> It's
> also the only quick way in IDLE to be sure that the text widget is
> scrolled all the way to the left, since there's no horizontal scrollbar.
> (Any idea why that is? I assumed it's to discourage long lines, but I
> don't know.)

The shell is configured to wrap lines if they get too long, but you
will notice that pressing 'end' doesn't really get you to the end of
line.
It should be easier to not wrap there too and add a horizontal
scrollbar (if needed) to all text widgets used by idle.