An example on how to use the new NavigationSplitView on iPad with global navigation

NavigationSplitViewExample.swift

import SwiftUI

enum HomeDestination: String, CaseIterable, Hashable {
  case hot, best, trending, new, top, rising
}
enum SubredditDestination: String, CaseIterable, Hashable {
  case news, diablo, pics, wtf, 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)
}

struct MainView: View {
  @State private var sidebarDestination: Destination = .subreddit(subreddit: .games)
  @State private var detailNavigation: Destination?
  
  var body: some View {
    NavigationSplitView {
      SidebarView(destination: $sidebarDestination)
    } content: {
      switch sidebarDestination {
      case .home(let destination):
        HomeView(destination: destination)
      case .subreddit(let subreddit):
        SubredditView(subreddit: subreddit, destination: $detailNavigation)
      case .user(let destination):
        AccountView(destination: destination)
      case .post(let post):
        PostView(post: post)
      }
    } detail: {
      NavigationStack {
        Group {
          if let detailNavigation {
            switch detailNavigation {
            case .post(let post):
              PostView(post: post)
              
            default:
              Text("Please select a post")
            }
          } else {
            Text("Please select a post")
          }
        }.navigationDestination(for: Destination.self) { destination in
          switch destination {
          case .user(let userDestination):
            AccountView(destination: userDestination)
          default:
            Text("Not supported here")
          }
        }
      }
    }

  }
}

struct SidebarView: View {
  @Binding var destination: Destination
  
  var body: some View {
    List(selection: $destination) {
      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")
          }
        }
      }
    }
  }
}


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"
}

struct SubredditView: View {
  let subreddit: SubredditDestination
  @Binding var destination: Destination?
  @State private var posts: [Post] = [Post(), Post(), Post(), Post(), Post(), Post(), Post(), Post()]
  
  var body: some View {
    List(posts, selection: $destination) { 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)
  }
}

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)
  }
}

struct MainView_Previews: PreviewProvider {
  static var previews: some View {
    MainView()
  }
}

Thanks for posting.
Using @State for selection causes this behaviour in .compact class as sidebar and content are recreated.
So, persisting the selections (by following Bringing robust navigation structure to your SwiftUI app) seems to resolve the issue.

Also, iOS beta 4 resolved the following issue which required a workaround for conditional views:

Fixed: Conditional views in columns of NavigationSplitView fail to update on some state changes. (91311311)

Updated code:
https://gist.github.com/nkalvi/4cdc746ab92b5da664621d61ac7690e9