@rl_dane hit the nail on the head.
This isn't really specific to completion. It's just general overlong input line editing behaviour.
The PD #KornShell and its derivatives (e.g. the #MirBSD Korn Shell) only have line editing with a single line that sideways scrolls.
ksh93, however, has a
set -o multiline
option for switching to a multiple-line line editing mode. (The Z and Bourne Again shells have similar.)
You might like the Watanabe shell. It's in ports.




