Title has the full question :
Does savePanel.beginWithCompletionHandler run in a different thread than the caller thread ?
Does savePanel handler execute in a specific thread
I think the "begin…" must be called on the main thread, and you can assume that the completion handlier is also called on the main thread. (It's implied by the documentation, which says that you might call orderOut in the completion handler, and that needs to be done on the main thread.)