PagingCollectionViewRotation.swift

// This incantation works well for a fullscreen, paging collection view.
// Non-fullscreen works too, with a little finesse.
extension MyCollectionViewController {

    private func updateCollectionViewLayout(forBoundingSize size: CGSize) { ... }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
    
        updateCollectionViewLayout(forBoundingSize: self.view.bounds.size)
    }
    
    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)

        updateCollectionViewLayout(forBoundingSize: size)

        if view.window == nil {
            view.frame.size = size
            view.layoutIfNeeded()
        } else {
            let indexPath = self.collectionView?.indexPathsForVisibleItems.last
            coordinator.animate(alongsideTransition: { ctx in
                self.collectionView?.layoutIfNeeded()
            }, completion: { _ in
                guard let indexPath = indexPath else { return }
                self.collectionView?.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: false)
            })
        }
        
    }
 
    // MARK: UICollectionViewDelegate
    override func collectionView(_ collectionView: UICollectionView, targetContentOffsetForProposedContentOffset proposedContentOffset: CGPoint) -> CGPoint {
        guard  let indexPath = collectionView.indexPathsForVisibleItems.last, let layoutAttributes = collectionView.collectionViewLayout.layoutAttributesForItem(at: indexPath) else {
            return proposedContentOffset
        }
        
        return CGPoint(x: layoutAttributes.center.x - (layoutAttributes.size.width / 2.0) - (flowLayout.minimumLineSpacing / 2.0), y: 0)
    }

}