In pp-display-expression there is this sexp:
(let ((temp-buffer-show-function old-show-function)
(old-selected (selected-window))
(window (display-buffer buf)))
^^^^^^^^^^^^^^^^^^^^
(goto-char (point-min)) ; expected by some hooks ...
(make-frame-visible (window-frame window))
(unwind-protect
(progn
(select-window window)
(run-hooks 'temp-buffer-show-hook))
(select-window old-selected)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(message "See buffer %s." out-buffer-name)))
An user may implement an overriding action for display-buffer that could
delete old-selected.