SwiftUI identity problem

navigationProblem.swift

import SwiftUI

struct CoverWithBoolView: View {
  @State var presentCover: Bool = false
  
  var body: some View {
    GeometryReader { proxy in
      ZStack {
        if proxy.size.height > proxy.size.width {
          Color.gray
          PortraitView()
        } else {
          Color.pink
          LandscapeView()
        }
      }
    }
  }
}

struct PortraitView: View {
  var body: some View {
    HStack {
      SelectionView(landscape: false)
      DetailsView(landscape: false)
    }
  }
}

struct LandscapeView: View {
  var body: some View {
    HStack {
      SelectionView(landscape: true)
      DetailsView(landscape: true)
    }
  }
}

struct DetailsView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      DetailInfoView()
    } else {
      VStack {
        DetailInfoView()
        UserView(landscape: landscape)
      }
    }
  }
}

struct SelectionView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      VStack {
        UserView(landscape: landscape)
        CategoriesView()
      }
    } else {
      CategoriesView()
    }
  }
}

struct CategoriesView: View {
  let listOfCategories: [String] = ["A", "list", "of", "categories"]

  var body: some View {
    List(listOfCategories, id:\.self) { item in
      Text(item)
    }
  }
}

struct DetailInfoView: View {
  let listOfInfo: [String] = ["Details", "about", "the", "selected", "info", "category"]

  var body: some View {
    List(listOfInfo, id:\.self) { item in
      Text(item)
    }
  }
}

struct UserView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      HStack {
        AvatarImageView()
        ActionsView()
      }
    } else {
      VStack {
        AvatarImageView()
        ActionsView()
      }
    }
  }
}

struct ActionsView: View {
  var body: some View {
    Text("View with actions for users")
  }
}

struct AvatarImageView: View {
  @State var presentCover: Bool = false

  var body: some View {
    Button("Show detail") {
      presentCover = true
    }
    .fullScreenCover(isPresented: $presentCover, content: {
      Text("Detail")
    })
  }
}

#Preview {
  CoverWithBoolView()
}

In your code above you explicitly remove the fullScreenCover view from the view hierarchy. If you do the same in UIKit, it also disappears.

This sounds like you are not using coordinators in SwiftUI, why not? If you can present an actual issue, I might be able to help you with that. Sounds a little like the state design of the "real code" is messed up, not SwiftUI. If you can show more info, I might be able to present a way to do this properly w/ SwiftUI. (but generally you can apply the same basic structure you'd use in UIKit, including coordinators if you want)

So we already learned that we cannot do the sheet in a subview that gets dropped since the sheet would get removed when the subview is (which is a feature, not a bug). The sheet has to be placed at the level at which we want to preserve it, using this we control when a sheet is auto-dismissed (i.e. to which view it is related to visually).
In the example we just put it at the top, but in other scenarios it might be within a navigation view content view, for example. So it gets dismissed when one navigates away from a page.

There are many different ways to approach this, but this is roughly what I would do:

import SwiftUI

@Observable @MainActor
final class Presenter {
  
  var presentCover = false
  
  func showCover() { presentCover = true }
}

struct ContentView: View {
  
  @State var presenter = Presenter()
  
  var body: some View {
    GeometryReader { proxy in
      ZStack {
        if proxy.size.height > proxy.size.width {
          Color.gray
          PortraitView()
        }
        else {
          Color.pink
          LandscapeView()
        }
      }
      .environment(presenter)
      .fullScreenCover(isPresented: $presenter.presentCover) {
        Text("Detail")
      }
    }
  }
}

struct PortraitView: View {
  var body: some View {
    HStack {
      SelectionView(landscape: false)
      DetailsView(landscape: false)
    }
  }
}

struct LandscapeView: View {
  var body: some View {
    HStack {
      SelectionView(landscape: true)
      DetailsView(landscape: true)
    }
  }
}

struct DetailsView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      DetailInfoView()
    } else {
      VStack {
        DetailInfoView()
        UserView(landscape: landscape)
      }
    }
  }
}

struct SelectionView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      VStack {
        UserView(landscape: landscape)
        CategoriesView()
      }
    } else {
      CategoriesView()
    }
  }
}

