The incriminated code is:

  // Update Rx window size, i.e. the flow control window
  if (tcpHeader.GetFlags () & TcpHeader::ACK)
    {
      UpdateWindowSize (tcpHeader);
    }

In one hand, it is correct because UpdateWindowSize manages also the scale of the window (if WindowScale is present), and so it should not be called when a SYN or SYN-ACK is received. However, this leaves m_rWnd set to 0 for SYN and SYN-ACK segments. The correct code would be:

  // Update Rx window size, i.e. the flow control window
  if (tcpHeader.GetFlags () & TcpHeader::ACK)
    {
      UpdateWindowSize (tcpHeader);
    }
  else if (tcpHeader.GetFlags () & TcpHeader::SYN)
    {
      /* The window field in a segment where the SYN bit is set (i.e., a <SYN>
       * or <SYN,ACK>) MUST NOT be scaled (from RFC 7323 page 9). But should be
       * saved anyway..
       */
      m_rWnd = tcpHeader.GetWindowSize ();
    }

Discovered & corrected while investigating on zero window state. Patching is easy.