That's actually XON/XOFF - in the old days when computers could send serial data to your VT100 or whatever faster than it could handle it (and we're talking 9600 baud here), the terminal would send XOFF (ctrl-S) telling the computer to stop sending - then, when the terminal caught up, it would send XON (ctrl-Q) to let the system know it could resume sending.

The advantage of doing it this way ('software flow control') is it worked with just TX and RX lines, rather than having to connect up additional wires to indicate when the terminal was busy ('hardware flow control').

Ah this is also a fun default feature of Powershell, except it works on mouse click.

there was an old riddle I saw on usenet, "there is a key I use for one of its intended purposes several times a day, but when it stopped working I never missed it. what key was that?". the answer was the caps lock key, one of whose intended purposes was reverting back to normal after accidentally pressing it to go into caps mode.

You can disable this misbegotten feature with

stty -ixon