Hello, thanks for the quick response! Background Info To clarify, we are building a browser app. We have the same behaviour as Safari: with the Duo unfolded, you can use Split Views to open two independent windows of Safari, side by side. This lets users browse the internet in two different windows, similar to how you can have separate windows for Safari on macOS. (For this reason, ArrangementView unfortunately does not satisfy our needs.) Our app has a feature for users who have opened 2+ independent instances of our app (i.e. iPhone Duo users using Split View and iPad users using Multitask). By tapping a button, users can "merge" all their open windows, copying all their browser tabs to one window, then close the extra windows. Afterward, the App Switcher should then only display one copy of the app. Demo I've uploaded a minimal reproducible example of the problem here: GitHub Demo You just have to run on iPhone Duo in Xcode 27 and you will see the error logged to the console. The README shows the steps to open two windows in Split View on the iPhone Duo. In Closing Seeing as I can open two distinct windows of my app on the iPhone Duo using Split View (just like you can with Safari), and thus create two independent connectedScenes, it seems odd that I cannot request one of these scenes to be destroyed with link text. 🤔 Is there another way we can programmatically close one of the windows for iPhone Duo users who have two opened using Split View? Thank you for your time!

Thanks for your thoughtful reply! It's much appreciated. I did follow your suggestion and opened an issue requesting the same treatment for notify. My bug number is FB20920858. P.S. Part of your answer above the horizontal line appears to have been cut off, but I was curious to hear what you were going to suggest! Regarding your suggestion for moving to Swift 6.2 and enabling default main actor isolation -- I did try this by copying my minimal example to a new Swift 6 project with Default Actor Isolation set to MainActor. Alas, it still crashes (albeit on a different line; it seems an internal thread assertion fails from the group.notify call itself rather than my MainActor.assertIsolated() in the closure body). Honestly, it's still what I expected given the missing @Sendable.