Why is a RealityView inside an ImmersiveSpace bound by what seems to be a Window?

I have an app that launches into an immersive space with a mixed immersion style.

It appears like the Reality View has bounds that resemble a window. I would expect the bounds to not exist because it's an ImmersiveSpace.

Why do they exist? And how can I remove these?

This is the entire code:

@main
struct RealityKitDebugViewApp: App {
    var body: some Scene {
        ImmersiveSpace {
            ContentView()
        }
    }
}

struct ContentView: View {
    
    @State var logMessages = [String]()
    
    var body: some View {
        RealityView { content, attachements in
            let root = Entity()
            content.add(root)
            
            guard let debugView = attachements.entity(for: "entity_debug_view") else { return }
            debugView.position = [0, 0, 0]
            root.addChild(debugView)
        } update: { content, attachements in
        } attachments: {
            Color.blue
                .tag("entity_debug_view")
        }
        .onAppear(perform: {
            self.logMessages.append("Hello World")
        })
    }
}