martin rudalics wrote:

> > To clarify: this was after I managed to get the steppers to appear and I
> > clicked on the up-stepper.
> 
> Does the slider of a one-line window's scroll bar also expand into the
> other window's scroll bar when you remove the steppers?

I'm not sure how to remove the steppers other than changing the theme.

If I change the theme to Adwaita (no steppers) and follow your test
instructions, the results depend on which version of Emacs I use.  With
my emacs-26 build, I get the whitespace that I mentioned previously.  If
I use (- 3 ...) instead of (- 2 ...), giving me a 2-line window, I get a
proper scrollbar that stops at the top window's modeline (see attached).

With the Emacs 25 that is bundled with Debian 9, following your
instructions gives me a scrollbar that extends over the modeline for the
top window, but no further (see attached).

I'll get to the other experiments later, hopefully today.

mike