diff --git a/src/heyarne/rect_packing/core.cljs b/src/heyarne/rect_packing/core.cljs
index 13d3342..be84301 100644
--- a/src/heyarne/rect_packing/core.cljs
+++ b/src/heyarne/rect_packing/core.cljs
@@ -50,20 +50,20 @@
 
 (defn main []
   (let [{:keys [frame rects]} @state
-        sorted (pack-rects-naive frame rects)]
+        sorted (map-indexed vector (pack-rects-naive frame rects))]
     [:main
      [:h1 "Visualization"]
      [:svg {:viewBox "-1 -1 501 501" :xmlns "http://www.w3.org/2000/svg"}
       [:rect {:width (geom/width frame)
               :height (geom/height frame)}]
-      (for [rect sorted]
-        [:rect {:width (geom/width rect) :height (geom/height rect)
-                :x (-> rect :p :x)
-                :y (-> rect :p :y)}])]
+      (for [[idx rect] sorted]
+        ^{:key idx} [:rect {:width (geom/width rect) :height (geom/height rect)
+                            :x (-> rect :p :x)
+                            :y (-> rect :p :y)}])]
      [:h2 "Rectangles"]
      [:ul
-      (for [rect sorted]
-        [:li [:pre (pr-str rect)]])]]))
+      (for [[idx rect] sorted]
+        ^{:key idx} [:li [:pre (pr-str rect)]])]]))
 
 (defn ^:dev/after-load init []
   (println "Initializing…")