Currently Shift-Up allows for a quick reply to the latest message, but what if someone just said something and the one you want is no longer the latest?

Repeated presses could move up through the message history, allowing to reply to a message that isn't the latest.

Likewise, Shift-Down (currently unused) could move down the list (and, optionally, all the way back to the main input field like Ctrl-Shift-L)