mu4e: support jumping to previous/next thread

in headers view, message view with '{' and '}.
Also update documentation.

Fixes #1523

mu4e/mu4e-headers.el

@@ -923,6 +923,9 @@ after the end of the search results."
     (define-key map (kbd "[") #'mu4e-headers-prev-unread)
     (define-key map (kbd "]") #'mu4e-headers-next-unread)
 
+    (define-key map (kbd "{") #'mu4e-headers-prev-thread)
+    (define-key map (kbd "}") #'mu4e-headers-next-thread)
+
     ;; change the number of headers
     (define-key map (kbd "C-+") #'mu4e-headers-split-view-grow)
     (define-key map (kbd "C--") #'mu4e-headers-split-view-shrink)
@@ -1467,7 +1470,7 @@ previous header."
 (defun mu4e~headers-prev-or-next-unread (backwards)
   "Move point to the next message that is unread (and
 untrashed). If BACKWARDS is non-`nil', move backwards."
-  (interactive)
+  (interactive "P")
   (or (mu4e-headers-find-if-next
        (lambda (msg)
          (let ((flags (mu4e-message-field msg :flags)))
@@ -1488,6 +1491,26 @@ untrashed)."
   (interactive)
   (mu4e~headers-prev-or-next-unread nil))
 
+
+(defun mu4e~headers-prev-or-next-thread (backwards)
+  "Move point to the top of the next thread.
+If BACKWARDS is non-`nil', move backwards."
+  (interactive "P")
+  (or (mu4e-headers-find-if-next
+       (lambda (msg)
+         (eq 0 (plist-get (plist-get msg :meta) :level)))
+       backwards)
+      (mu4e-message (format "No %s thread found"
+                            (if backwards "previous" "next")))))
+
+(defun mu4e-headers-prev-thread ()
+  "Move point to the previous thread."
+  (interactive) (mu4e~headers-prev-or-next-thread t))
+
+(defun mu4e-headers-next-thread ()
+  "Move point to the previous thread."
+  (interactive) (mu4e~headers-prev-or-next-thread nil))
+
 (defun mu4e-headers-split-view-grow (&optional n)
   "In split-view, grow the headers window.
 In horizontal split-view, increase the number of lines shown by N.

mu4e/mu4e-view.el

@@ -193,14 +193,13 @@ previous header."
   (mu4e--view-in-headers-context
    (mu4e~headers-move (- (or n 1)))))
 
-(defun mu4e--view-prev-or-next-unread (backwards)
+(defun mu4e--view-prev-or-next (func backwards)
   "Move point to the next or previous message.
 Go to the previous message if BACKWARDS is non-nil.
 unread message header in the headers buffer connected with this
 message view. If this succeeds, return the new docid. Otherwise,
 return nil."
-  (mu4e--view-in-headers-context
-   (mu4e~headers-prev-or-next-unread backwards))
+  (mu4e--view-in-headers-context (funcall func backwards))
   (mu4e-select-other-view)
   (mu4e-headers-view-message))
 
@@ -208,13 +207,25 @@ return nil."
   "Move point to the previous unread message header.
 If this succeeds, return the new docid. Otherwise, return nil."
   (interactive)
-  (mu4e--view-prev-or-next-unread t))
+  (mu4e--view-prev-or-next #'mu4e~headers-prev-or-next-unread t))
 
 (defun mu4e-view-headers-next-unread ()
   "Move point to the next unread message header.
 If this succeeds, return the new docid. Otherwise, return nil."
   (interactive)
-  (mu4e--view-prev-or-next-unread nil))
+  (mu4e--view-prev-or-next #'mu4e~headers-prev-or-next-unread nil))
+
+(defun mu4e-view-headers-prev-thread()
+  "Move point to the previous thread.
+If this succeeds, return the new docid. Otherwise, return nil."
+  (interactive)
+  (mu4e--view-prev-or-next #'mu4e~headers-prev-or-next-thread t))
+
+(defun mu4e-view-headers-next-thread()
+  "Move point to the previous thread.
+If this succeeds, return the new docid. Otherwise, return nil."
+  (interactive)
+  (mu4e--view-prev-or-next #'mu4e~headers-prev-or-next-thread nil))
 
 
 ;;; Interactive functions
@@ -907,6 +918,8 @@ This is useful for advising some Gnus-functionality that does not work in mu4e."
 
     (define-key map (kbd "[") #'mu4e-view-headers-prev-unread)
     (define-key map (kbd "]") #'mu4e-view-headers-next-unread)
+    (define-key map (kbd "{") #'mu4e-view-headers-prev-thread)
+    (define-key map (kbd "}") #'mu4e-view-headers-next-thread)
 
     ;; switching from view <-> headers (when visible)
     (define-key map "y" #'mu4e-select-other-view)

mu4e/mu4e.texi

@@ -978,7 +978,8 @@ key          description
 ===========================================================
 n,p          view the next, previous message
 ],[          move to the next, previous unread message
-y            select the message view (if it's visible)
+},{          move to the next, previous thread
+y            select the message view (if visible)
 RET          open the message at point in the message view
 
 searching
@@ -1304,7 +1305,8 @@ key          description
 ==============================================================
 n,p          view the next, previous message
 ],[          move to the next, previous unread message
-y            select the headers view (if it's visible)
+},{          move to the next, previous thread
+y            select the headers view (if visible)
 
 RET          scroll down
 M-RET        open URL at point / attachment at point