bug#68352: 29.1; gnus-message-citation-mode removes values from font-lock-keywords

[Morgan Willcock <morgan@ice9.digital>]

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I've attached a patch which is essentially the same as Eric's suggestion
except that, the original comment is kept, the HOW argument of
`font-lock-add-keywords' is set to t, and the code is restructured to
avoid any font-lock changes until `font-lock-mode' is active.

-- 
Morgan Willcock

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From 9fa84932aa0ad7c37b20d68ca358ee779d225f09 Mon Sep 17 00:00:00 2001
From: Morgan Willcock <morgan@ice9.digital>
Date: Wed, 10 Jan 2024 21:29:29 +0000
Subject: [PATCH] Avoid font-lock reset in `gnus-message-citation-mode'

* lisp/gnus/gnus-cite.el (gnus-message-citation-mode): Use
`font-lock-add-keywords' and `font-lock-remove-keywords' instead of
modifying font-lock defaults.  Make no font-lock changes until
`font-lock-mode' is active.
---
 lisp/gnus/gnus-cite.el | 36 +++++++++++-------------------------
 1 file changed, 11 insertions(+), 25 deletions(-)

diff --git a/lisp/gnus/gnus-cite.el b/lisp/gnus/gnus-cite.el
index 04abdfc0d1b..3fde9baa0fe 100644
--- a/lisp/gnus/gnus-cite.el
+++ b/lisp/gnus/gnus-cite.el
@@ -1122,31 +1122,17 @@ gnus-message-citation-mode
 When enabled, it automatically turns on `font-lock-mode'."
   :lighter ""
   (when (derived-mode-p 'message-mode)
-    ;; FIXME: Use font-lock-add-keywords!
-    (let ((defaults (car font-lock-defaults))
-	  default) ;; keywords
-      (while defaults
-	(setq default (if (consp defaults)
-			  (pop defaults)
-			(prog1
-			    defaults
-			  (setq defaults nil))))
-	(if gnus-message-citation-mode
-	    ;; `gnus-message-citation-keywords' should be the last
-	    ;; elements of the keywords because the others are unlikely
-	    ;; to have the OVERRIDE flags -- XEmacs applies a keyword
-	    ;; having no OVERRIDE flag to matched text even if it has
-	    ;; already other faces, while Emacs doesn't.
-	    (set (make-local-variable default)
-		 (append (default-value default)
-			 gnus-message-citation-keywords))
-	  (kill-local-variable default))))
-    ;; Force `font-lock-set-defaults' to update `font-lock-keywords'.
-    (setq font-lock-set-defaults nil)
-    (font-lock-set-defaults)
-    (if font-lock-mode
-	(font-lock-flush)
-      (gnus-message-citation-mode (font-lock-mode 1)))))
+    (if (not font-lock-mode)
+        (gnus-message-citation-mode (font-lock-mode 1))
+      (if gnus-message-citation-mode
+	  ;; `gnus-message-citation-keywords' should be the last
+	  ;; elements of the keywords because the others are unlikely
+	  ;; to have the OVERRIDE flags -- XEmacs applies a keyword
+	  ;; having no OVERRIDE flag to matched text even if it has
+	  ;; already other faces, while Emacs doesn't.
+	  (font-lock-add-keywords nil gnus-message-citation-keywords t)
+        (font-lock-remove-keywords nil gnus-message-citation-keywords))
+      (font-lock-flush))))
 
 (defun turn-on-gnus-message-citation-mode ()
   "Turn on `gnus-message-citation-mode'."
-- 
2.39.2