesAutoresizingMaskIntoConstraints = true
I noticed that in the xib file, the imageView already has constrains set up. When the line of code is commented out, the view still works.
To better understand the code from the demo, I created a minimal project with only a NSScrollView added under the default view controller's view. Here the NSImageView is created programmatically. If the constrains in h and v are activated, the window will be very big and unresponsive to resizing and dragging.
class ViewController: NSViewController {
#IBOutlet weak var scrollView: NSScrollView!
#IBOutlet weak var clipView: NSClipView!
override func viewDidLoad() {
super.viewDidLoad()
let path = "/Library/Desktop Pictures/Snow.jpg"
let url = URL(fileURLWithPath: path, isDirectory: false)
let image = NSImage(contentsOf: url)
let imageView = NSImageView(image: image!)
imageView.translatesAutoresizingMaskIntoConstraints = false
clipView.addSubview(imageView)
let h = NSLayoutConstraint.constraints(
withVisualFormat: "H:|[imageView]|",
options: NSLayoutConstraint.FormatOptions(rawValue: 0),
metrics: nil,
views: ["imageView" : imageView])
let v = NSLayoutConstraint.constraints(
withVisualFormat: "V:|[imageView]|",
options: NSLayoutConstraint.FormatOptions(rawValue: 0),
metrics: nil,
views: ["imageView" : imageView])
clipView.documentView = imageView
// NSLayoutConstraint.activate([h, v].flatMap({$0}))
}
}
My questions are:
Why and how do the constrains in my project cause the problem, while the constrains in the demo don't.
Are the constrains in the official demo necessary?