UserSelection.swift

// In UserProfileViewController (the master) we need:

// 1
protocol UserProfileFeedSelectionDelegate: AnyObject {
    func postSelectedAt(_ indexPath: IndexPath)
}
// 2
weak var delegate: UserProfileFeedSelectionDelegate?
// 3
lazy private var contentDetailViewController: ContentDetailViewcontroller = {
  let detailViewController = ContentDetailViewcontroller.instantiate(from: "Main")
  delegate = detailViewController
  return detailViewController
}()

extension UserProfileViewController: GridCollectionViewDelegate {
    
    func cellDidSelect(_ indexPath: IndexPath) {
        // 4
        guard let secondaryContentNavigationController = splitViewController?.secondaryViewController as? NavigationController, 
        let secondaryContentViewController = secondaryContentNavigationController.topViewController as? ContentDetailViewcontroller 
        else {
             // 5
            let detailNavigationController = NavigationController(rootViewController: contentDetailViewController)
            splitViewController?.showDetailInNavigationControllerIfNeeded(detailNavigationController, sender: self)
            delegate?.postSelectedAt(indexPath)
            return
        }
        // 6
        splitViewController?.showDetailInNavigationControllerIfNeeded(secondaryContentViewController, sender: self)
        delegate?.postSelectedAt(indexPath)
    }
}

// In ContentDetailViewcontroller (the detail) we need:

extension ContentDetailViewcontroller: UserProfileFeedSelectionDelegate {
    // 7
    func postSelectedAt(_ indexPath: IndexPath) {
        verticalFeedTableView?.scrollTo(indexPath, animated: true)
    }
}