CompositionalLayoutBug.swift

/*
Collection view with UICollectionViewCompositionalLayout 
resets section's content offset on vertical scroll
if visibleItemsInvalidationHandler is set.

Even if it's empty
*/

import UIKit

class ViewController: UIViewController {
  private(set) lazy var collectionView: UICollectionView = {
    $0.contentInsetAdjustmentBehavior = .never
    $0.showsVerticalScrollIndicator = false
    $0.showsHorizontalScrollIndicator = false
    $0.backgroundColor = .white
    $0.register(
      CardCell.self,
      forCellWithReuseIdentifier: CardCell.reuseID
    )

    $0.dataSource = self
    return $0
  }(UICollectionView(frame: .zero, collectionViewLayout: collectionViewLayout))

  func createContentSection() -> NSCollectionLayoutSection {
    let item = NSCollectionLayoutItem(
      layoutSize: NSCollectionLayoutSize(
        widthDimension: .fractionalWidth(1),
        heightDimension: .fractionalHeight(1)
      )
    )

    let group = NSCollectionLayoutGroup.horizontal(
      layoutSize: NSCollectionLayoutSize(widthDimension: .fractionalWidth(0.7), heightDimension: .fractionalHeight(0.5)),
      subitem: item,
      count: 1)

    let section = NSCollectionLayoutSection(group: group)
    section.contentInsets = NSDirectionalEdgeInsets(top: 50, leading: 50, bottom: 50, trailing: 50)
    section.orthogonalScrollingBehavior = .groupPagingCentered
    section.interGroupSpacing = 10

    // COMMENT THIS TO FIX CONTENT OFFSET RESET:
    section.visibleItemsInvalidationHandler = { visibleItems, offset, environment in
      //Empty or any actions like transforming scale while scrolling, etc
    }

    section.contentInsets = NSDirectionalEdgeInsets(top: 30, leading: 0, bottom: 0, trailing: 0)
    return section
  }

  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .white
    view.addSubview(collectionView)
    collectionView.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activate([
      collectionView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor),
      collectionView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor),
      collectionView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
      collectionView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
    ])
  }

  private var collectionViewLayout: UICollectionViewLayout {
    UICollectionViewCompositionalLayout(section: createContentSection())
  }
}

extension ViewController: UICollectionViewDataSource {
  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    5
  }

  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: CardCell.reuseID,
                                                  for: indexPath) as! CardCell
    cell.titleLabel.text = String(indexPath.row)
    return cell
  }
}

// MARK: - Cell

class CardCell: UICollectionViewCell {
  static let reuseID = "CardCell"

  let titleLabel: UILabel = {
    $0.font = UIFont.systemFont(ofSize: 40)
    $0.textAlignment = .center
    $0.textColor = .white
    $0.adjustsFontSizeToFitWidth = true
    $0.minimumScaleFactor = 0.1
    return $0
  }(UILabel())

  override init(frame: CGRect) {
    super.init(frame: frame)
    commonInit()
  }

  required init?(coder: NSCoder) {
    super.init(coder: coder)
    commonInit()
  }

  private func commonInit() {
    backgroundColor = UIColor.gray
    layer.cornerRadius = 40
    layer.masksToBounds = true
    contentView.addSubview(titleLabel)
    titleLabel.translatesAutoresizingMaskIntoConstraints = false

    NSLayoutConstraint.activate([
      titleLabel.leftAnchor.constraint(equalTo: contentView.leftAnchor),
      titleLabel.rightAnchor.constraint(equalTo: contentView.rightAnchor),
      titleLabel.topAnchor.constraint(equalTo: contentView.topAnchor),
      titleLabel.bottomAnchor.constraint(equalTo: contentView.bottomAnchor),
    ])
  }
}