code: func (r *receiver) nonZeroWindow() {
  	if (r.rcvAcc-r.rcvNxt)>>r.rcvWndScale != 0 {
  		// We never got around to announcing a zero window size, so we
  		// don't need to immediately announce a nonzero one.
  		return
  	}
  
  	// Immediately send an ack.
  	r.ep.snd.sendAck()
  }
docs: // nonZeroWindow is called when the receive window grows from zero to nonzero;
  // in such cases we may need to send an ack to indicate to our peer that it can
  // resume sending data.