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

[Juri Linkov]

>> I do not see anything wrong on the Org side.
>> Maybe Emacs should not apply :extent t attribute to the newline when the
>> text in fontified line is hidden?
>
> IIUC, this is bug#52587?

And bug#53981.

> The conclusion there, AFAIR, was "it's unfortunate, but that's the
> design for now; let's rediscuss when we see some patches for outline.el
> (or the specific modes that use extended backgrounds)".

Here is an experimental patch for outline.el
that demonstrates a possible way to fix this:

diff --git a/lisp/outline.el b/lisp/outline.el
index a646f71db8..73ae707821 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -930,7 +930,8 @@ outline-flag-region
     ;; We use `front-advance' here because the invisible text begins at the
     ;; very end of the heading, before the newline, so text inserted at FROM
     ;; belongs to the heading rather than to the entry.
-    (let ((o (make-overlay from to nil 'front-advance)))
+    (let ((o (make-overlay from (1+ to))))
+      (overlay-put o 'display "\n")
       (overlay-put o 'evaporate t)
       (overlay-put o 'invisible 'outline)
       (overlay-put o 'isearch-open-invisible