Remove 1px line at top of UISearchController in large titles UINavigationBar

I'm transitioning from a view with a large style UINavigationItem into a view that has a large style UINavigationItem and a UISearchController. I've customized the background colour of the UINavigationBar.

For some reason, there's a 1px line or space between the UINavigationBar and the UISearchController. I can tell it's from the NavigationBar, because if I scroll so that the search bar sticks to the top, the line disappears. I'm just not sure what's generating this line. I'm not using shadows, or anything fancy, other than setting the barTintColor and the tintColor. If I don't use large style titles, I see the line, but only when transitioning between views. It's like when transitioning the UISearchController just doesn't stick right to the UINavigationBar.

Any help in figuring out where this line is coming from is appreciated.

Update: On some further investigation, it seems this only happens when navigationItem.hidesSearchBarWhenScrolling is set to false. If it initially is hidden, then the animation from the previous view is smooth, and showing the bar is also smooth.

Testing the navigation item and search bar setup you've described, I could only reproduce the 1px/point line during the transition animations.

Workaround: This line is actually a background layout view (forLastBaselineLayout) used by UINavigationBar ending up visible due to the buggy framing of UISearchBar. Set its background to match your navigation bar color to hide it in hind site

// MARK: UINavigationControllerDelegate

public func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {
    viewController.navigationController?.navigationBar
        .forLastBaselineLayout.backgroundColor = .red // TODO:
}

Make sure to set the object implementing UINavigationControllerDelegate is set as navigationController.delegate to receive the above delegate call.

In addition (not part of the question): UISearchBar seems to behave strangely when transitioning (push and pop) between controllers especially when one has/show a searchBar but not the other. Workaround, I found it visually more pleasing to temporarily hide the search bar from presenting controllers. Here is the code I use:

public func navigationController(_ navigationController: UINavigationController, willShow viewController: UIViewController, animated: Bool) {
    if let sender = navigationStack.popLast() {
        prepareSearchBarForTransition(from: sender)
    }
    navigationStack = navigationController.children
}

/// Set only from `navigationController: willShow`
private var navigationStack: [UIViewController] = []

func prepareSearchBarForTransition(from sender: UIViewController) {
    if #available(iOS 11.0, *) {
        let searchController = sender.navigationItem.searchController
        sender.navigationItem.searchController = nil
        weak var weakSender = sender
        let navigationTransitionDuration: TimeInterval = 0.33
        DispatchQueue.main.asyncAfter(deadline: .now() + navigationTransitionDuration) {
            /// Reset the search bar.
            weakSender?.navigationItem.searchController = searchController
        }
    } else {
        // TODO: Check if the above workaround is neccessary for < iOS 11
    }
}