messageBuffer.write(data)

    var b0 = formatOpcode or B0_FLAG_FIN
    if (perMessageDeflate && data.size >= minimumDeflateSize) {
      val messageDeflater =
        this.messageDeflater
          ?: MessageDeflater(noContextTakeover).also { this.messageDeflater = it }
      messageDeflater.deflate(messageBuffer)
      b0 = b0 or B0_FLAG_RSV1
    }
    val dataSize = messageBuffer.size
    sinkBuffer.writeByte(b0)