code: func (s *sendStream) popStreamFrame(maxBytes protocol.ByteCount) (*wire.StreamFrame, bool /* has more data to send */) {
  	s.mutex.Lock()
  	completed, frame, hasMoreData := s.popStreamFrameImpl(maxBytes)
  	s.mutex.Unlock()
  
  	if completed {
  		s.sender.onStreamCompleted(s.streamID)
  	}
  	return frame, hasMoreData
  }
docs: // popStreamFrame returns the next STREAM frame that is supposed to be sent on this stream
  // maxBytes is the maximum length this frame (including frame header) will have.