Selecting WindowGroup to open at app startup.

Do you need two WindowGroups? Can you put the if statement inside one WG, and apply it to the Views instead?

@AppStorage("showFirstView") private var showFirstView = true
var body: some Scene {
  WindowGroup(id: "wg") {
    if showFirstView {
      ViewOne()
    } else {
      ViewTwo()
    }
  }
}

I have no idea if this will work.

The problem with putting the if inside one window group is that I want both windows to behave differently. For example, they have different size and position, one shows the title bar and the other doesn't. Also, the content of the windows is completely different and has different size. if it's a single window it doesn't resize when its view changes.

Try this:

WindowGroup(id: showFirstView ? "wg1" : "wg2") {
  if(showFirstView) {
    ViewOne()
  } else {
    ViewTwo()
  }
}

Hang on a second... Don't you define a single View inside a WindowGroup? From this page:

// Here's where you define two different WindowGroups
WindowGroup("Message", id: "message", for: UUID.self) { $uuid in
    MessageDetail(uuid: uuid)
}

WindowGroup("Account", id: "account-info", for: UUID.self) { $uuid in
    AccountDetail(uuid: uuid)
}


// Then, use both the identifier and a value to open the window
struct ActionButtons: View {
    var messageID: UUID
    var accountID: UUID

    @Environment(\.openWindow) private var openWindow

    var body: some View {
        HStack {
            Button("Open message") {
                openWindow(id: "message", value: messageID)
            }
            Button("Edit account information") {
                openWindow(id: "account-info", value: accountID)
            }
        }
    }
}

So, you define your different window types, and then open them as required, so your's might be:

var body: some Scene {
  WindowGroup(id: "wg1") {
      ViewOne()
  }
  WindowGroup(id: "wg2") {
      ViewTwo()
  }
}

Then you open a window at startup with:

openWindow(id: showFirstView ? "wg1" : "wg2")