An demo of preferredCompactColumn

Day16.swift

import SwiftUI

struct Day16: View {
    @State private var sidebarSelection: Int?
    @State private var path = NavigationPath()
    
    /// Acts as a floor on what column is on top when collapsed
    @State private var topColumn: NavigationSplitViewColumn = .content
    @State private var vis: NavigationSplitViewVisibility = .all
    var body: some View {
        NavigationSplitView(
            columnVisibility: $vis, preferredCompactColumn: $topColumn
        ) {
            List(selection: $sidebarSelection) {
                NavigationLink("Push 30", value: 30)
            }
        } content: {
            if let sidebarSelection {
                NavigationLink("Push View-Destination") {
                    NavigationLink("Push 'Hello!'", value: "Hello!")
                        .navigationDestination(for: String.self) {
                            DismissibleView(
                                text: "Pushed \($0)", color: .orange)
                        }
                }
            } else {
                Text("Make a selection")
            }
        } detail: {
            NavigationStack(path: $path) {
                Text("Root")
            }
        }
        .overlay(alignment: .bottomLeading) {
            Text("Preferred Top: \(topColumn.description)")
                .font(.headline)
        }
        
    }
}

private struct DismissibleView: View {
    var text: String
    var color: Color
    @Environment(\.dismiss) var dismiss
    
    var body: some View {
        ZStack {
            color
            VStack {
                Text(text)
                Button("Dismiss") { dismiss() }
            }
        }
    }
}

private extension NavigationSplitViewColumn {
    var description: String {
        switch self {
        case .content: "content"
        case .detail: "detail"
        case .sidebar: "sidebar"
        default: "unknown"
        }
    }
}