[Swift] CollectionViewの最後のセルがレイアウトバグって苦労した

import UIKit
class SomeViewController: UIViewController {
    @IBOutlet private weak var collectionView: UICollectionView!
    /// collectionViewCellのPadding
    private let sectionInsets = UIEdgeInsets(top: 10.0, left: 7.0, bottom: 10.0, right: 7.0)
    // 1行あたりのアイテム数
    private let itemsPerRow: CGFloat = 3

    override func viewDidLoad() {
        super.viewDidLoad()
        setupCollectionView()
    }

    /// collectionViewの設定
    private func setupCollectionView() {
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.register(UINib(nibName: "SomeCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "SomeCollectionViewCell")
        collectionView.register(UINib(nibName: NSStringFromClass(UICollectionViewCell.self), bundle: nil), forCellWithReuseIdentifier: NSStringFromClass(UICollectionViewCell.self))
    }
}

extension SomeViewController: UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "SomeCollectionViewCell", for: indexPath)
        cell.setup(hoge: fuga)
        return cell
    }

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

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let paddingSpace = sectionInsets.left * (itemsPerRow + 1)
        let availableWidth = view.frame.width - paddingSpace
        let widthPerItem = availableWidth / itemsPerRow
        return CGSize(width: widthPerItem, height: widthPerItem * 1.4)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return sectionInsets
    }

    // セルの行間の設定
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return sectionInsets.left
    }
}