bug#59065: 28.2; Mapping default face affects extended faces

[Al Haji-Ali]

I'll admit this "bug" is a convoluted one. I am noticing a weird behaviour 
where extended background colour to the whole window instead of a single line 
when a face a remapped.

To re-produce an example:

0. Run `emacs -Q`

1. Create a buffer with this content and ensure `org-mode` is on
------------------------------------------------------------
* One
#+begin_src emacs-lisp
#+end_src
* Two
#+begin_src emacs-lisp
#+end_src
------------------------------------------------------------

2. Call `org-narrow-to-subtree` on "One".

3. Change the face `org-block-end-line` to have an extended background on the 
end-line of the org-block

(set-face-attribute
   'org-block-end-line nil
   :extend t
   :background "#4e5079")

4. Create a any font remap like this trivial one

(face-remap-add-relative 'default 'default)

or even this one

(face-remap-add-relative
        'default
        '(:filtered (:window not-real-filter t) not-real-face)

5. Notice how the background colour on the last line of the narrowed buffer 
extends over the whole window instead of just the line.

Best regards,
-- Al