macOS Catalyst `requestSceneSessionActivation` requires multiwindow adoption

Question

When trying to open a new window in a macOS Catalyst application, I'm using this enum to describe an instance of NSUserActivity:

enum Activity: String {
  case list
  case settings

  var type: String {
    "com.example.app.\(rawValue)"
  }

  var userActivity: NSUserActivity {
    NSUserActivity(activityType: type)
  }
}

Then to open a new window, I use this code in a button tap handler, as described in this raywenderlich.com tutorial:

UIApplication
  .shared
  .requestSceneSessionActivation(
    nil,
    userActivity: Activity.settings.userActivity,
    options: nil,
    errorHandler: nil
  )

However, this doesn't work and I always get this error message in the debugger console:

[Scene] Calling -[UIApplication requestSceneSessionActivation:] requires multiwindow adoption.

What does this error message mean and what are possibly missing steps here to make support for multiple windows work in a Catalyst app?

Answer 1

What actually worked is setting UIApplicationSupportsMultipleScenes key within UIApplicationSceneManifest dictionary to YES in your Info.plist:

    <key>UIApplicationSceneManifest</key>
    <dict>
        <key>UIApplicationSupportsMultipleScenes</key>
        <true/>
    </dict>

Or if you prefer Xcode GUI, check out this screenshot:

Unfortunately, this setting doesn't seem to be documented in the raywenderlich.com tutorial or in any of Apple's guides on Catalyst. It only seems to be documented on this Developer reference page.