>> when xref-find-definitions pops up the xref window, usually it
>> contains just 2 lines taking half of the screen where most space is
>> uselessly empty.
>
> We have fit-window-to-buffer for these situations.
>
>> The distinction between internal and public window functions is quite fuzzy.
>
> To my mind, internal functions shouldn't be used outside of the file
> that defines them.

This patch addresses all these concerns.

display-buffer--maybe-at-bottom can be renamed to
display-buffer-maybe-at-bottom without a deprecation alias
because it was added in Emacs 27.