Scrolling doesn't work in one-line terminals

[soeren]

Hi,

I am using a readline-enabled program in a one-line terminal created,
for instance, using `tmux split-window -l 1`. Unfortunately, horizontal
scrolling does not seem to work in these terminals. For example, when
the input exceeds the current line scrolling backwards with the left
arrow key will result in no input being displayed.

To reproduce:

        1. Create a one-line terminal using `tmux split-window -l 1`,
           requires tmux >= 2.9.
        2. Type bogus characters until the current line has been filled
           and horizontal scrolling is forced.
        3. Try seeing the previous input by scrolling left. For
           instance, with the left arrow key or C-a.

Result: The previous input will not be shown correctly.

I can reproduce this with different readline version and different
operating systems. The following screen cast, demonstrating this issue,
was created on Alpine Linux edge with readline-8.0:

        https://user.informatik.uni-bremen.de/~tempel/readline-one-line.gif

Cheers,
Sören