Bug: Random "vertical jumps" while scrolling up in UICollectionViewController

CustomCollectionViewCell.swift

import UIKit

class CustomCollectionViewCell: UICollectionViewCell {
    private lazy var label: UILabel = {
        let view = UILabel()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.font = UIFont.boldSystemFont(ofSize: 48)
        view.textColor = .yellow
        view.textAlignment = .center
        return view
    }()
    
    private lazy var labelHeightConstraint: NSLayoutConstraint = {
        return label.heightAnchor.constraint(equalToConstant: 0)
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        initialize()
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private func initialize() {
        contentView.backgroundColor = .red
        contentView.addSubview(label)
        
        NSLayoutConstraint.activate([
            label.topAnchor.constraint(equalTo: contentView.topAnchor),
            label.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
            label.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
            label.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
            labelHeightConstraint
        ])
    }
    
    func configure(index: Int, height: CGFloat) {
        label.text = String(index)
        labelHeightConstraint.constant = height
    }
}

CustomViewController.swift

import UIKit

class CustomViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {
    enum Section {
        case main
    }

    struct CustomItem: Hashable
    {
        let uuid = UUID()
        
        static func ==(lhs: CustomItem, rhs: CustomItem) -> Bool {
            return lhs.uuid == rhs.uuid
        }

        func hash(into hasher: inout Hasher) {
            hasher.combine(uuid)
        }
    }
    
    weak var delegate: ViewControllerChildDelegate?
    
    private let layout: UICollectionViewCompositionalLayout = {
        let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .estimated(200))
        let item = NSCollectionLayoutItem(layoutSize: itemSize)

        let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .estimated(200))
        let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])

        let section = NSCollectionLayoutSection(group: group)
        section.interGroupSpacing = 20

        let layout = UICollectionViewCompositionalLayout(section: section)
        return layout
    }()
    
    private lazy var dataSource: UICollectionViewDiffableDataSource<Section, AnyHashable> = {
        let dataSource = UICollectionViewDiffableDataSource<Section, AnyHashable>(collectionView: collectionView) { (collectionView, indexPath, item) -> UICollectionViewCell? in
            let cell = self.cell(in: collectionView, for: indexPath, item: item)
            return cell
        }

        return dataSource
    }()
    
    private let heights: [CGFloat] = [100, 150, 200, 250, 300, 350, 400, 450, 500, 550]
    
    init() {
        super.init(collectionViewLayout: layout)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView.backgroundColor = .orange
        
        collectionView.register(CustomCollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
        
        update()
    }
    
    override func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        delegate?.childScrollViewWillBeginDragging(with: scrollView.contentOffset.y)
    }
    
    override func scrollViewDidScroll(_ scrollView: UIScrollView) {
        delegate?.childScrollViewDidScroll(to: scrollView.contentOffset.y)
    }
    
    private func cell(in collectionView: UICollectionView, for indexPath: IndexPath, item: AnyHashable) -> UICollectionViewCell {
        switch item {
        case is CustomItem:
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CustomCollectionViewCell
            cell.configure(index: indexPath.row, height: heights[indexPath.row])
            return cell
        default:
            return UICollectionViewCell()
        }
    }
    
    private func update() {
        var snapshot = NSDiffableDataSourceSnapshot<Section, AnyHashable>()
        
        snapshot.appendSections([.main])
        
        for _ in 0..<10 {
            snapshot.appendItems([CustomItem()], toSection: .main)
        }
        
        dataSource.apply(snapshot, animatingDifferences: false)
    }
}

ViewController.swift

import UIKit

protocol ViewControllerChildDelegate: AnyObject {
    func childScrollViewWillBeginDragging(with offset: CGFloat)
    func childScrollViewDidScroll(to offset: CGFloat)
}

class ViewController: UIViewController {
    private lazy var pageViewController: UIPageViewController = {
        let viewController = UIPageViewController(transitionStyle: .scroll, navigationOrientation: .horizontal, options: [:])
        viewController.delegate = self
        viewController.dataSource = self
        return viewController
    }()
    
    private lazy var pagerView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .brown
        return view
    }()
    
    private let pagerViewHeight: CGFloat = 44
    private var lastContentOffset: CGFloat = 0
    
    lazy var pagerViewTopAnchorConstraint: NSLayoutConstraint = {
        return pagerView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor)
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        title = "xxx"

        view.addSubview(pagerView)
        
        addChild(pageViewController)
        view.addSubview(pageViewController.view)
        pageViewController.didMove(toParent: self)
        pageViewController.view.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            pagerViewTopAnchorConstraint,
            pagerView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            pagerView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            pagerView.heightAnchor.constraint(equalToConstant: pagerViewHeight),
            
            pageViewController.view.topAnchor.constraint(equalTo: pagerView.bottomAnchor),
            pageViewController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            pageViewController.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            pageViewController.view.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor)
        ])
        
        let viewController = CustomViewController()
        viewController.delegate = self
        
        pageViewController.setViewControllers(
            [viewController],
            direction: .forward,
            animated: false,
            completion: nil
        )
    }
}

extension ViewController: UIPageViewControllerDataSource, UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        return nil
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        return nil
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
    }
}

extension ViewController: ViewControllerChildDelegate {
    func childScrollViewWillBeginDragging(with offset: CGFloat) {
        lastContentOffset = offset
    }
    
    func childScrollViewDidScroll(to offset: CGFloat) {
        if lastContentOffset > offset {
            view.layoutIfNeeded()
            pagerViewTopAnchorConstraint.constant = 0
            UIView.animate(withDuration: 0.3, animations: { [weak self] in
                self?.view.layoutIfNeeded()
            })
        } else if lastContentOffset < offset {
            view.layoutIfNeeded()
            pagerViewTopAnchorConstraint.constant = -pagerViewHeight
            UIView.animate(withDuration: 0.3, animations: { [weak self] in
                self?.view.layoutIfNeeded()
            })
        }
    }
}

ViewController should be embedded in a navigation-controller.