diff --git a/script/run.ts b/script/run.ts
index ef7dea8..659d2ac 100644
--- a/script/run.ts
+++ b/script/run.ts
@@ -260,19 +260,27 @@ namespace Flat {
 
 export function fadeTo(newPane: Pane): void {
   for (const pane of allPanes) {
-    const here = pane == newPane;
     const elem = document.getElementById(pane)!;
-    if (here) {
+    if (pane == newPane) {
+      // show new pane
+      //
+      // - "entering" state is displayed and z-index 1, but opacity 0
+      // - "active" state is fully showing
+      //
+      // "entering" exists because firefox doesn't animate a
+      // transition out of `display: none`
       elem.dataset.state = 'entering';
       setTimeout(() => elem.dataset.state = 'active');
-      // elem.addEventListener('transitionend',
-      //                       () => elem.dataset.state = 'active',
-      //                       {once: true});
       history.replaceState(null, '🦎', `#${pane}`);
     } else if (elem.dataset.state == 'active') {
-      elem.addEventListener('transitionend',
-                            () => elem.dataset.state = 'hidden',
-                            {once: true});
+      // hide old pane
+      //
+      // - "leaving" is z-index -1 and opacity 0
+      // - "hidden" is `display: none`
+      //
+      // separate for the same reason as above
+      function hide() { elem.dataset.state = 'hidden'; }
+      elem.addEventListener('transitionend', hide, {once: true});
       elem.dataset.state = 'leaving';
     }
   }