AVR Freaks

The behavior of the TXC flag hasn't changed from very old devices (eg mega8 and tiny2313).
Why is it a problem now?
I think the bad thing is the "ignorant" code.

I think that the operation of TXC is not unique to AVR and is equivalent to general uc.
And shouldn't you complain to the person who created it if optiboot isn't suitable for it?
"Optiboot is ignorant code."

Who is the culprit who cleared TXCIF with your code?
Why didn't you give him a way to make it clear?

Found code, may not be what you are using, don't know. When using putch you clear TXCIF so it is always clear whenever a byte is put in hardware, and is the only place the flag is cleared. When you eventually reach getch, you wait until the txcif flag is set. You cannot get to putch in any state that would be a problem if following the protocol, so nothing to worry about with putch.

Since you probably use getch before putch, as I said before you can get the txcif flag initially set by putting a byte in txdataL, and a baud of 0 works so basically no transmit. There are other ways, like using normal baud but no tx output enabled, and I guess you pick whatever is easiest.

void putch (char ch) {
  rs485_txon();  // turn on and leave on till we're done xmitting.
  while (0 == (MYUART.STATUS & USART_DREIF_bm));
  MYUART.TXDATAL = ch;
  MYUART.STATUS = USART_TXCIF_bm; //clear TXCIF
}

uint8_t getch (void) {

  while( (MYUART.STATUS & USART_TXCIF_bm) == 0 ){} //wait for TXCIF

this next function needs to change as we are doing the txcif waiting here
  rs485_txoff();    // To receive, turn off transmitter

  uint8_t ch, flags;
  while (!(MYUART.STATUS & USART_RXCIF_bm));
  flags = MYUART.RXDATAH;
  ch = MYUART.RXDATAL;
  if ((flags & USART_FERR_bm) == 0)
    watchdogReset();
#ifdef LED_DATA_FLASH
  LED_PORT.IN |= LED;
#endif

  return ch;
}

edit- this is what I have in my bootloader when I init the uart (txcif is set)-

            SI void
uartOn      ()
            {
            USART.CTRLB = (USART_RXEN_bm|USART_TXEN_bm);
            USART.TXDATAL = 0; //set TXCIF (w/baud val 0, no bytes go out)
            }

and the flag gets clear on any transmit-

            // blocking, tx 1 byte
            SI void
putByte     (u8 c)
            {
            while( ! isTxEmpty() );
            USART.TXDATAL = c;
            clearTxDone();
            }

and the autobaud waits until all tx is done before it starts as it messes with baud (autobaud is done for every 'packet')-

            SI void
autobaud    ()
            {
            while( ! isTxDone() );      // wait until tx done

            ...