bug#59141: 28.1.90; Face :extend when all the line but trailing \n is invisible

[Juri Linkov]

> we could add ellipses in a more clean way: as a new value of the user
> option 'outline-minor-mode-use-buttons'.  So when it's customized to
> 'ellipsis', the ellipsis overlay will be created in a new pcase branch
> in outline--insert-button.

Unfortunately, this is not possible to do because of a bug
in overlay handling.  Here is a short test case:

(progn
  (insert "ab")
  (let ((o (make-overlay 1 2)))
    (overlay-put o 'after-string "!"))
  (let ((o (make-overlay 2 3)))
    (overlay-put o 'invisible t)))

The problem is that "!" from 'after-string' is not displayed,
even when it's on the text that is not invisible.