The library emits no signal when allHistoryLoaded() becomes apparent

[KitsuneRal]

One most easily reproduced case is a freshly created room: all events have just come with the sync but in order to know for sure the library has to call /messages in an attempt to get the history. /messages will surely return no events, with an early exit in addHistoricalMessageEvents as a result. allHistoryLoaded() would return true now but clients are not given a heads up about that in any form.