TrackerQueue uses "int" data type for numbering deliveries and managing the window size.
tests are not coded for circular arithmetic so weirdness will start after 2^31 deliveries.
Quick fix is to make these labels long.
Slightly less quick fix is to use circular arithmetic.
if (seq < _lwm || seq > _hwm) return null;
if (((seq - _lwm) < 0) || ((seq - _hwm) > 0)) return null;
if (seq < _lwm || seq > _hwm) return;
change as above.