mu4e: when in-reply-to: is removed, remove references: too

When user removes the In-Reply-To header, also remove the (hidden)
References header when sending the message, effectively making the
message appear at the top-level.

Mention in the doc, NEWS.

NEWS.org

@@ -21,7 +21,12 @@
      one.
    - allow for transforming mailing-list names for display, using
      `mu4e-mailing-list-patterns'.
-   - some optimizations in indexing
+   - some optimizations in indexing (~30% faster in some cases)
+   - new variable mu4e-user-agent-string, to customize the User-Agent:
+     header.
+   - when removing the "In-reply-to" header from replies, mu4e will
+     also remove the (hidden) References header, effectively creating
+     a new message-thread.
   
 ** 0.9.13
 

mu4e/mu4e-compose.el

@@ -264,6 +264,14 @@ Ie. either 'name <email>' or 'email')."
   (add-hook 'completion-at-point-functions
     'mu4e~compose-complete-contact nil t))
 
+
+(defun mu4e~remove-refs-maybe ()
+  "Remove the References: header if the In-Reply-To header is
+missing. This allows the user to effectively start a new
+message-thread by removing the In-Reply-To header."
+  (unless (message-fetch-field "in-reply-to")
+    (message-remove-header "References")))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 (defvar mu4e-compose-mode-map nil
   "Keymap for \"*mu4e-compose*\" buffers.")
@@ -303,6 +311,8 @@ Ie. either 'name <email>' or 'email')."
     ;; setup the fcc-stuff, if needed
     (add-hook 'message-send-hook
       (lambda () ;; mu4e~compose-save-before-sending
+	;; when in-reply-to was removed, remove references as well.
+	(mu4e~remove-refs-maybe)
 	;; for safety, always save the draft before sending
 	(set-buffer-modified-p t)
 	(save-buffer)
@@ -621,6 +631,8 @@ end of the buffer."
 (define-key mu4e-compose-mode-map
   (vector 'remap 'end-of-buffer) 'mu4e-compose-goto-bottom)
 
+
+
 (provide 'mu4e-compose)
 
 ;; Load mu4e completely even when this file was loaded through

mu4e/mu4e.texi

@@ -533,8 +533,8 @@ updates - for example when receiving new mail. See
 writing mail and inherits the setup for sending mail as well.
 
 For sending mail using @abbr{SMTP}, @t{mu4e} uses @t{smtpmail}
-(@inforef{Top,,smtpmail}). This package supports many different ways to send
-mail; please refer to its documentation for the details.
+(@inforef{Top,,smtpmail}). This package supports many different ways to
+send mail; please refer to its documentation for the details.
 
 Here, we only provide some simple examples - for more, see @ref{Example
 configurations}.
@@ -3444,9 +3444,13 @@ You can drag-and-drop from your desktop; alternatively, you can use @t{dired}
 @item @emph{@t{mu4e} seems to remove myself from the @t{Cc:}-list; how can I
 prevent that?}  Set @code{mu4e-compose-keep-self-cc} to @t{t} in your
 configuration.
+@item @emph{How can I start a new message-thread from a reply?} Remove the @t{In-Reply-To} header,
+and @t{mu4e} automatically removes the (hidden) @t{References} header as
+well when sending it. This makes the message show up as a top-level
+message rather than as a response.
 @item @emph{How can I sign or encrypt messages?} You can do so using @command{emacs}'
-MIME-support -- check the @t{Attachments}-menu while composing a message. Also
-see @ref{Signing and encrypting}.
+MIME-support -- check the @t{Attachments}-menu while composing a
+message. Also see @ref{Signing and encrypting}.
 @item @emph{Can I use @t{BBDB} with @t{mu4e}?} Yes, with the current
 (2015-06-23) development release of BBDB
 @url{http://savannah.nongnu.org/projects/bbdb/}, or releases of BBDB