Re: New package `outline-occur'

>> 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.
>
> So 'display-buffer-base-alist' would conceptually be set by applications
> only, override 'display-buffer-base-action' and could be overridden by
> 'display-buffer-alist'.  Which one would prevail when an ACTION argument
> and a 'display-buffer-base-alist' entry both apply?

Maybe display-buffer-base-alist is not the best name
since it should be able to override an ACTION argument
(this is like completion-metadata-get prefers the category
over the direct metadata).

So the new variable should be like display-buffer-alist
(and use the same display-buffer-assq-regexp) whereas
display-buffer-alist is indented for user customization,
but the new variable is for applications to let-bind it.
Maybe a new app-action after user-action:

- display-buffer-overriding-action
- user-action (by display-buffer-assq-regexp)
- app-action (by display-buffer-assq-regexp)
- special-action
- action
- extra-action
- display-buffer-base-action
- display-buffer-fallback-action

This will allow overriding actions only for some categories
like was necessary for bug#81233, etc.