This pull request replaces our client-side style transitions with `<style>` tags over async `<link rel=stylesheet>` tags. This should fix some edge cases users see with Chrome accidentally causing a FOUC.

This also removes the need to perform an async operation before starting the render, which should remove any perceivable navigation delay.

---

Fixes #16289

This pull request reuses existing `<link rel=stylesheet>` tags if their `href` matches instead of recreating it. This is in effort to fix an edge case where the browser will FOUC on the tag swap.

This behavior should be sufficiently covered by all the existing CSS cases, as misbehavior would result in the resulting CSS styles being incorrect.

This pull request correctly tracks render cancelation behavior. Prior to this PR, we'd have an unhandled rejection that left the app in a bad state and no routeChangeError event was fired.

---

Closes #16424
Fixes #16445

This fixes an edge case where every dozen or so transitions you'll see a flash depending on what's happening on the main thread at the time.

I'm not sure it's possible to test for this case, so we'll just have to do more field testing with this.

This fixes an error that was occurring in ie11 due to `forEach` being called on `querySelectorAll` before it was massaged to an array.

x-ref: https://github.com/vercel/next.js/pull/16126
Fixes #16283

This pull request adds a test case for the reproduction provided in #12445. This bug is specifically caused when loading the next page before navigation has actually occurred.

---

Fixes #12445

Fixes #16166

Fixes https://github.com/vercel/next.js/issues/15755

Caught this while reviewing router code for https://github.com/vercel/next.js/pull/15710

`pageProps` should always be defined to ensure everything is working as expected although to prevent a breaking change this adds an additional check before attempting to access `pageProps` before hydration. It also adds tests to prevent regressing on this

Closes: https://github.com/vercel/next.js/issues/15647

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Preparation for a larger rewrite

* Avoid adding basePath when it's not needed

When using the `basePath` setting, on pages with params it will fire a router change. This will pass the url pathname in the `as` param using the `getUrl()` function. This means the `as` path will be sent through already including the `basePath`, leading to `/basePath/basePath/path` which will cause the router to throw an error.

* lint

* Add test case and ensure removal

* Make sure to re-add before changeState
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

Fixes #9326

Initial work to use chunkhashes instead of buildid for the page files in production. This does not change the calculation of the filename itself initially.

Was going through _document and noticed some variable shadowing going on. Added a rule for it to our eslint configuration and went through all warnings with @Timer.

This waits for the render to be committed to DOM before we render the route change complete event (no longer sync in new React).

We have tests that ensure this resolves.

---

Closes #12938

* updates relayer, uses web-vital lib, removes fid polyfill and removes unstable prefix from relayer

* simplify logic, fix null check

* update docs

* Update tests

* Rename relayWebVitals to reportWebVitals
Co-authored-by: Nhousseindjirdeh <houssein@google.com>

* measures fid

* updates typings, fixes logic, updates per review comments

* update to es5

* separate clearMeasures

* use relayer

* creates fid polyfll render helper + simplifies measure

* switch to dynamic import

* creates fid experimental flag

* removes unecessary time-to-first-input metric

* removes hydration measure removes

* default flag to false
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

* Update to use separate export instead of attaching to Component to identify GS(S)P pages

* Handle initialProps being undefined

* add new types of performance monitoring

* adding LCP

* adding cls to perf relayer

* add test for cls and lcp

* nit fixes

* re-organizing code

* fixing tests for lcp

* updating size limits in test

* Update head-manager to compress better

* Decrease size limit test

* Enable polyfillsOptimization

* Update sizes

* Commons is no longer there

* Update test

This adds a `isFallback` property to detect if the page is being rendered in "fallback" mode or normal mode.
Accessed via the `useRouter()` hook.

---

Closes #10527