Random "vertical jumps" while scrolling up in UICollectionViewController – Ios swift

I have a UIViewController which includes a UIPageViewController which itself includes a UICollectionViewController.

Above the UIPageViewController: a UIView (named pagerView) which moves up or down depending on the vertical scrolling-offset of the UICollectionViewController.

Finally, the UIPageViewController’s top anchor is constrained to pagerView‘s bottom anchor.

The problem is that scrolling up is not continuous, the UICollectionViewController sometimes "jumps" (it can be a few hundreds of points.)

In order to reproduce: scroll to the bottom (row 9) then start to scroll up and see how it jumps to row 7.

The source of the UIViewController:

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()
            })
        }
    }
}

The whole code can be viewed in this gist. There is also a XCode project to download.

And also: a video demonstrating the problem.