It still doesn’t work to drag items between two drop destinations inside a List. It works fine if I change the List to a VStack. It's so embarrassing, it's mind boggling.

Same here. Did you manage to find the issue?

It still happens even after clearing any MainActor in its way. The assert happens in this assembly: libdispatch.dylib`_dispatch_assert_queue_fail: ... 0x103ae8274 <+68>: add x9, x9, #0xe52 ; "BUG IN CLIENT OF LIBDISPATCH: Assertion failed: " 0x103ae8278 <+72>: stp x9, x8, [sp] 0x103ae827c <+76>: adrp x1, 63 0x103ae8280 <+80>: add x1, x1, #0xe1d ; "%sBlock was %sexpected to execute on queue [%s (%p)]" 0x103ae8284 <+84>: sub x0, x29, #0x18 0x103ae8288 <+88>: bl 0x103b23d60 ; symbol stub for: asprintf 0x103ae828c <+92>: ldur x19, [x29, #-0x18] 0x103ae8290 <+96>: str x19, [sp] 0x103ae8294 <+100>: adrp x0, 63 0x103ae8298 <+104>: add x0, x0, #0xe88 ; "%s" 0x103ae829c <+108>: bl 0x103b1fed4 ; _dispatch_log 0x103ae82a0 <+112>: adrp x8, 101 0x103ae82a4 <+116>: str x19, [x8, #0x1c0] -> 0x103ae82a8 <+120>: brk #0x1 Is that something I control or is it a bug for Apple?

Thank you, Ziqiao, your example helped me trace the issue after a lot of trial and error. Regarding appleExerciseTime (and other apple* types), it seemed to me like those types are reserved for the Apple Watch, is that correct? If so, is there any way for me as a 3rd party app+accessory to make the other activity rings show or affect them in any way without an Apple Watch?

As you suspected, I unfortunately don’t have a reproducible case, as this appears to be a rare race condition (though it has affected approximately 250 of my users). Here’s an example of the most frequent crash (truncated; full stack trace attached): Crashed: com.apple.main-thread 0 libsystem_kernel.dylib 0xc2d4 __pthread_kill + 8 1 libsystem_pthread.dylib 0x859c pthread_kill + 268 2 libsystem_c.dylib 0x77b08 abort + 128 3 libswiftCore.dylib 0x3ccae4 swift::fatalError(unsigned int, char const*, ...) + 134 4 libswiftCore.dylib 0x3ccb04 swift::warningv(unsigned int, char const*, char*) + 30 5 libswiftCore.dylib 0x3c66cc tryCastFromClassToObjCBridgeable(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetMetadata<swift::InProcess> const*&, swift::TargetMetadata<swift::InProcess> const*&, bool, bool) + 254 6 libswiftCore.dylib 0x3c8408 tryCastToObjectiveCClass(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetMetadata<swift::InProcess> const*&, swift::TargetMetadata<swift::InProcess> const*&, bool, bool) + 88 7 libswiftCore.dylib 0x3c5d30 tryCast(swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::OpaqueValue*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetMetadata<swift::InProcess> const*&, swift::TargetMetadata<swift::InProcess> const*&, bool, bool) + 992 8 libswiftCore.dylib 0x3c57dc swift_dynamicCast + 208 9 SwiftUI 0x91540 PlatformViewHost.updateNestedHosts(_:colorSchemeChanged:) + 332 10 SwiftUI 0x91110 PlatformViewHost.updateEnvironment(_:viewPhase:) + 272 11 SwiftUI 0x27f148 specialized PlatformViewHost.init(_:host:environment:viewPhase:importer:) + 1288 12 SwiftUI 0x27ec28 specialized PlatformViewHost.__allocating_init(_:host:environment:viewPhase:importer:) + 92 13 SwiftUI 0x27eb88 closure #1 in closure #1 in closure #4 in closure #1 in PlatformViewChild.updateValue() + 504 14 SwiftUI 0x27e984 partial apply for closure #1 in closure #1 in closure #4 in closure #1 in PlatformViewChild.updateValue() + 48 15 SwiftUI 0x27e90c RepresentableContextValues.asCurrent<A>(do:) + 188 16 SwiftUI 0x27e804 closure #1 in closure #4 in closure #1 in PlatformViewChild.updateValue() + 176 17 SwiftUI 0x27e72c partial apply for closure #4 in closure #1 in PlatformViewChild.updateValue() + 128 18 SwiftUICore 0x1640c partial apply for closure #1 in _withObservation<A>(do:) + 48 19 SwiftUICore 0x14148 closure #1 in _withObservation<A>(do:)partial apply + 16 20 SwiftUICore 0x128c8 withUnsafeMutablePointer<A, B, C>(to:_:) + 160 21 SwiftUICore 0x7c08c StatefulRule.withObservation<A>(do:) + 872 22 SwiftUICore 0x7bb8c StatefulRule.withObservation<A>(do:) + 72 ... There’s no reference to any specific code of mine—no clues at all. How would you recommend I tackle this crash? Full Crash Stacktrace

Also having this problem in OSX Monterey 12.6.