test: scope no-js preview visibility gate

scripts/verify-no-js-preview.mjs

@@ -2,20 +2,27 @@ import { readFile } from "node:fs/promises";
 import { resolve } from "node:path";
 
 const outputPath = resolve(".next/server/app/index.html");
-const html = await readFile(outputPath, "utf8");
+const source = await readFile(resolve("src/app/page.tsx"), "utf8");
 
 // The placeholder is the first frame for a newly-created project. It must stay
 // visible when a client chunk is delayed or rejected; later product pages are
 // free to replace it and choose their own bounded animation policy.
-if (html.includes("Fluw template")) {
+if (source.includes("Fluw template")) {
+  const html = await readFile(outputPath, "utf8");
   const main = html.match(/<main\b[\s\S]*?<\/main>/i)?.[0];
   if (!main) {
     throw new Error("Fluw placeholder is missing its main landmark");
   }
+  const criticalElement = main.match(
+    /<[^>]*\bdata-fluw-preview-critical="true"[^>]*>/i,
+  )?.[0];
+  if (!criticalElement) {
+    throw new Error("Fluw placeholder is missing its preview-critical element");
+  }
 
   if (
     /style="[^"]*(?:opacity\s*:\s*0(?:[;"]|\s)|visibility\s*:\s*hidden|display\s*:\s*none)/i.test(
-      main,
+      criticalElement,
     )
   ) {
     throw new Error(