maxQueue

how far a subscriber may fall behind under RealtimeOverflow.QUEUE before the server gives up on it and reports the range through PubSubMessageListener.onMessageDrop. It counts delivery batches, not messages. null (the default) leaves it to the server; N >= 1 asks for N, clamped to the server's maximum, and whichever subscriber subscribes to the topic first fixes it for everyone. Below 1, or combined with RealtimeOverflow.DROP, is rejected.