docs: @Override
    public void flushBufferStopAtMark() throws IOException {
      final int end = pos;
      pos = mark;
      flushBuffer(true);
      out.flush();
      startPacket(0);
  
      System.arraycopy(buf, mark, buf, pos, end - mark);
      pos += end - mark;
      mark = -1;
      bufferContainDataAfterMark = true;
    }