An example on how to use the new NavigationSplitView on iPad with global navigation - updated to work by following "Bringing robust navigation structure to your SwiftUI app"

NavigationSplitViewExample.swift

// 2024-11-06: Simple test based on "Bringing robust navigation structure to your SwiftUI app"
// https://developer.apple.com/documentation/swiftui/bringing_robust_navigation_structure_to_your_swiftui_app
// Xcode Version 16.1, iOS 17+

// Opens to a sheet first instead of NavigationSplitView

import SwiftUI

@main
struct TestSplitViewApp: App {
    private var navigationModel = NavigationModel(sidebarDestination: .subreddit(subreddit: .games))

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .environment(navigationModel)
    }
}


@Observable
final class NavigationModel {
    var sidebarDestination: Destination?
    var detailPath: [Destination]

    var detailNavigation: Destination?
    var columnVisibility: NavigationSplitViewVisibility

    var showSplitView: Bool = false
    var showLoginView: Bool = false

    init(sidebarDestination: Destination? = nil, detailPath: [Destination] = [], detailNavigation: Destination? = nil, columnVisibility: NavigationSplitViewVisibility = .doubleColumn) {
        self.sidebarDestination = sidebarDestination
        self.detailPath = detailPath
        self.detailNavigation = detailNavigation
        self.columnVisibility = columnVisibility
    }

    var selectedDetail: Destination? {
        get { detailPath.first }
        set { detailPath = [newValue].compactMap { $0 } }
    }
}

struct LoginView: View {
    @Environment(\.dismiss) private var dismiss
    @Environment(NavigationModel.self) var navigationModel

    var body: some View {
        VStack {
            Text("Login")
                .padding()
                .navigationBarTitle("Login")
            Button("Continue") {
                navigationModel.showSplitView = true
                dismiss()
            }
        }
    }
}

struct ContentView: View {
    @Environment(NavigationModel.self) var navigationModel

    var body: some View {
        @Bindable var navigationModel = navigationModel

        Group {
            if navigationModel.showSplitView {
                MainView()
            } else {
                EmptyView()
            }
            
        }
        .onAppear {
            navigationModel.showLoginView = true
        }
        .sheet(isPresented: $navigationModel.showLoginView) {
            LoginView()
        }


    }
}

struct MainView: View {
    @Environment(NavigationModel.self) private var navigationModel

    var body: some View {
        @Bindable var navigationModel = navigationModel
        NavigationSplitView(
            columnVisibility: $navigationModel.columnVisibility
        ) {
            SidebarView()
        } content: {
            Group {
                switch navigationModel.sidebarDestination {
                case .home(let destination):
                    HomeView(destination: destination)
                        .navigationTitle(navigationModel.sidebarDestination!.caseName.capitalized)
                case .subreddit(let subreddit):
                    SubredditView(subreddit: subreddit)
                case .user(let destination):
                    AccountView(destination: destination)
                        .navigationTitle(navigationModel.sidebarDestination!.caseName.capitalized)
                case .post(let post):
                    PostView(post: post)
                case .none:
                    EmptyView()
                }
            }


        } detail: {
            NavigationStack {
                Group {
                    if case .subreddit = navigationModel.sidebarDestination {
                        if let detailNavigation = navigationModel.selectedDetail {
                            if case .post(let post) = detailNavigation {
                                PostView(post: post)
                            }
                        } else {
                            Text("Please select a post")
                        }
                    } else {
                        EmptyView()
                    }
                }.navigationDestination(for: Destination.self) { destination in
                    switch destination {
                    case .user(let userDestination):
                        AccountView(destination: userDestination)
                    default:
                        Text("Not supported here")
                    }
                }
                .toolbar {
                    Button("Login") {
                        navigationModel.showSplitView = false
                        navigationModel.showLoginView = true
                    }
                }
            }
        }
    }
}

struct SidebarView: View {
    @Environment(NavigationModel.self) var navigationModel

    var body: some View {
        @Bindable var navigationModel = navigationModel
        List(selection: $navigationModel.sidebarDestination) {
            Section("Home") {
                ForEach(HomeDestination.allCases, id: \.self) { homeItem in
                    NavigationLink(value: Destination.home(home: homeItem)) {
                        Label(homeItem.rawValue.capitalized, systemImage: "globe")
                    }
                }
            }

