+act - #15501 - Adding support for a Non-blocking, bounded, MPMC mailbox...

... called 'NonBlockingBoundedMailbox'

See #15501 
Since it is an approximation in any case, may I suggest:
~~~java
public final int size() {
  final int deq = getDeq().count;
  final int enq = getEnq().count;
  return Math.min(enq - deq, capacity);
}
~~~