diff --git a/config.el b/config.el
index 0cd73fd..cbb661d 100644
--- a/config.el
+++ b/config.el
@@ -185,6 +185,13 @@
 
 ;; Configure undo-tree.
 (after! undo-tree
+  ;; Doom's default is to popup on the left, but undo-tree can get really wide.
+  ;; It would actually be better to open it in its own window as is the default,
+  ;; but then the diff window has annoying graphical glitches which for some
+  ;; reason don't happen when a popup is used with the diff window configuration
+  ;; hack further below.
+  (set-popup-rule! undo-tree-visualizer-buffer-name
+    :slot 2 :side 'top :size 20 :select t :quit t)
   ;; Doom just gives up on the diff visualizer, because it is impossible to work
   ;; around undo-tree's window management opinions in a consistent way. For now,
   ;; hack it to work in some sensible way.