Fix org-fontify-whole-*-line by setting face extension (bug#42184)

* lisp/org-faces.el (org-block): Set background extension beyond
end-of-line.
* lisp/org-compat.el (org--set-faces-extend): New function to
temporarily (re)set :extend for Emacs≥27.
* lisp/org.el (org-mode): Call it to set the extend attribute of
relevant faces to the correct value.

lisp/org-compat.el

@@ -101,6 +101,14 @@ is nil)."
   (defun org-time-convert-to-list (time)
     (seconds-to-time (float-time time))))
 
+(defun org--set-faces-extend (faces extend-p)
+  "Set the :extend attribute of FACES to EXTEND-P.
+
+This is a no-op for Emacs versions lower than 27, since face
+extension beyond end of line was not controllable."
+  (when (fboundp 'set-face-extend)
+    (mapc (lambda (f) (set-face-extend f extend-p)) faces)))
+
 
 ;;; Emacs < 26.1 compatibility
 

lisp/org-faces.el

@@ -393,7 +393,8 @@ follows a #+DATE:, #+AUTHOR: or #+EMAIL: keyword."
   "Face for #+TITLE:, #+AUTHOR:, #+EMAIL: and #+DATE: keywords."
   :group 'org-faces)
 
-(defface org-block '((t :inherit shadow))
+(defface org-block `((t :inherit shadow
+			,@(and (>= emacs-major-version 27) '(:extend t))))
   "Face text in #+begin ... #+end blocks.
 For source-blocks `org-src-block-faces' takes precedence."
   :group 'org-faces

lisp/org.el

@@ -4940,7 +4940,11 @@ The following commands are available:
   ;; Try to set `org-hide' face correctly.
   (let ((foreground (org-find-invisible-foreground)))
     (when foreground
-      (set-face-foreground 'org-hide foreground))))
+      (set-face-foreground 'org-hide foreground)))
+  ;; Set face extension as requested.
+  (org--set-faces-extend '(org-block-begin-line org-block-end-line)
+                         org-fontify-whole-block-delimiter-line)
+  (org--set-faces-extend org-level-faces org-fontify-whole-heading-line))
 
 ;; Update `customize-package-emacs-version-alist'
 (add-to-list 'customize-package-emacs-version-alist