Re: New package `outline-occur'

>> But still we need a better way for a command to override the
>> default behavior (popup or not) using some let-bound variable.
>
> You can let-bind `display-buffer-base-action`.
> We've discussed this a few times in the past already,
> since `display-buffer-base-action` is also a custom var
> and thus it's best not to let-bind it.

display-buffer-base-action customizes only an action
whereas we need both condition and action parts like in
display-buffer-alist to be able to match a category
in the condition part.  Therefore we need a new variable, e.g.
display-buffer-base-alist.

By analogy with completion category options, the new variable
display-buffer-base-alist will correspond to
completion-category-defaults, and the existing option
display-buffer-alist corresponds to
completion-category-overrides for user customization.

Then a command could let-bind it by changing default settings
only for one category like:

(defun occur-outline (regexp &optional nlines region)
  (interactive (occur-read-primary-args))
  (let ((display-buffer-base-alist
         (append `(((and (category . occur)
                         ;; optional:
                         (this-command . occur-outline))
                    nil
                    (post-command-select-window . t)))
                 display-buffer-base-alist)))
    (occur-1 regexp nlines (list (current-buffer)))))

PS: this assumes the following change as well:

diff --git a/lisp/replace.el b/lisp/replace.el
index e7407869ca2..be8b4d1baff 100644
--- a/lisp/replace.el
+++ b/lisp/replace.el
@@ -2062,7 +2062,7 @@ occur-1
 	  (setq occur-revert-arguments (list regexp nlines bufs))
           (if (= count 0)
               (kill-buffer occur-buf)
-            (display-buffer occur-buf)
+            (display-buffer occur-buf '(nil (category . occur)))
             (when occur--final-pos
               (set-window-point
                (get-buffer-window occur-buf 'all-frames)