New customizable variable, notmuch-tree-thread-symbols, that allows
tweaking of how trees in a forest are represented.  For instance, one
can now choose to use an hyphen rather than a white space as a prefix,
or replace the character(s) used to draw arrows.
---
 emacs/notmuch-tree.el | 44 +++++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/emacs/notmuch-tree.el b/emacs/notmuch-tree.el
index b48a132d..485e89a8 100644
--- a/emacs/notmuch-tree.el
+++ b/emacs/notmuch-tree.el
@@ -74,6 +74,30 @@
       notmuch-unthreaded-show-out
     notmuch-tree-show-out))
 
+(defcustom notmuch-tree-thread-symbols
+  '((prefix . " ")
+    (top . "─")
+    (top-tee . "┬")
+    (vertical . "│")
+    (vertical-tee . "├")
+    (bottom . "╰")
+    (arrow . "►"))
+  "Strings used to draw trees in notmuch tree results.
+Symbol keys denote where the corresponding string value is used:
+`prefix' is used at the top of the tree, followed by `top' if it
+has no children or `top-tee' if it does; `vertical' is a bar
+connecting with a response down the list skipping the current
+one, while `vertical-tee' marks the current message as a reply to
+the previous one; `bottom' is used at the bottom of threads.
+Finally, the `arrrow' string in the list is used as a pointer to
+every message.
+
+Common customizations include setting `prefix' to \"-\", to see
+equal-length prefixes, and `arrow' to an empty string or to a
+different kind of arrow point."
+  :type '(alist :key-type symbol :value-type string)
+  :group 'notmuch-tree)
+
 (defcustom notmuch-tree-result-format
   `(("date" . "%12s  ")
     ("authors" . "%-20s")
@@ -980,20 +1004,20 @@ message together with all its descendents."
 	(replies (cadr tree)))
     (cond
      ((and (< 0 depth) (not last))
-      (push "├" tree-status))
+      (push (alist-get 'vertical-tee  notmuch-tree-thread-symbols) tree-status))
      ((and (< 0 depth) last)
-      (push "╰" tree-status))
+      (push (alist-get 'bottom notmuch-tree-thread-symbols) tree-status))
      ((and (eq 0 depth) first last)
-      ;; Choice between these two variants is a matter of taste.
-      ;; (push "─" tree-status))
-      (push " " tree-status))
+      (push (alist-get 'prefix notmuch-tree-thread-symbols) tree-status))
      ((and (eq 0 depth) first (not last))
-      (push "┬" tree-status))
+      (push (alist-get 'top-tee notmuch-tree-thread-symbols) tree-status))
      ((and (eq 0 depth) (not first) last)
-      (push "╰" tree-status))
+      (push (alist-get 'bottom notmuch-tree-thread-symbols) tree-status))
      ((and (eq 0 depth) (not first) (not last))
-      (push "├" tree-status)))
-    (push (concat (if replies "┬" "─") "►") tree-status)
+      (push (alist-get 'vertical-tee notmuch-tree-thread-symbols) tree-status)))
+    (push (concat (alist-get (if replies 'top-tee 'top) notmuch-tree-thread-symbols)
+		  (alist-get 'arrow notmuch-tree-thread-symbols))
+	  tree-status)
     (setq msg (plist-put msg :first (and first (eq 0 depth))))
     (setq msg (plist-put msg :tree-status tree-status))
     (setq msg (plist-put msg :orig-tags (plist-get msg :tags)))
@@ -1002,7 +1026,7 @@ message together with all its descendents."
     (pop tree-status)
     (if last
 	(push " " tree-status)
-      (push "│" tree-status))
+      (push (alist-get 'vertical notmuch-tree-thread-symbols 3) tree-status))
     (notmuch-tree-insert-thread replies (1+ depth) tree-status)))
 
 (defun notmuch-tree-insert-thread (thread depth tree-status)
-- 
2.33.0
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