There's an easily reproducible SwiftUI bug on macOS where an app's UI state no longer updates/re-renders for "Designed for iPad" apps (i.e. ProcessInfo.processInfo.isiOSAppOnMac == true). The bug occurs in Xcode and also if the app is running independent of Xcode.

The bug occurs when:

1. the user Hides the app (i.e. it goes into the background)
2. the user puts the Mac to sleep (e.g. Apple menu > Sleep)
3. a total of ~60 seconds transpires (i.e. macOS puts the app into the "suspended state")
4. when the app is brought back into the foreground the UI no longer updates properly

The only way I have found to fix this is to manually open a new actual full app window via File > New, in which case the app works fine again in the new window.

The following extremely simple code in a default Xcode project illustrates the issue:

import SwiftUI

@main
struct staleApp: App {
  @State private var isBright = true
  var body: some Scene {
    WindowGroup() {
      ZStack {
        (isBright ? Color.white : Color.black).ignoresSafeArea()
        Button("TOGGLE") { isBright.toggle(); print("TAPPED") }
      }
    }
  }
}

For the code above, after Hiding the app and putting the computer to sleep for 60 seconds or more, the button no longer swaps views, although the print statements still appear in the console upon tapping the button. Also, while in this buggy state, i can get the view to update to the current state (i.e. the view triggered by the last tap) by manually dragging the corner of the app window to resize the window. But after resizing, the view again does not update upon button tapping until I resize the window again.

so it appears the diff engine is mucked or that the Scene or WindowGroup are no longer correctly running on the main thread

I have tried rebuilding the entire view hierarchy by updating .id() on views but this approach does NOT work. I have tried many other options/hacks but have not been able to reset the 'view engine' other than opening a new window manually or by using: @Environment(.openWindow) private var openWindow

openWindow could be a viable solution except there's no way to programmatically close the old window for isiOSAppOnMac (@Environment(.dismissWindow) private var dismissWindow doesn't work for iOS)