mu4e-view: extract shared headers function refact

Extract the header insertion code from mu4e--view-gnus-display-mime into
a new mu4e--view-insert-headers function.

Add a fallback label in mu4e--view-gnus-insert-header for fields not in
mu4e-header-info, such as :user-agent.

mu4e/mu4e-view.el

@@ -660,15 +660,12 @@ Note that for some messages, this can trigger high CPU load."
   (setq gnus-article-emulate-mime (not gnus-article-emulate-mime))
   (mu4e-view-refresh))
 
-(defun mu4e--view-gnus-display-mime (msg)
+(defun mu4e--view-insert-headers (msg &optional raw-headers)
-  "Like `gnus-display-mime', but include mu4e headers to MSG."
+  "Insert mu4e headers for MSG into the current buffer at point.
-  (lambda (&optional ihandles)
+RAW-HEADERS, when non-nil, is an alist of (FIELD . VALUE) strings
-    (gnus-display-mime ihandles)
+for standard RFC headers (From, To, Cc, etc.) that should be
-    (unless ihandles
+rendered directly.  When nil, those fields are left for Gnus to
-      (save-restriction
+render.  After inserting, highlight the headers."
-        (article-goto-body)
-        (forward-line -1)
-        (narrow-to-region (point) (point))
   (dolist (field mu4e-view-fields)
     (let ((fieldval (mu4e-message-field msg field)))
       (pcase field
@@ -679,29 +676,51 @@ Note that for some messages, this can trigger high CPU load."
            (mu4e--view-gnus-insert-header field (format "<%s>" msgid))))
         (':mailing-list
          (let ((list (plist-get msg :list)))
-                 (if list (mu4e-get-mailing-list-shortname list) "")))
+           (when list
+             (mu4e--view-gnus-insert-header
+              field (mu4e-get-mailing-list-shortname list)))))
         ((or ':flags ':labels ':tags)
          (let ((items (mapconcat (lambda (item)
                                   (if (symbolp item)
                                       (symbol-name item)
-                                           item)) fieldval ", ")))
+                                    item))
+                                fieldval ", ")))
            (mu4e--view-gnus-insert-header field items)))
         (':size (mu4e--view-gnus-insert-header
                  field (mu4e-display-size fieldval)))
         ((or ':subject ':to ':from ':cc ':bcc ':from-or-to
-                   ':user-agent ':date ':attachments
+             ':user-agent ':date)
-                   ':signature ':decryption)) ;; handled by Gnus
+         ;; Standard fields: insert from raw-headers if available,
+         ;; otherwise they are handled by Gnus.
+         (when-let* ((raw (and raw-headers (cdr (assq field raw-headers)))))
+           (mu4e--view-gnus-insert-header field raw)))
+        ((or ':attachments ':signature ':decryption)) ;; skip
         (_
          (mu4e--view-gnus-insert-header-custom msg field)))))
+  ;; Highlight the header block we just inserted
   (let ((gnus-treatment-function-alist
          '((gnus-treat-highlight-headers
             gnus-article-highlight-headers))))
-          (gnus-treat-article 'head))))))
+    (gnus-treat-article 'head)))
+
+(defun mu4e--view-gnus-display-mime (msg)
+  "Like `gnus-display-mime', but include mu4e headers to MSG."
+  (lambda (&optional ihandles)
+    (gnus-display-mime ihandles)
+    (unless ihandles
+      (save-restriction
+        (article-goto-body)
+        (forward-line -1)
+        (narrow-to-region (point) (point))
+        (mu4e--view-insert-headers msg)))))
 
 (defun mu4e--view-gnus-insert-header (field val)
   "Insert a header FIELD with value VAL."
   (let* ((info (cdr (assoc field mu4e-header-info)))
-         (key (plist-get info :name))
+         (key (or (plist-get info :name)
+                  ;; Fallback for fields not in mu4e-header-info
+                  ;; (e.g. :user-agent): derive from the keyword name.
+                  (capitalize (substring (symbol-name field) 1))))
          (help (plist-get info :help)))
     (if (and val (> (length val) 0))
         (insert (propertize (concat key ":") 'help-echo help)