struct CategoriesView: View {
  let listOfCategories = ["A", "list", "of", "categories"]

  var body: some View {
    List(listOfCategories, id:\.self) { item in
      Text(item)
    }
  }
}

struct DetailInfoView: View {
  let listOfInfo = ["Details", "about", "the", "selected", "info", "category"]

  var body: some View {
    List(listOfInfo, id:\.self) { item in
      Text(item)
    }
  }
}

struct UserView: View {
  var landscape: Bool

  var body: some View {
    if landscape {
      HStack {
        AvatarImageView()
        ActionsView()
      }
    }
    else {
      VStack {
        AvatarImageView()
        ActionsView()
      }
    }
  }
}

struct ActionsView: View {
  var body: some View {
    Text("View with actions for users")
  }
}

struct AvatarImageView: View {
  @Environment(Presenter.self) var presenter

  var body: some View {
    Button("Show detail", action: presenter.showCover)
  }
}

#Preview {
  ContentView()
}

A few more best practices: Regardless of the particular issue, it is often a good idea not to change identity, if possible. E.g. the switching between HStack and VStack can be done using this in an identity preserving ways:

struct UserView: View {
  var landscape: Bool

  var body: some View {
    (landscape ? AnyLayout(HStackLayout()) : AnyLayout(VStackLayout())) {
      AvatarImageView()
      ActionsView()
    }
} 

Also instead of such:

  var body: some View {
    if landscape {
      VStack {
        UserView(landscape: landscape)
        CategoriesView()
      }
    } else {
      CategoriesView()
    }
  }

do this:

  var body: some View {
      VStack {
        if landscape { UserView(landscape: true) }
        CategoriesView()
      }
  }

Here is another simple version where the overlay content is provided by the calling leaf view (could also be easily adjusted to move the detail content selection to the presenter, same idea):

import SwiftUI

extension Optional {
  
  var isSet: Bool {
    set { if !newValue { self = .none } }
    get {
      switch self {
        case .some(_): true
        case .none: false
      }
    }
  }
}

@Observable @MainActor
final class Presenter {
  
  var presentedView : AnyView?
  
  func showCover<V: View>(@ViewBuilder content: () -> V) {
    presentedView = AnyView(content())
  }
}

struct ContentView: View {
  
  @State var presenter = Presenter()
  
  var body: some View {
    GeometryReader { proxy in
      ZStack {
        let landscape = proxy.size.height <= proxy.size.width
        landscape ? Color.gray : Color.pink
        SwitchingView(landscape: landscape)
      }
      .environment(presenter)
      .fullScreenCover(isPresented: $presenter.presentedView.isSet) {
        presenter.presentedView
      }
    }
  }
}

struct SwitchingView: View {
  let landscape : Bool
  var body: some View {
    HStack {
      SelectionView(landscape: landscape)
      DetailsView(landscape: landscape)
    }
  }
}

struct DetailsView: View {
  var landscape: Bool

  var body: some View {
    VStack {
      DetailInfoView()
      if !landscape { UserView(landscape: landscape) }
    }
  }
}

struct SelectionView: View {
  var landscape: Bool

  var body: some View {
    VStack {
      if landscape { UserView(landscape: landscape) }
      CategoriesView()
    }
  }
}

struct CategoriesView: View {
  let listOfCategories = ["A", "list", "of", "categories"]

  var body: some View {
    List(listOfCategories, id:\.self) { item in
      Text(item)
    }
  }
}

struct DetailInfoView: View {
  let listOfInfo = ["Details", "about", "the", "selected", "info", "category"]

  var body: some View {
    List(listOfInfo, id:\.self) { item in
      Text(item)
    }
  }
}

struct UserView: View {
  var landscape: Bool

  var body: some View {
    (landscape ? AnyLayout(HStackLayout()) : AnyLayout(VStackLayout())) {
      AvatarImageView()
      ActionsView()
    }
  }
}

struct ActionsView: View {
  var body: some View {
    Text("View with actions for users")
  }
}

struct AvatarImageView: View {
  @Environment(Presenter.self) var presenter

  var body: some View {
    Button("Show detail") {
      presenter.showCover {
        Text("Detail Provided View")
      }
    }
  }
}

#Preview {
  ContentView()
}