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

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merged 2 commits into from Aug 11, 2022
Merged

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

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fix: ensure hidden iframe apps render in development mode
Aug 11, 2022
a5522d9
Merge branch 'canary' into fix-39340
ijjk Aug 11, 2022
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18 changes: 15 additions & 3 deletions packages/next/client/dev/fouc.ts
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@@ -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
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Summary of the issue:

  1. Hidden iframes are not guaranteed to ever run callbacks schedule through requestAnimationFrame. This table breaks it down browser to browser. For example, on Chrome, window.requestAnimationFrame is available, but the callback is never called in a hidden cross-origin iframe.
  2. displayContent (below) is passed as an option to beforeRender which is specifically await-ed before hydrating the app. If displayContent never resolves, the app never hydrates.

// 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) {
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Even though the issue is with hidden iframes, I think it's safe to scope this check to iframes in general (given that this is a dev feature. Open to feedback here and can also detect if it's hidden or not here.

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;
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