Ah yes, I am seeing that now for the first time here, and I have a pretty good idea of what to look for. Definitely caused by Hijack, I think, and only recently being noticed due to Hijack injecting extra lines at the serial port.

I think it is somehow tossing data into the output FIFO without re-enabling the serial port TX interrupt in the cases where that interrupt is not already enabled.

Hitting ENTER is a great clue, because that would trigger a serial RX interrupt, and the processing thereof would also re-enable TX.

So I know where to look for it.