This changes `seen' to mean that the user viewed `enough' of the whole
message: more precisely, a message is deemed seen if the top of the
message and either the bottom of the message or a point at least some
customisable number of lines into the message have each been visible
in the buffer at some point.

This is placed into the post-command-hook infrastructure introudced in
the previous patch.
---
 emacs/notmuch-show.el |   74 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 7d0ac5d..6451314 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -211,6 +211,20 @@ For example, if you wanted to remove an \"unread\" tag and add a
   :type '(repeat string)
   :group 'notmuch-show)
 
+(defcustom notmuch-show-seen-lines-needed 0.75
+  "Control which messages get marked seen.
+
+A message is marked seen if both the top of the message and a
+point far \"enough\" down in the message have each been visible
+in the buffer at some point. This parameter controls the
+definition of enough.  Seeing the bottom of message is always
+deemed enough, but additionally...it an integer n then at least n
+lines of message must be visible in the window. If it is a float
+x then at least that proportion of the window must contain the
+message."
+  :type 'number
+  :group 'notmuch-show)
+
 (defface notmuch-show-deleted-tag-face
   '((t :strike-through "red" :inherit 'notmuch-tag-face))
   "Face for tags that have been removed"
@@ -1551,9 +1565,65 @@ marked as unread, i.e. the tag changes in
     (apply 'notmuch-show-tag-message
 	   (notmuch-tag-change-list notmuch-show-mark-read-tags unread))))
 
+(defun notmuch-show-update-seen (top-or-bottom)
+  "Update seen status of current message
+
+Mark that we have seen the TOP-OR-BOTTOM of current message."
+  (let* ((current (notmuch-show-get-prop :seen-local))
+	 new)
+    (unless (eq current 'both)
+      (if (eq top-or-bottom 'top)
+	  (if (eq current 'bottom)
+	      (setq new 'both)
+	    (setq new 'top))
+	(if (eq current 'top)
+	    (setq new 'both)
+	  (setq new 'bottom)))
+      (unless (eq current new)
+	(notmuch-show-set-prop :seen-local new))
+      (when (eq new 'both)
+	(notmuch-show-mark-read)))))
+
+(defun notmuch-show-do-message-seen (start end)
+  "Update seen status for the current message.
+
+A message is seen if both the top and enough of the rest of the
+message have been visible in the buffer.  Enough means either the
+bottom of the message or a point in the message more than
+LINES-NEEDED lines into the message. LINES-NEEDED is
+`notmuch-show-seen-lines-needed` if that is an integer and that
+times the current window height if it is a float."
+  (let* ((lines-needed (if (integerp notmuch-show-seen-lines-needed)
+			   notmuch-show-seen-lines-needed
+			 (truncate (* notmuch-show-seen-lines-needed (window-body-height)))))
+	 (top (notmuch-show-message-top))
+	 (bottom (notmuch-show-message-bottom)))
+    (when (notmuch-show-message-visible-p)
+      (when (>= top start)
+	(notmuch-show-update-seen 'top))
+      (when (or (<= bottom end)
+		(> (count-screen-lines top end) lines-needed))
+	(notmuch-show-update-seen 'bottom)))))
+
 (defun notmuch-show-do-seen (start end)
-  "Update seen status for all messages between start and end."
-  )
+  "Update seen status for all messages between start and end.
+
+A message is seen if both the top and enough of the rest of the
+message have been visible in the buffer. See
+`notmuch-show-do-message-seen` for the definition of enough. Seen
+is a buffer local property. The unread status is removed from all
+seen messages when the user quits the show buffer."
+  (save-excursion
+    (goto-char start)
+    (while (and (or (notmuch-show-do-message-seen start end) t)
+		(< (notmuch-show-message-bottom) end)
+		(notmuch-show-goto-message-next)))
+
+    ;; This is a work around because emacs gives weird answers for
+    ;; window-end if the buffer ends with invisible text.
+    (when (and (pos-visible-in-window-p (point-max))
+	       (notmuch-show-message-visible-p))
+      (notmuch-show-update-seen 'bottom))))
 
 (defun notmuch-show-command-hook ()
   (when (eq major-mode 'notmuch-show-mode)
-- 
1.7.9.1