ksh (all variants I tried) has another issue: That thing about
horizontally scrolling the command line.

Why?

Is it something historical for saving screen estate?

It's rather impractical, I think. When typing very long commands, well,
I'd like to see it all. I want to get an overview. Why hide it?

It also forces the current line to be redrawn when the width of the
window changes. I have a BEL character in my PS1, so I get urgency hints
on windows all the time. Not ksh's fault, of course, but still annoying.