mu4e: experimental :thread-subject field (one subject per thread)

with :thread-subject field, we attempt to only show one subject per
thread, somewhat like mutt does it.

the current implementation is straightforward, but does not take into
account whether the thread-subject is currently visible on the screen,
which is a bit tricky to implement

mu4e/mu4e-headers.el

@@ -434,6 +434,19 @@ date. The formats used for date and time are
 	  (format-time-string mu4e-headers-date-format date))))))
 
 
+(defsubst mu4e~headers-thread-subject (msg)
+  "Get the subject if it is the first one in a thread; otherwise,
+return the thread-prefix without the subject-text. In other words,
+show the subject of a thread only once, similar to e.g. 'mutt'."
+  (let ((tinfo  (mu4e-message-field msg :thread))
+	 (subj (mu4e-msg-field msg :subject)))
+    (concat ;; prefix subject with a thread indicator
+      (mu4e~headers-thread-prefix tinfo)
+      (if (or (zerop (plist-get tinfo :level))
+	    (plist-get tinfo :empty-parent))
+	(truncate-string-to-width subj 600) ""))))
+
+
 (defsubst mu4e~headers-mailing-list (list)
   "Get some identifier for the mailing list."
   (if list
@@ -467,6 +480,7 @@ if provided, or at the end of the buffer otherwise."
 		  ;; work-around: emacs' display gets really slow when lines are too long;
 		  ;; so limit subject length to 600 
 		  (truncate-string-to-width val 600)))
+	      (:thread-subject (mu4e~headers-thread-subject msg))
 	      ((:maildir :path :message-id) val)
 	      ((:to :from :cc :bcc) (mu4e~headers-contact-str val))
 	      ;; if we (ie. `user-mail-address' is the 'From', show

mu4e/mu4e-vars.el

@@ -658,6 +658,11 @@ mu4e-compose-mode."
 	 :shortname "Tags"
 	 :help "Tags for the message"
 	 :sortable nil))
+     (:thread-subject .
+       ( :name "Subject"
+	 :shortname "Subject"
+	 :help "Subject of the thread"
+	 :sortable :subject))
      (:to .
        ( :name "To"
 	 :shortname "T"

mu4e/mu4e.texi

@@ -779,11 +779,15 @@ the @t{:date} field instead.
 @code{mu4e-headers-date-format} and @code{mu4e-headers-time-format},
 respectively. In the example, we use @code{:human-date}, which shows when the
 time when the message was sent today, and the date otherwise.
+@item By default, the subject is shown using the @t{:subject} field;
+however, it is also possible to use @t{:thread-subject}, which shows
+the subject of a thread only once, similar to the display of the
+@t{mutt} e-mail client.
 @item The header field used for sorting is indicated by ``@t{V}'' or
-``@t{^}''@footnote{or you can use little graphical triangles; see variable
-@code{mu4e-use-fancy-chars}}, corresponding to the sort order (descending or
-ascending, respectively). You can influence this by a mouse click, or
-@key{O}. Not all fields allow sorting.
+``@t{^}''@footnote{or you can use little graphical triangles; see
+variable @code{mu4e-use-fancy-chars}}, corresponding to the sort order
+(descending or ascending, respectively). You can influence this by a
+mouse click, or @key{O}. Not all fields allow sorting.
 @item Instead of showing the @t{From:} and @t{To:} fields separately, you
 can use From/To (@t{:from-or-to} in @code{mu4e-headers-fields} as a more
 compact way to convey the most important information: it shows @t{From:}
@@ -2984,10 +2988,10 @@ customize.
 ;; (better only use that for the last field.
 ;; These are the defaults:
 (setq mu4e-headers-fields
-    '( (:date          .  25)
+    '( (:date          .  25)    ;; alternatively, use :human-date
        (:flags         .   6)
        (:from          .  22)
-       (:subject       .  nil)))
+       (:subject       .  nil))) ;; alternatively, use :thread-subject
 
 ;; program to get mail; alternatives are 'fetchmail', 'getmail'
 ;; isync or your own shellscript. called when 'U' is pressed in