emacs: show: add an update seen function to post-command-hook

Add a function for updating seen messages to the
post-command-hook. This function calls a customizable (by eg
defcustom) function with parameters the start and end of the current
window and that function can decide what to mark read based on that
and the current point.

Since this is in the post-command-hook it should get called after most
user actions (exceptions include user resizing the window) so it
should be possible to make sure the seen status gets updated whether
the user uses notmuch commands like next-message or normal emacs
commands like scroll-up.

It removes all of the old mark read/seen points but introduces a
simple example function that just marks the current message read if it
is open. This function has one small subtlety: it makes sure it
doesn't mark the same message read twice (in the same instance of the
same buffer); otherwise the post-command-hook makes it impossible for
a user to manually mark a message unread.

This fixes the current bugs (imo) that closed messages can be marked
read, and that opening a closed message does not mark it read.

Another advantage of using the post-command-hook any programmatic use
with point passing through a message will not mark it read.

emacs/notmuch-show.el

@@ -222,6 +222,10 @@ For example, if you wanted to remove an \"unread\" tag and add a
   :type '(repeat string)
   :group 'notmuch-show)
 
+(defcustom notmuch-show-mark-read-function #'notmuch-show-seen-current-message
+  "Function to control which messages are marked read."
+  :type 'function
+  :group 'notmuch-show)
 
 (defmacro with-current-notmuch-show-message (&rest body)
   "Evaluate body with current buffer set to the text of current message"
@@ -1156,6 +1160,8 @@ function is used."
   (let ((inhibit-read-only t))
 
     (notmuch-show-mode)
+    (add-hook 'post-command-hook #'notmuch-show-command-hook nil t)
+
     ;; Don't track undo information for this buffer
     (set 'buffer-undo-list t)
 
@@ -1544,6 +1550,23 @@ 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-seen-current-message (start end)
+  "Mark the current message read if it is open.
+
+We only mark it read once: if it is changed back then that is a
+user decision and we should not override it."
+  (when (and (notmuch-show-message-visible-p)
+	     (not (notmuch-show-get-prop :seen)))
+	(notmuch-show-mark-read)
+	(notmuch-show-set-prop :seen t)))
+
+(defun notmuch-show-command-hook ()
+  (when (eq major-mode 'notmuch-show-mode)
+    ;; We need to redisplay to get window-start and window-end correct.
+    (redisplay)
+    (save-excursion
+      (funcall notmuch-show-mark-read-function (window-start) (window-end)))))
+
 ;; Functions for getting attributes of several messages in the current
 ;; thread.
 
@@ -1679,9 +1702,7 @@ If a prefix argument is given and this is the last message in the
 thread, navigate to the next thread in the parent search buffer."
   (interactive "P")
   (if (notmuch-show-goto-message-next)
-      (progn
+      (notmuch-show-message-adjust)
-	(notmuch-show-mark-read)
-	(notmuch-show-message-adjust))
     (if pop-at-end
 	(notmuch-show-next-thread)
       (goto-char (point-max)))))
@@ -1692,7 +1713,6 @@ thread, navigate to the next thread in the parent search buffer."
   (if (= (point) (notmuch-show-message-top))
       (notmuch-show-goto-message-previous)
     (notmuch-show-move-to-message-top))
-  (notmuch-show-mark-read)
   (notmuch-show-message-adjust))
 
 (defun notmuch-show-next-open-message (&optional pop-at-end)
@@ -1707,9 +1727,7 @@ to show, nil otherwise."
     (while (and (setq r (notmuch-show-goto-message-next))
 		(not (notmuch-show-message-visible-p))))
     (if r
-	(progn
+	(notmuch-show-message-adjust)
-	  (notmuch-show-mark-read)
-	  (notmuch-show-message-adjust))
       (if pop-at-end
 	  (notmuch-show-next-thread)
 	(goto-char (point-max))))
@@ -1722,9 +1740,7 @@ to show, nil otherwise."
     (while (and (setq r (notmuch-show-goto-message-next))
 		(not (notmuch-show-get-prop :match))))
     (if r
-	(progn
+	(notmuch-show-message-adjust)
-	  (notmuch-show-mark-read)
-	  (notmuch-show-message-adjust))
       (goto-char (point-max)))))
 
 (defun notmuch-show-open-if-matched ()
@@ -1735,8 +1751,7 @@ to show, nil otherwise."
 (defun notmuch-show-goto-first-wanted-message ()
   "Move to the first open message and mark it read"
   (goto-char (point-min))
-  (if (notmuch-show-message-visible-p)
+  (unless (notmuch-show-message-visible-p)
-      (notmuch-show-mark-read)
     (notmuch-show-next-open-message))
   (when (eobp)
     ;; There are no matched non-excluded messages so open all matched
@@ -1744,8 +1759,7 @@ to show, nil otherwise."
     (notmuch-show-mapc 'notmuch-show-open-if-matched)
     (force-window-update)
     (goto-char (point-min))
-    (if (notmuch-show-message-visible-p)
+    (unless (notmuch-show-message-visible-p)
-	(notmuch-show-mark-read)
       (notmuch-show-next-open-message))))
 
 (defun notmuch-show-previous-open-message ()
@@ -1755,7 +1769,6 @@ to show, nil otherwise."
 		  (notmuch-show-goto-message-previous)
 		(notmuch-show-move-to-message-top))
 	      (not (notmuch-show-message-visible-p))))
-  (notmuch-show-mark-read)
   (notmuch-show-message-adjust))
 
 (defun notmuch-show-view-raw-message ()