I would like to make something like the following work:

import SwiftUI

@main
struct TestWindowApp: App {
  init() {
    _showWindow = .constant(false)
  }

  var body: some Scene {

    MenuBarExtra {
      Button("Show window") { showWindow = true }
    } label: {
      Text(">_<")
    }

    if showWindow {
      Window("window", id: "main") {
        Text("Hello, SwiftUI!")
          .padding()
      }
    }
  }

  @Binding
  var showWindow: Bool
}

This doesn’t compile.

I tried using WindowGroup instead of Window which compiles if the condition is in the WindowGroup. I first found WindowGroup to work unreliably when MenuBarExtra is present, and when it works it shows an empty window when the showWindow condition is not satisfied:

WindowGroup {
  if showWindow {
    Text("Hello, SwiftUI!")
      .padding()
  }
}

I’m quite confident I could get the desired behavior with creating an NSWindow when showWindow is true, but I would prefer not mixing AppKit with SwiftUI.