fix: keep empty preview visible before hydration

scripts/verify-no-js-preview.mjs

@@ -0,0 +1,27 @@
+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");
+
+// 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")) {
+  const main = html.match(/<main\b[\s\S]*?<\/main>/i)?.[0];
+  if (!main) {
+    throw new Error("Fluw placeholder is missing its main landmark");
+  }
+
+  if (
+    /style="[^"]*(?:opacity\s*:\s*0(?:[;"]|\s)|visibility\s*:\s*hidden|display\s*:\s*none)/i.test(
+      main,
+    )
+  ) {
+    throw new Error(
+      "Fluw placeholder hides content before client hydration; keep the first frame visible",
+    );
+  }
+}
+
+console.log("No-JS preview visibility verified");