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.