            Section("Subreddit") {
                ForEach(SubredditDestination.allCases, id: \.self) { subreddit in
                    NavigationLink(value: Destination.subreddit(subreddit: subreddit)) {
                        Label(subreddit.rawValue.capitalized, systemImage: "globe")
                    }
                }
            }

            Section("Account") {
                ForEach(UserDestination.allCases, id: \.self) { userDestination in
                    NavigationLink(value: Destination.user(user: userDestination)) {
                        Label(userDestination.rawValue.capitalized, systemImage: "globe")
                    }
                }
            }
        }
        .navigationTitle("Categories")
    }
}

struct PostView: View {
    let post: Post

    var body: some View {
        VStack {
            Text(post.title)
                .font(.title)
            Text(post.preview)
            NavigationLink(value: Destination.user(user: .comments)) {
                Text("See some sub navigation")
            }
        }
    }
}

struct AccountView: View {
    let destination: UserDestination

    var body: some View {
        Text(destination.rawValue.capitalized)
    }
}

struct HomeView: View {
    let destination: HomeDestination

    var body: some View {
        Text(destination.rawValue.capitalized)
    }
}

enum HomeDestination: String, CaseIterable, Hashable {
    case hot, best, trending, new, top, rising
}

enum SubredditDestination: String, CaseIterable, Hashable {
    case news, diablo, pics, games, movies
}

enum UserDestination: String, CaseIterable, Hashable {
    case profile, inbox, posts, comments, saved
}

enum Destination: Hashable {
    case home(home: HomeDestination)
    case subreddit(subreddit: SubredditDestination)
    case user(user: UserDestination)
    case post(post: Post)

    var caseName: String {
        switch self {
        case .home:
            "Home"
        case .subreddit:
            "Subreddit"
        case .user:
            "Account"
        case .post:
            "Post"
        }
    }
}

struct Post: Identifiable, Hashable {
    let id = UUID()
    let title = "A post title"
    let preview = "Some wall of text to represent the preview of a post that nobody will read if the title is not a clickbait"
}

extension Post {
    static var posts: [Post] = [Post(), Post(), Post(), Post(), Post(), Post(), Post(), Post()]
}

struct SubredditView: View {
    let subreddit: SubredditDestination
    @Environment(NavigationModel.self) var navigationModel

    var body: some View {
        @Bindable var navigationModel = navigationModel
        List(Post.posts, selection: $navigationModel.selectedDetail) { post in
            NavigationLink(value: Destination.post(post: post)) {
                HStack {
                    VStack(alignment: .leading) {
                        Text(post.title)
                            .font(.title3)
                            .fontWeight(.semibold)
                        Text(post.preview)
                            .font(.callout)
                    }
                }
            }
        }.navigationTitle(subreddit.rawValue.capitalized)
    }
}

#Preview("LoginView") {
    @Previewable var navigationModel = NavigationModel(sidebarDestination: .subreddit(subreddit: .games))
    LoginView()
        .environment(navigationModel)
}

#Preview("SidebarView") {
    @Previewable var navigationModel = NavigationModel(sidebarDestination: .subreddit(subreddit: .games))
    SidebarView()
        .environment(navigationModel)
}

#Preview("PostView") {
    @Previewable var navigationModel = NavigationModel(sidebarDestination: .subreddit(subreddit: .games))
    PostView(post: .posts.first!)
        .environment(navigationModel)
}

Hi @SF-Simon , I've updated the sample using Observable and added a simple opening sheet instead of NavigationStack. Tested on iPad simulator and a real device. Please see whether it crashes.

BTW, I was happy to see Apple sample code has been recently updated; please check it out - it is a good one.
Bringing robust navigation structure to your SwiftUI app | Apple Developer Documentation

Thank you so much.