mu4e/thread: only when available

Fixes #2479.

mu4e/mu4e-headers.el

@@ -868,8 +868,11 @@ after the end of the search results."
                 mu4e--search-msgid-target nil)
           (when (mu4e~headers-docid-at-point)
             (mu4e~headers-highlight (mu4e~headers-docid-at-point)))))
-    ;; run-hooks
-    (run-hooks 'mu4e-headers-found-hook))
+  ;; maybe enable thread folding
+  (when mu4e-search-threads
+    (mu4e-thread-mode))
+  ;; run-hooks
+  (run-hooks 'mu4e-headers-found-hook))
 
 
 ;;; Marking
@@ -932,12 +935,6 @@ after the end of the search results."
     (define-key map (kbd "<C-kp-subtract>")
                 #'mu4e-headers-split-view-shrink)
 
-    ;; threads
-    (define-key map (kbd "<S-left>")  #'mu4e-thread-goto-root)
-    (define-key map (kbd "<tab>")     #'mu4e-thread-fold-toggle-goto-next)
-    (define-key map (kbd "<C-tab>")   #'mu4e-thread-fold-toggle-goto-next)
-    (define-key map (kbd "<backtab>") #'mu4e-thread-fold-toggle-all)
-
     ;; switching to view mode (if it's visible)
     (define-key map "y" #'mu4e-select-other-view)
 

mu4e/mu4e-thread.el

@@ -32,10 +32,14 @@
 ;; Similarly, when a thread has marked messages, the folding stops at the first
 ;; marked message.
 
+;; Note, you can only use these functions when threads are available, roughly
+;; when `mu4e-search-threads' in non-nil.
+
 ;;; Usage example:
 ;;
-;; This enforces folding after a new search
-;; (add-hook 'mu4e-headers-found-hook #'mu4e-thread-fold-apply-all)
+;; After a search, mu4e-thread-mode will be enable when threads
+;; are available; so, to automatically sort them:
+;;  (add-hook 'mu4e-thread-mode-hook #'mu4e-thread-fold-apply-all)
 
 ;;; Code:
 
@@ -273,5 +277,19 @@ Reset individual folding states."
     (mu4e-thread-unfold)
     (mu4e-thread-goto-next)))
 
+(define-minor-mode mu4e-thread-mode
+  "Mode for thread-support."
+  :global nil
+  :init-value nil ;; disabled by default
+  :group 'mu4e
+  :lighter ""
+  :keymap
+  (let ((map (make-sparse-keymap)))
+    (define-key map (kbd "<S-left>")  #'mu4e-thread-goto-root)
+    (define-key map (kbd "<tab>")     #'mu4e-thread-fold-toggle-goto-next)
+    (define-key map (kbd "<C-tab>")   #'mu4e-thread-fold-toggle-goto-next)
+    (define-key map (kbd "<backtab>") #'mu4e-thread-fold-toggle-all)
+    map))
+
 (provide 'mu4e-thread)
 ;;; mu4e-thread.el ends here

mu4e/mu4e.texi

@@ -1118,15 +1118,19 @@ Folding stops at the @emph{first unread message}, unless you set
 the folding stops at the first marked message. Marking folded messages is not
 allowed as it is too error-prone.
 
+Thread-mode functionality is only available with @code{mu4e-search-threads}
+enabled; this triggers a minor mode @code{mu4e-thread-mode} in the headers-view.
+For now, this functionality is not available in the message view, due to the
+conflicting key bindings.
+
 If you want to automatically fold all threads after a query, you can use a hook:
 @lisp
- (add-hook 'mu4e-headers-found-hook #'mu4e-thread-fold-apply-all)
+  (add-hook 'mu4e-thread-mode-hook #'mu4e-thread-fold-apply-all)
 @end lisp
 
-By default, single-child threads are @emph{not} collapsed, since it would simply
-result in replacing a single line with the collapsed one. However, if, for
-consistency you also want to fold those, you can use
-@t{mu4e-thread-fold-single-children}.
+By default, single-child threads are @emph{not} collapsed, since it would result
+in replacing a single line with the collapsed one. However, if, for consistency,
+you also want to fold those, you can use @t{mu4e-thread-fold-single-children}.
 
 @node HV Custom headers
 @section Custom headers