I’ve discovered while looking into a bug report two issues related to progress notification handlers.

1: The progress notifier is registered AFTER the changes have been pulled down, signifying a race condition and that if the changeset is large enough, the handler may register in time.

2: If a Realm is up to date or no data exists in a collection, the sync protocol will only send down a 'mark' message. This means that{{connection.receive_download_message}} won't be called. The consequence of this is that if a user relies on the progress handler to show a loading screen in their app the screen will show indefinitely.

@tkaye407 @mpobrien @simonask @jedelbo