I notice that, if I perform add/ expand animation within an UIScrollView, it will cause unwanted scrolling behavior, when the UIScrollView fill with enough content to become scroll-able.

As you can see in the following animation, initially, the add/ expand animation works just fine.

When we have added enough item till the UIScrollView scrollable, whenever a new item is added, and UIScrollView will first perform scroll down, and then scroll up again!

My expectation is that, the UIScrollView should remain static, when add/ expand animation is performed.

Here's the code which performs add/ expand animation.

Add/ expand animation

@IBAction func add(_ sender: Any) {
    let customView = CustomView.instanceFromNib()
    
    customView.hide()
    stackView.addArrangedSubview(customView)
    // Clear off horizontal swipe in animation caused by addArrangedSubview
    stackView.superview?.layoutIfNeeded()
    
    customView.show()
    // Perform expand animation.
    UIView.animate(withDuration: 1) {
        self.stackView.superview?.layoutIfNeeded()
    }
}

Here's the constraint setup of the UIScrollView & added custom view item

Constraint setup

Custom view

class CustomView: UIView {

    private var zeroHeightConstraint: NSLayoutConstraint!
    
    @IBOutlet weak var borderView: UIView!
    @IBOutlet weak var stackView: UIStackView!
    
    override func awakeFromNib() {
        super.awakeFromNib()
        
        borderView.layer.cornerRadius = stackView.frame.height / 2
        borderView.layer.masksToBounds = true
        borderView.layer.borderWidth = 1
        
        zeroHeightConstraint = self.safeAreaLayoutGuide.heightAnchor.constraint(equalToConstant: 0)
        zeroHeightConstraint.isActive = false
    }
    
    func hide() {
        zeroHeightConstraint.isActive = true
    }
    
    func show() {
        zeroHeightConstraint.isActive = false
    }
}

Here's the complete source code

The "add view" function now becomes (I added a Bool so we can skip the animation on the initial add in viewDidLoad()):

func addCustomView(_ animated: Bool) {
    let customView = CustomView.instanceFromNib()
    
    stackView.addArrangedSubview(customView)
    customView.isHidden = true
    
    if animated {
        DispatchQueue.main.async {
            UIView.animate(withDuration: 1) {
                customView.isHidden = false
            }
        }
    } else {
        customView.isHidden = false
    }
}

And we can get rid of all of the hide() / show() and zeroHeightConstraint in the custom view class:

class CustomView: UIView {
    
    @IBOutlet weak var borderView: UIView!
    @IBOutlet weak var stackView: UIStackView!
    
    override func awakeFromNib() {
        super.awakeFromNib()
        
        borderView.layer.masksToBounds = true
        borderView.layer.borderWidth = 1
        
    }
    override func layoutSubviews() {
        super.layoutSubviews()
        borderView.layer.cornerRadius = borderView.bounds.height * 0.5
    }
}

Since it's a bit difficult to clearly show everything here, I forked your project with the changes: https://github.com/DonMag/add-expand-animation-in-scroll-view