diff --git a/tests/debug.html b/tests/debug.html
index 98b106f9c..d564c239d 100644
--- a/tests/debug.html
+++ b/tests/debug.html
@@ -285,6 +285,7 @@
     });
 
     map.on('load', async function () {
+        map.showTileBoundaries = true;
         const catalog = await fetch('http://0.0.0.0:3000/catalog');
         const sources = (await catalog.json()).tiles;
         // Set up the corresponding toggle button for each layer.