In an iOS app, I wanted to place the content view under the navigation bar like in ZStack, so that the content view does not shift when the navigation bar hides / shows.

XCode: 15.3

iOS: 15 or 16

Currently I have this code:

import SwiftUI

@main
struct ViewDismissApp: App {
    var body: some Scene {
        WindowGroup {
            MyTabView()
        }
    }
}

struct MyTabView: View {
    var body: some View {
        TabView {
            NavigationView {
                RandomView(title: "tab 1")
            }
            .tabItem {
                Text("Tab 1")
            }

            NavigationView {
                RandomView(title: "tab 2")
            }
            .tabItem {
                Text("Tab 2")
            }
        }
    }
}

struct RandomView: View {
    let title: String
    
    var body: some View {
        NavigationLink {
            ChildView()
        } label: {
            Text(title)
        }
    }
}

struct ChildView: View {
    @State private var showNavigationbar = true
    @State private var hideTabbar = true

    var body: some View {
        mainBody()
            .frame(maxWidth: .infinity, maxHeight: .infinity)
            .background{Color.green}
            .toolbar(hideTabbar ? .hidden : .visible, for: .tabBar)
            .onAppear {
                hideTabbar = true
            }
            .onDisappear {
                hideTabbar = false
            }
    }
    
    private func mainBody() -> some View {
        VStack {
            Text("Tap here to hide NavigationBar")
        }
        .onTapGesture(perform: {
            showNavigationbar.toggle()
        })
        .navigationBarHidden(!showNavigationbar)
        .navigationBarTitleDisplayMode(.inline)
        .background{Color.yellow}
    }
}

The ChildView would visually move up and own when the navigation bar hides / shows. Like in this screenshot:

When the user tapped the text, the navigation bar hides and the content view extends upwards and the text moves up as well:

How can I make the content view (in green) extends beneath the navigation bar in the first place (i.e. always) ?