An example of perfectly paging horizontal UICollectionViewController with overflowing cells. Works great with Storyboard — no need to set any specific attributes, just add this Class to the Controller and set your desired size for the cells like you would normally.

PagingCollectionViewController.swift

import UIKit

private let reuseIdentifier = "Cell"

class CollectionViewController: UICollectionViewController {

    /* Custom scrollView for paging */
    let pagingScrollView = UIScrollView()
    
    /* Return item size */
    var itemSize: CGSize {
        let layout = self.collectionViewLayout as! UICollectionViewFlowLayout
        return layout.itemSize
    }
    
    /* Return spacing: Interitem */
    var interItemSpacing: CGFloat {
        let layout = self.collectionViewLayout as! UICollectionViewFlowLayout
        return layout.minimumInteritemSpacing
    }
    
    /* Get section inset */
    var sectionInset: UIEdgeInsets {
        let layout = self.collectionViewLayout as! UICollectionViewFlowLayout
        return layout.sectionInset
    }
    
    override func scrollViewDidScroll(_ scrollView: UIScrollView) {
        
        // Override native UICollectionView scrollView events
        if scrollView == self.pagingScrollView {
            var contentOffset = scrollView.contentOffset
            
            // Include offset from left
            if let contentOffsetX = self.collectionView?.contentInset.left {
                contentOffset.x = contentOffset.x - contentOffsetX
                self.collectionView?.contentOffset = contentOffset
            }
        }
    }
    
    // MARK: - Initialize scrollView with proper size and frame
    func initScrollView() {
        
        // Get UICollectionView frame
        if let collectionViewFrame = self.collectionView?.frame {
            
            // Set proper frame
            self.pagingScrollView.frame = collectionViewFrame
            self.pagingScrollView.isHidden = true
            
            // Enable paging
            self.pagingScrollView.isPagingEnabled = true

            // Set delegate
            self.pagingScrollView.delegate = self
            
            // Set bounds for scrollView
            self.pagingScrollView.bounds = CGRect(x: 0, y: 0, width: self.itemSize.width + self.interItemSpacing, height: collectionViewFrame.size.height)
            
            // Number of items in UICollectionView
            if let numberOfItemsInCollectionView = self.collectionView?.numberOfItems(inSection: 0) {
                
                // Calculcate full width (with spacing) for contentSize
                let collectionViewWidth = CGFloat(numberOfItemsInCollectionView) * (self.itemSize.width + self.interItemSpacing)
                
                // Set contentSize
                self.pagingScrollView.contentSize = CGSize(width: collectionViewWidth, height: view.frame.size.height)
            }
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Initialize Paging scrollView
        self.initScrollView()
        
        // Add custom paging scrollView
        self.view.addSubview(self.pagingScrollView)
        
        // Disable standard gesture recognizer for UICollectionView scrollView and add custom
        self.collectionView?.addGestureRecognizer(self.pagingScrollView.panGestureRecognizer)
        self.collectionView?.panGestureRecognizer.isEnabled = false
    }
    
    
    // MARK: UICollectionViewDataSource
    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
        
        return cell
    }
}

Excellent work, thanks!

just add this Class to the Controller and set your desired size for the cells like you would normally.
how?