New Rmail option 'rmail-show-message-set-modified'

* lisp/mail/rmail.el (rmail-show-message-set-modified): New
option.
(rmail-show-message-1): If 'rmail-show-message-set-modified' is
non-nil, don't reset the buffer's modified state.  (Bug#45941)

* etc/NEWS: Announce the new option.

etc/NEWS

@@ -1138,6 +1138,11 @@ bindings, will be aborted, and Emacs will not ask you whether to
 enlarge 'max-specpdl-size' to complete the rendering.  The default is
 t, which preserves the original behavior.
 
+---
+*** New user option 'rmail-show-message-set-modified'.
+If set non-nil, showing an unseen message will set the Rmail buffer's
+modified flag.
+
 ** Apropos
 
 *** New commands 'apropos-next-symbol' and 'apropos-previous-symbol'.

lisp/mail/rmail.el

@@ -2723,6 +2723,12 @@ See also `unrmail-mbox-format'."
   :version "24.4"
   :group 'rmail-files)
 
+(defcustom rmail-show-message-set-modified nil
+  "If non-nil, displaying an unseen message marks the Rmail buffer as modified."
+  :type 'boolean
+  :group 'rmail
+  :version "28.1")
+
 (defun rmail-show-message-1 (&optional msg)
   "Show message MSG (default: current message) using `rmail-view-buffer'.
 Return text to display in the minibuffer if MSG is out of
@@ -2750,6 +2756,8 @@ The current mail message becomes the message displayed."
 	;; Mark the message as seen, but preserve buffer modified flag.
 	(let ((modiff (buffer-modified-p)))
 	  (rmail-set-attribute rmail-unseen-attr-index nil)
+          (and rmail-show-message-set-modified
+               (setq modiff t))
 	  (unless modiff
 	    (restore-buffer-modified-p modiff)))
 	;; bracket the message in the mail