Disclosure Group In a List

I'm converting a file viewer app to use SwiftUI. It uses an NSOutlineView to show the objects in the file and their component parts. DisclosureGroup seems like the thing to use. I have a List of DisclosureGroups, one for each of the top-level objects. The disclosure shows the component parts.

This does not behave correctly. I've attached a little sample program that shows what I'm trying to do.

I have built and run this little app on both macOS and iOS. It behaves differently and incorrectly on both platforms.

I tried making the top-level view a VStack; that behaves better, but still doesn't do what I expect.