UICollectionViewCompositionalLayout subclass that can stack a pinned collection-level header with pinned section-level headers.

StackedHeaderCompositionalLayout.swift

class StackedHeaderCompositionalLayout: UICollectionViewCompositionalLayout {
    let collectionHeaderKind: String
    let sectionHeaderKind: String

    init(
        section: NSCollectionLayoutSection,
        configuration: UICollectionViewCompositionalLayoutConfiguration,
        collectionHeaderKind: String,
        sectionHeaderKind: String
    ) {
        self.collectionHeaderKind = collectionHeaderKind
        self.sectionHeaderKind = sectionHeaderKind

        super.init(section: section, configuration: configuration)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let layoutAttributes = super.layoutAttributesForElements(in: rect)

        let collectionHeaderAttributes = layoutAttributes?.first(where: { $0.representedElementKind == collectionHeaderKind })
        collectionHeaderAttributes?.zIndex = 100

        let collectionHeaderFrame = collectionHeaderAttributes?.frame ?? .zero

        layoutAttributes?.forEach { attributes in
            guard attributes.representedElementKind == sectionHeaderKind else {
                return
            }

            attributes.zIndex = 50

            // The positition of the header to pin it to the top, while allowing it to scroll down.
            let pinnedTop = max(collectionHeaderFrame.maxY, attributes.frame.origin.y)

            let section = attributes.indexPath.section
            let nextSection = IndexPath(row: 0, section: section + 1)
            if let nextAttributes = layoutAttributesForSupplementaryView(ofKind: sectionHeaderKind, at: nextSection) {
                let nextHeaderTop = nextAttributes.frame.origin.y
                let offsetPinnedTop = min(pinnedTop, nextHeaderTop - attributes.frame.height)
                attributes.frame.origin.y = offsetPinnedTop
            } else {
                attributes.frame.origin.y = pinnedTop
            }
        }

        return layoutAttributes
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        true
    }
}