> Would these work despite of the 'fit-window-to-buffer' problem you
> reported earlier?

Actually I see no problem anymore with using display-buffer-in-direction
in the following patch.

Only a small plea: could you please add an alias ‘(direction . bottom)’
as a shorthand for ‘(direction . below) (window . main)’

Similarly:
top = (direction . above) (window . main)
bottom = (direction . below) (window . main)
leftmost = (direction . left) (window . main)
rightmost = (direction . right) (window . main)