Section Does Not Seem To Expand/Collapse

This is obviously a user error, but I've been working on different possibilities for a couple of days and nothing works. The problem is my Section in the following code doesn't expand or collapse when I click on the chevron:

`class AstroCat { var title: String var contents: [ String ] var isExpanded: Bool

init(title: String, contents: [String], isExpanded: Bool) {
    self.title = title
    self.contents = contents
    self.isExpanded = isExpanded
}

}

struct TestView: View {

@Binding var isShowingTargetSelection: Bool
@State var catalog: AstroCat
@State private var expanded = false

var body: some View {
    NavigationStack {
        List {
            Section(catalog.title, isExpanded: $catalog.isExpanded) {
                ForEach(catalog.contents, id: \.self) { object in
                    Text(object)
                }
            }
        }
        .navigationTitle("Target")
        .listStyle(.sidebar)
    }
}

}

#Preview {

struct TestPreviewContainer : View {
    @State private var value = false
    @State var catalog = AstroCat(title: "Solar System", contents: ["Sun", "Mercury", "Venus", "Earth"], isExpanded: true)

    var body: some View {
       TestView(isShowingTargetSelection: $value, catalog: catalog)
    }
}

return TestPreviewContainer()

}`

If I change the "isExpanded: $catalog.isExpanded" to just use the local variable "expanded", then it works, so I think I have the basic SwiftUI pieces correct. But using a boolean inside of the class doesn't seem to work (the section just remains expanded or collapsed based on the initial value of the class variable).

Any hints? Am I not specifying the binding correctly? (I've tried a bunch of alternatives)

Thanks for the help, Robert