How to get collapsable sections as shown in 'Stacks, Grids, and Outlines in SwiftUI'?

I tried to replicate the code as shown in Stacks, Grids, and Outlines in SwiftUI (12:35).

They make it feel like it's a very easy thing to do. Just insert a Section and as you could see in the video it will immediately become a collapsable view. Add a header to the section and it will get the bolder text style. I wasn't able to replicate this.

xCode Version 12.0 beta (12A6159)

Below you can find the code I used.

The expected result would be nicely styled, collapsable section headers.

The actual result is a non-collapsable section with a weird underline under the name and no arrow.

Code Block struct ContentView: View {
  let model: Model = Model()
   
  @State private var selection: Set<String> = []
  var body: some View {
    NavigationView {
      List(selection: $selection) {
        ForEach(model.canvases) { canvas in
          Section(header: Text(canvas.name)) {
            OutlineGroup(canvas.graphics, children: \.children) { graphic in
              Label(graphic.name, systemImage: graphic.icon)
            }
          }
        }
      }
      .listStyle(SidebarListStyle())
      .navigationTitle("Graphics")
    }
  }
}
struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
  }
}

It uses this simple model to hold the canvases and graphics:

Code Block 
struct Model: Identifiable {
  let id: String = UUID().uuidString
  let canvases: [Canvas] = [
    Canvas(name: "Main", graphics: [
      Graphic(name: "Rectangle", icon: "rectangle.fill"),
      Graphic(name: "Oval", icon: "circle.fill"),
      Graphic(name: "Group", icon: "rectangle.3.offgrid", children: [
        Graphic(name: "Rectangle", icon: "rectangle.fill"),
        Graphic(name: "Rectangle", icon: "rectangle.fill"),
        Graphic(name: "Rectangle", icon: "rectangle.fill")
      ]),
      Graphic(name: "Group", icon: "rectangle.3.offgrid", children: [
        Graphic(name: "Rectangle", icon: "rectangle.fill"),
        Graphic(name: "Rectangle", icon: "rectangle.fill"),
        Graphic(name: "Rectangle", icon: "rectangle.fill")
      ])
    ]),
    Canvas(name: "Highlights", graphics: [
      Graphic(name: "Triangle", icon: "triangle.fill"),
      Graphic(name: "Triangle", icon: "triangle.fill")
    ]),
    Canvas(name: "Version 2", graphics: [
      Graphic(name: "Rectangle", icon: "triangle.fill"),
      Graphic(name: "Oval", icon: "circle.fill"),
      Graphic(name: "Triangle", icon: "triangle.fill")
    ]),
  ]
}
struct Canvas: Identifiable {
  var id: String = UUID().uuidString
  var name: String
  let graphics: [Graphic]
}
struct Graphic: Identifiable {
  var id: String = UUID().uuidString
  var name: String
  var icon: String
  var children: [Graphic]?
}