If a string value is assigned to notmuch-show-header-line, it's used
as a format string to be passed passed to format-spec with `%s`
substituted by the message's subject.  As before, t means displaying
the subject and nil not using any header line.

Signed-off-by: jao <jao@gnu.org>
---
 emacs/notmuch-show.el | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 7c1f02c9..f31c15e6 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -85,8 +85,18 @@ visible for any given message."
   :group 'notmuch-show)
 
 (defcustom notmuch-show-header-line t
-  "Show a header line with the current message's subject."
-  :type 'boolean
+  "Show a header line in notmuch show buffers.
+
+If t (the default), the header line will contain the current
+message's subject. If a string, this value is interpreted as a
+format string to be passed to `format-spec` with `%s` as the
+substitution variable for the message's subject.  E.g., to
+display the subject trimmed to a maximum of 80 columns, you could
+use \"%>-80s\" as format.  If this variabale is set to nil, no
+header is displayed."
+  :type '(choice (const :tag "No header" ni)
+                 (const :tag "Subject" t)
+                 (string :tag "Format"))
   :group 'notmuch-show)
 
 (defcustom notmuch-show-relative-dates t
@@ -1351,11 +1361,14 @@ If no messages match the query return NIL."
        (lambda () (notmuch-show-set-prop :orig-tags (notmuch-show-get-tags))))
       ;; Set the header line to the subject of the first message.
       (when notmuch-show-header-line
-	(setq header-line-format
-	      (replace-regexp-in-string "%" "%%"
-					(notmuch-sanitize
-					 (notmuch-show-strip-re
-					  (notmuch-show-get-subject))))))
+        (let* ((s (replace-regexp-in-string "%" "%%"
+                                            (notmuch-sanitize
+                                             (notmuch-show-strip-re
+                                              (notmuch-show-get-subject)))))
+               (h (cond ((stringp notmuch-show-header-line)
+                         (format-spec notmuch-show-header-line `((?s . ,s))))
+			(notmuch-show-header-line s))))
+          (setq header-line-format h)))
       (run-hooks 'notmuch-show-hook)
       (if state
 	  (notmuch-show-apply-state state)
-- 
2.36.0

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