Stop child NSWindow disappearing when moved to another screen

Question

On macOS Monetrey, when I move a child NSWindow to another screen (by manually dragging it), it disappears. Minimal repro using SwiftUI:

import SwiftUI

struct ContentView: View {
    var body: some View {
        Button("Hello World", action:  openChild)
            .padding(50)
    }
    
    func openChild()  {
        let win = NSWindow();
        let parent = NSApp.mainWindow
        parent?.addChildWindow(win, ordered: NSWindow.OrderingMode.above)
        win.makeKeyAndOrderFront(self)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

(I know this may not be the "correct" SwiftUI way of doing things, it's just to demonstrate the problem in as little code as possible - this happens in low level Obj-C code in reality)

The interesting thing is if you:

• Show the child window
• Move it to another screen (it disappears)
• Select "Window -> Move to [Name of other screen]"

The child window reappears and can now be moved freely between screens.

From user reports I understand that this behavior doesn't occur on Big Sur.

I know there are applications that show child windows and allow them to be moved between screens, what is the correct incantation to allow this?