Stop splicing anonymous faces in font-lock-append-text-property

This is the same fix as 2019-04-29 "Refrain from splicing anonymous
faces in text properties", which was only applied to
font-lock-prepend-text-property.

* lisp/font-lock.el (font-lock-append-text-property): Distinguish list
of faces from property list.
* test/lisp/font-lock-tests.el: New test suite.

(Bug#35476)

1 files changed, 6 insertions, 1 deletions

diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 7ff4e606fa6..95ca2f99c21 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1417,7 +1417,12 @@ Optional argument OBJECT is the string or buffer containing the text."
 Arguments PROP and VALUE specify the property and value to append to the value
 already in place.  The resulting property values are always lists.
 Optional argument OBJECT is the string or buffer containing the text."
-  (let ((val (if (listp value) value (list value))) next prev)
+  (let ((val (if (and (listp value) (not (keywordp (car value))))
+                 ;; Already a list of faces.
+                 value
+               ;; A single face (e.g. a plist of face properties).
+               (list value)))
+        next prev)
     (while (/= start end)
       (setq next (next-single-property-change start prop object end)
             prev (get-text-property start prop object))