org-fold: Refactor fontifying newlines after folds

* lisp/org.el (org-activate-folds): New fontification function to
arrange faces at newlines after folds to be the same as face before
the fold.
(org-set-font-lock-defaults): Add the new fontification function.
* lisp/org-fold-core.el (org-fold-core-region): Refresh fontification
of newlines after folds when folding/unfolding.

Reported-by: StrawberryTea <look@strawberrytea.xyz>
Link: https://orgmode.org/list/87plwoqrfv.fsf@strawberrytea.xyz

lisp/org-fold-core.el

@@ -1034,26 +1034,19 @@ If SPEC-OR-ALIAS is omitted and FLAG is nil, unfold everything in the region."
     (when spec (org-fold-core--check-spec spec))
     (with-silent-modifications
       (org-with-wide-buffer
-       ;; Arrange face property of newlines after all the folds
-       ;; between FROM and TO to match the first character before the
-       ;; fold; not the last as per Emacs defaults.  This makes
-       ;; :extend faces span past the ellipsis.
-       ;; See bug#65896.
-       (if flag ; folding
-           (when (equal ?\n (char-after to))
-             (put-text-property to (1+ to) 'face (get-text-property from 'face)))
-         ;; unfolding
-         (dolist (region (org-fold-core-get-regions :from from :to to :specs spec))
-           (when (equal ?\n (char-after (cadr region)))
-             (if-let ((specs (remq spec (org-fold-core-get-folding-spec 'all (1- (cadr region))))))
-                 ;; There are multiple folds, re-apply 'face according
-                 ;; to the folds that will stay after unfolding SPEC.
-                 (put-text-property
-                  (cadr region) (1+ (cadr region)) 'face
-                  (get-text-property
-                   (car (org-fold-core-get-region-at-point (car specs) (1- (cadr region))))
-                   'face))
-               (font-lock-flush (cadr region) (1+ (cadr region)))))))
+       ;; Arrange fontifying newlines after all the folds between FROM
+       ;; and TO to match the first character before the fold; not the
+       ;; last as per Emacs defaults.  This makes :extend faces span
+       ;; past the ellipsis.  See bug#65896.  The face properties are
+       ;; assigned via `org-activate-folds'.
+       (when (equal ?\n (char-after to))
+         (font-lock-flush to (1+ to)))
+       ;; Re-fontify beginning of the fold when unfolding - we may
+       ;; unfold inside an existing fold, with FROM begin a newline
+       ;; after spliced fold.
+       (unless flag
+         (when (equal ?\n (char-after from))
+           (font-lock-flush from (1+ from))))
        (when (eq org-fold-core-style 'overlays)
          (if org-fold-core--keep-overlays
              (mapc
@@ -1114,13 +1107,13 @@ If SPEC-OR-ALIAS is omitted and FLAG is nil, unfold everything in the region."
                      (setq pos (next-single-char-property-change pos 'invisible nil to)))))))
            (when (eq org-fold-core-style 'text-properties)
 	     (remove-text-properties from to (list (org-fold-core--property-symbol-get-create spec) nil)))
-           ;; FROM..TO may represent only a part of the fold.  Transfer
-           ;; 'face from the new char before fold, if there is any.
-           (when-let ((trailing-fold (org-fold-core-get-region-at-point spec to)))
-             (when (equal ?\n (char-after (cdr trailing-fold)))
-               (put-text-property
-                (cdr trailing-fold) (1+ (cdr trailing-fold))
-                'face (get-text-property to 'face))))))))))
+           ;; Re-calculate trailing faces for all the folds revealed
+           ;; by unfolding.
+           (unless flag
+             ;; unfolding
+             (dolist (region (org-fold-core-get-regions :from from :to to :specs spec))
+               (when (equal ?\n (char-after (cadr region)))
+                 (font-lock-flush (cadr region) (1+ (cadr region))))))))))))
 
 (cl-defmacro org-fold-core-regions (regions &key override clean-markers relative)
   "Fold every region in REGIONS list in current buffer.

lisp/org.el

@@ -5797,6 +5797,24 @@ highlighting was done, nil otherwise."
     (org-rear-nonsticky-at (match-end 1))
     t))
 
+(defun org-activate-folds (limit)
+  "Arrange trailing newlines after folds to inherit face before the fold."
+  (let ((next-unfolded-newline (search-forward "\n" limit 'move)))
+    (while (and next-unfolded-newline (org-fold-folded-p) (not (eobp)))
+      (setq next-unfolded-newline (search-forward "\n" limit 'move)))
+    (when next-unfolded-newline
+      (org-with-wide-buffer
+       (when (and (> (match-beginning 0) (point-min))
+                  (org-fold-folded-p (1- (match-beginning 0))))
+         (put-text-property
+          (match-beginning 0) (match-end 0)
+          'face
+          (get-text-property
+           (org-fold-previous-visibility-change
+            (1- (match-beginning 0)))
+           'face)))
+       t))))
+
 (defun org-outline-level ()
   "Compute the outline level of the heading at point.
 
@@ -5978,7 +5996,8 @@ needs to be inserted at a specific position in the font-lock sequence.")
           (progn
             (unless (null org-cite-activate-processor)
               (org-cite-try-load-processor org-cite-activate-processor))
-            '(org-cite-activate)))))
+            '(org-cite-activate))
+          '(org-activate-folds))))
     (setq org-font-lock-extra-keywords (delq nil org-font-lock-extra-keywords))
     (run-hooks 'org-font-lock-set-keywords-hook)
     ;; Now set the full font-lock-keywords