カスタムNavigationViewStyle例

import SwiftUI

public struct AllVisibleNavigationViewStyle: NavigationViewStyle {
    struct ControllerModifier: ViewModifier {
        struct ControllerView: UIViewControllerRepresentable {
            class ViewController: UIViewController {
                var perform: (UIViewController) -> Void = { _ in }
                override func viewDidAppear(_ animated: Bool) {
                    super.viewDidAppear(animated)
                    perform(self)
                }
            }
            var perform: (UIViewController) -> Void
            func makeUIViewController(context: Self.Context) -> UIViewController {
                let vc = ViewController()
                vc.perform = perform
                return vc
            }
            func updateUIViewController(_ uiViewController: UIViewController, context: Self.Context) {}
        }
        var perform: (UIViewController) -> Void
        func body(content: Content) -> some View {
            content.overlay (
                ControllerView(perform: perform).frame(width: 0, height: 0)
            )
        }
    }
    public func _body(configuration: _NavigationViewStyleConfiguration) -> some View {
        NavigationView { configuration.content }
            .modifier(ControllerModifier(perform: { vc in
                guard let svc = vc.parent?.children.first as? UISplitViewController else { return }
                svc.preferredDisplayMode = .allVisible
            }))
    }
    public init(){}
}

struct ContentView: View {
    struct Greet: View {
        var text: String
        var body: some View {
            Text(text).navigationBarTitle("Secondary")
        }
    }
    var body: some View {
        NavigationView {
            List(0..<100) { i in
                NavigationLink(destination: Greet(text: "hello \(i)")) {
                    Text("greeting \(i)")
                }
            }.navigationBarTitle("Primary")
        }.navigationViewStyle(AllVisibleNavigationViewStyle())
    }
}

import PlaygroundSupport

PlaygroundPage.current.wantsFullScreenLiveView = false
PlaygroundPage.current.setLiveView(ContentView())