The problem is that we can only return to the proper thread/queue when we know what that thread/queue is. Additionally, when running tests, we are calling from a concurrent queue, so even knowing the queue, we cannot safely update the buffer with the new scopes, as only a serial queue can give us exclusive access.
The first time we ask for the shared spell checker it does some setup involving AppKit (creating the shared spelling panel?). While we already ensure that the spell checker is only called from the main queue, it seems this setup code must run on thread 0, which, when we are not in a Cocoa application, is not necessarily the thread used for the main queue.
