Maybe it serves me right that I can't delete the post above! It might make me think thrice before posting here ... Ignore all above: I found a crash log with a clue in it and it turns out this is in fact a bizarre async issue happening during my AU initialisation that wasn't caught before probably because it's in a bit of Objc code.

Quinn? Hopefully this little poke will get some attentiion :) Is this a bug, is it a known bug? Our bug report has not received any reply since January? Are we going to have to file a code level request on this one?

So a whole day's work on this one yields a result of sorts: Setting the value of a parameter at the time the tree is created appears to satisfy auvaltool's condition of a "default" value. Really Apple ... would createParameterWithIdentifier ... min,max,default ... have been so difficult ?

It would be nice to have this confirmed but it looks like our solution is to create an app group. It seems we can read and write as we need to from the group container and the that directory is always available to the audio unit even when running in Logic. ( via containerURL forSecurityApplicationGroupIdentifier ) It would be nice if someone could confirm this is the right thing to do ! :) So to reiterate our issue: We were using the File Manager to obtain a URL to a folder in user domain, lets say the Documents folder ... When the AU is hosted in a sandboxed host FileManager supplies a URL to a folder inside the sandbox container. (as expected) When the same AU is hosted in a non-sandboxed host FileManager supplies a URL to a folder inside the user folder (ie. ~/Documents) That inconsistency is a problem. Solved by creating an App Group and asking FileManager to supply the URL for the group folder. ?

But you'll find that if you explicitly call provider.loadPreviewImage(...) your previewImageHandler will actually get called. However, even though the previewImageHandler can provide a perfectly good image, even after it returns the new data SwiftUI seems to ingore it and continues to use the image from the original view! :( I too would like an answer to the question: Why does SwiftUI ignore attempts to add a custom drag image?