(setq display-buffer-base-action '((display-buffer-use-least-recent-window) (not-this-window . t)))

The explicit not-this-window is required to get display-buffer-pop-up-window

to trigger in a single window single frame case. It might be worth adding a note

to that effect to the docstring?

> Commenting out these lru-time settings will make
> it use as window for *bar* the window it previously used for *foo*.

This results in the behavior of the example matching that of xemacs.

With the lru-time option enabled the display multiple buffers case seems

to be served (I assume there is some other mechanism that can be used

to restore the previous window configuration in such cases that is not

included in this example).

The addition of lru-frames to decouple from reusable-frames is also good.

It produces the desired behavior in the context of multiple frames.

Many thanks!

Tom