Using NSCollectionLayoutSize with .estimated dimensions in horizontal orthogonal sections, creates layout issues. The cells & supplementary views have layout conflicts, the scroll behavior is sub optimal and spacing is not as expected Working with Xcode: 12.4 , Simulator: iOS 14.4 Layout bug: [LayoutConstraints] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. ( "NSLayoutConstraint:0x6000011266c0 UIView:0x7fc6c4617020.height == 80 (active)", "NSLayoutConstraint:0x600001126530 V:|-(0)-[UIView:0x7fc6c4617020] (active, names: '|':UIView:0x7fc6c4616d10 )", "NSLayoutConstraint:0x6000011261c0 UIView:0x7fc6c4617020.bottom == UIView:0x7fc6c4616d10.bottom (active)", "NSLayoutConstraint:0x600001121360 'UIView-Encapsulated-Layout-Height' UIView:0x7fc6c4616d10.height == 50 (active)" ) Will attempt to recover by breaking constraint NSLayoutConstraint:0x6000011266c0 UIView:0x7fc6c4617020.height == 80 (active) Code to reproduce: import UIKit class ViewController: UIViewController { lazy var collectionView: UICollectionView = { let layout = createLayout() let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout) collectionView.translatesAutoresizingMaskIntoConstraints = false collectionView.dataSource = self collectionView.backgroundColor = .systemBackground collectionView.register(Cell.self, forCellWithReuseIdentifier: "cell") collectionView.register(HeaderView.self, forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: "header") return collectionView }() private func createLayout() - UICollectionViewCompositionalLayout { let sectionProvider = { (section: Int, layoutEnvironment: NSCollectionLayoutEnvironment) - NSCollectionLayoutSection? in return self.horizontalLayout(layoutEnvironment: layoutEnvironment) } let config = UICollectionViewCompositionalLayoutConfiguration() config.interSectionSpacing = 8 let layout = UICollectionViewCompositionalLayout(sectionProvider: sectionProvider, configuration: config) return layout } private func supplementaryHeader() - NSCollectionLayoutBoundarySupplementaryItem { let titleSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .estimated(50)) let titleSupplementary = NSCollectionLayoutBoundarySupplementaryItem( layoutSize: titleSize, elementKind: UICollectionView.elementKindSectionHeader, alignment: .top) return titleSupplementary } private func horizontalLayout(layoutEnvironment: NSCollectionLayoutEnvironment) - NSCollectionLayoutSection { let size = NSCollectionLayoutSize(widthDimension: .estimated(120), heightDimension: .estimated(50)) let item = NSCollectionLayoutItem(layoutSize: size) let group = NSCollectionLayoutGroup.horizontal(layoutSize: size, subitems: [item]) let section = NSCollectionLayoutSection(group: group) section.orthogonalScrollingBehavior = .continuous section.interGroupSpacing = 8 section.contentInsets = NSDirectionalEdgeInsets(top: 16, leading: 16, bottom: 16, trailing: 16) section.boundarySupplementaryItems = [supplementaryHeader()] return section } override func viewDidLoad() { super.viewDidLoad() view.addSubview(collectionView) NSLayoutConstraint.activate([ collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor), collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor), collectionView.topAnchor.constraint(equalTo: view.topAnchor), collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor) ]) } } // MARK: UICollectionViewDataSource extension ViewController: UICollectionViewDataSource { func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) - UICollectionViewCell { let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) return cell } func numberOfSections(in collectionView: UICollectionView) - Int { return 25 } func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) - Int { return 4 } func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) - UICollectionReusableView { switch kind { case UICollectionView.elementKindSectionHeader: let header: HeaderView = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, withReuseIdentifier: "header", for: indexPath) as! HeaderView return header default: fatalError() } } } class Cell: UICollectionViewCell { lazy var view: UIView = { let view = UIView() view.translatesAutoresizingMaskIntoConstraints = false view.backgroundColor = .systemRed return view }() override init(frame: CGRect) { super.init(frame: frame) configure() } required init?(coder: NSCoder) { fatalError("not implemented") } func configure() { contentView.addSubview(view) view.heightAnchor.constraint(equalToConstant: 80).isActive = true view.widthAnchor.constraint(equalToConstant: 100).isActive = true NSLayoutConstraint.activate([ view.leadingAnchor.constraint(equalTo: contentView.leadingAnchor), view.trailingAnchor.constraint(equalTo: contentView.trailingAnchor), view.topAnchor.constraint(equalTo: contentView.topAnchor), view.bottomAnchor.constraint(equalTo: contentView.bottomAnchor) ]) } } class HeaderView: UICollectionReusableView { lazy var view: UIView = { let view = UIView() view.translatesAutoresizingMaskIntoConstraints = false view.backgroundColor = .systemTeal return view }() override init(frame: CGRect) { super.init(frame: frame) configure() } required init?(coder: NSCoder) { fatalError("not implemented") } func configure() { addSubview(view) view.heightAnchor.constraint(equalToConstant: 60).isActive = true NSLayoutConstraint.activate([ view.leadingAnchor.constraint(equalTo: self.leadingAnchor), view.trailingAnchor.constraint(equalTo: self.trailingAnchor), view.topAnchor.constraint(equalTo: self.topAnchor), view.bottomAnchor.constraint(equalTo: self.bottomAnchor) ]) } }