diff --git a/config.org b/config.org
index 5a2df05..88205ec 100644
--- a/config.org
+++ b/config.org
@@ -2375,6 +2375,17 @@ Now that we have this word list, we can also plug it into ~cape-dict~ and get pr
   (add-hook! 'completion-at-point-functions :local :depth 40 #'cape-dict))
 #+end_src
 
+** Undo Tree
+
+#+call: confpkg("Pkg: undo-tree")
+
+#+begin_src emacs-lisp
+(after! undo-tree
+  (setq undo-tree-visualizer-diff nil))
+
+(set-popup-rule! "^ \\*undo-tree\\*" :slot 2 :side 'right :size 30 :select t :quit nil)
+#+end_src
+
 ** VTerm
 
 #+call: confpkg("Pkg: vterm")
@@ -4141,8 +4152,7 @@ If a side session is open, we'll configure =winum= to record it as having the in
 Operators being in italics looks ugly, so let's fix that.
 
 #+begin_src emacs-lisp
-(after! haskell-mode
-  (custom-set-faces! '(haskell-operator-face :slant normal)))
+(custom-set-faces! '(haskell-operator-face :slant normal))
 #+end_src
 
 ** Idris
@@ -4216,16 +4226,15 @@ The Doom module is very outdated, so I'll be overriding it.
 Operators being in italics looks ugly in this mode too, but due to Idris's more complicated syntax highlighting system we have to do a bit more work than for Haskell. There's also the issue of the semantic highlighting faces, which don't match our theme colors.
 
 #+begin_src emacs-lisp
-(after! idris-mode
-  (custom-set-faces!
-    '((idris-operator-face idris-colon-face idris-equals-face) :slant normal)
+(custom-set-faces!
+  '((idris-operator-face idris-colon-face idris-equals-face) :slant normal)
 
-    ;; Semantic highlighting
-    `(idris-semantic-type-face :foreground ,(doom-color 'blue))
-    `(idris-semantic-data-face :foreground ,(doom-color 'red))
-    `(idris-semantic-bound-face :foreground ,(doom-color 'magenta) :slant italic)
-    `(idris-semantic-function-face :foreground ,(doom-color 'dark-green))
-    `(idris-semantic-postulate-face :foreground ,(doom-color 'yellow))))
+  ;; Semantic highlighting
+  `(idris-semantic-type-face :foreground ,(doom-color 'blue))
+  `(idris-semantic-data-face :foreground ,(doom-color 'red))
+  `(idris-semantic-bound-face :foreground ,(doom-color 'magenta) :slant italic)
+  `(idris-semantic-function-face :foreground ,(doom-color 'dark-green))
+  `(idris-semantic-postulate-face :foreground ,(doom-color 'yellow)))
 #+end_src
 
 ** Indentation