fix: ensure hidden iframe apps render in development mode (#39514)

packages/next/client/dev/fouc.ts

@@ -1,12 +1,24 @@
-// This wrapper function is used to avoid raising a Trusted Types violation.
-const safeSetTimeout = (callback: () => void) => setTimeout(callback)
+// This wrapper function is used to safely select the best available function
+// to schedule removal of the no-FOUC styles workaround. requestAnimationFrame
+// is the ideal choice, but when used in iframes, there are no guarantees that
+// the callback will actually be called, which could stall the promise returned
+// from displayContent.
+//
+// See: https://www.vector-logic.com/blog/posts/on-request-animation-frame-and-embedded-iframes
+const safeCallbackQueue = (callback: () => void) => {
+  if (window.requestAnimationFrame && window.self === window.top) {
+    window.requestAnimationFrame(callback)
+  } else {
+    window.setTimeout(callback)
+  }
+}
 
 // This function is used to remove Next.js' no-FOUC styles workaround for using
 // `style-loader` in development. It must be called before hydration, or else
 // rendering won't have the correct computed values in effects.
 export function displayContent(): Promise<void> {
   return new Promise((resolve) => {
-    ;(window.requestAnimationFrame || safeSetTimeout)(function () {
+    safeCallbackQueue(function () {
       for (
         var x = document.querySelectorAll('[data-next-hide-fouc]'),
           i = x.length;