title: SwiftUI list line separatorStyle not work correctly with navigation
body: <p>I have try to hide and show separator line on 2 views
  the first view I implement as below</p>
  
  <pre><code>struct FirstView: View {
     var body: some View {
        NavigationView{
           VStack{
              List{
                 Text("test1")
                 Text("test2")
              }
              .onAppear {UITableView.appearance().separatorStyle = .none}
              .onDisappear { UITableView.appearance().separatorStyle = .singleLine }
              NavigationLink(destination: NextView()) {
                  Text("next view")
              }
           }
        }.navigationViewStyle(StackNavigationViewStyle())
     }
  }
  </code></pre>
  
  <p>This will display none line on FirstView. 
  when navigate to NextView which also have list the separate line is not show up.
  After that navigate back to FirstView the separate line show up.</p>
  
  <pre><code>struct NextView: View {
     var body: some View {
        List{
            Text("test1")
            Text("test2")
        } 
     }
  }
  </code></pre>
  
  <p>I am not sure is this bug or I coded wrong on somewhere.</p>