+act - Adds a non-blocking, bounded, MPSC mailbox with tests.

I read it again and realize your version actually suffers from a different issue from mine :-)
Given that consumer index is always small than producer index the order matters:

        final long currentConsumerIndex = getDeq().count; at T0
        final long currentProducerIndex = getEnq().count; at T1 -> producer increments, size might be more that capacity
        size = (int)(currentProducerIndex - currentConsumerIndex);

But the way your computation happens:

        final long currentProducerIndex = getEnq().count; T0
        final long currentConsumerIndex = getDeq().count; T1 consumer increments -> size is decreasing -> you may report a negative size
        size = (int)(currentProducerIndex - currentConsumerIndex);