> * Command's stdout is printed correctly, but it stderr is ends up
>   weirdly interleaved.

That is trivial to solve by dup'ing the descriptors to be the same pipe,
but I think this is the least of the problems this tooling has.

> * Command's output (stdout+stderr) are only displayed _after_ the
>   command has fully completed, it is not real time. I'd like to see the
>   output as it happens.

The entire thing is syncronous, which is not what one expects.  As a
user, I expect this to behave more like how a tmux screen would.  This
is a simple curses program with syncronous re-do loop.  I think it
should be an sophisticated program runner, with an internal memory view,
which occasionally uses curses to display.  I've shopped around some
ideas and we'll see if someone bites.