Hi there, I’ve also been receiving crash reports to my app that seem to point to the same cause as yours. NSOutlineView on macOS 15 crashes in NSConcreteMapTable, [_TtCs12_SwiftObject isEqual:] (and isSubclass in my cases). I've also sent Feedback with my case FB16913684. FB16913684: NSOutlineView crashes by checking isSubclass internally on macOS 15

Sent feedback: FB18401945.

If this is the case, I'd like to see the feature in SwiftUI in future SDK. So I sent a feature request: FB18743468.