Use olivetti-mode to improve Org tree-slide presentations

This is motivated by a post by Daniel Liden (https://gitlab.com/dliden),
see https://www.danliden.com/posts/20231217-emacs-hooks.html (courtesy
of https://irreal.org/blog/?p=11849).

init.el

@@ -472,6 +472,20 @@
              mc/mark-previous-like-this
              mc/mark-all-like-this))
 
+(use-package olivetti
+  :ensure t
+  :commands (olivetti-mode)
+  :preface (progn
+             (defun turn-on-olivetti-mode ()
+               "Turn on `olivetti-mode'."
+               (interactive)
+               (olivetti-mode 1))
+             (defun turn-off-olivetti-mode ()
+               "Turn off `olivetti-mode'."
+               (interactive)
+               (olivetti-mode -1)))
+  :init (setq-default olivetti-body-width 0.618034))
+
 (use-package undo-tree
   :ensure t
   :commands (global-undo-tree-mode
@@ -1243,7 +1257,9 @@
   :bind (:map org-tree-slide-mode-map
               ("<C-down>" . org-tree-slide-display-header-toggle)
               ("<C-right>" . org-tree-slide-move-next-tree)
-              ("<C-left>" . org-tree-slide-move-previous-tree)))
+              ("<C-left>" . org-tree-slide-move-previous-tree))
+  :hook ((org-tree-slide-play . turn-on-olivetti-mode)
+         (org-tree-slide-stop . turn-off-olivetti-mode)))
 
 (use-package org-roam
   :init (progn