Currently there is no way to add multiple meta tags with the same name attribute to the head of a page. This PR modifies the Head component to allow multiple meta tags with the same name if they have unique keys. 

This is important for integrating with certain services like Google Scholar and Swiftype.

Fixes #10183

This PR fixes a bug where `next/babel` would accidentally enable development transforms for a production build (`next build`).

This is tested by the two updated unit tests (which removed a workaround for this bug, and one now properly enables dev transforms).

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Fixes #18929
Fixes #19001
x-ref #19046
x-ref #17032

This PR removes some left over traces of the `next/data` experiment, as it has been superseded by RSC.

Fixes #20500

This updates the blog-starter-typescript example to use the latest version of Tailwind CSS (2.0.2)

Notes:
Followed the upgrade guide here: https://tailwindcss.com/docs/upgrading-to-v2 
And Next / PostCSS guide here: https://nextjs.org/docs/advanced-features/customizing-postcss-config

* Update error when internal href and external as are used

* Update to handle error outside invariant

* Update err.sh components

In order to get this example working, I needed to remove the invocation as described here: https://github.com/vercel/next-plugins/issues/309#issuecomment-674376348

This PR fixes a bug where we'd accidentally pass-through the user-provided `srcSet` if the image was lazy, just to then replace it when we hydrate.

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Fixes #19041

### Don‘t use rem in react-dev-overlay.
If we create a nextjs project and use css rem. We could set the html fontsize to any 'px'.  The react-dev-overlay components styles will be affected so that the style becomes larger or smaller.

![image](https://user-images.githubusercontent.com/10840654/95490394-c189a800-09ca-11eb-816a-8bcaf6bcb657.png)
## So i suggest change the rem to px.
## After modification：
![image](https://user-images.githubusercontent.com/10840654/95490703-2e9d3d80-09cb-11eb-8369-7ec650fa6c52.png)

---

Fixes #18680

This bundles ally.js into Next.js itself to upgrade a dependency they have pinned.

I tried every other major focus trap solution, even those used by some modal libraries, and they all failed.

`ally.js` is the only library that can do it correctly, so we're going to stick with it.

I also removed the `maintain/disabled` as we have a backdrop that would effectively result in the same. This reduces CPU strain.

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Fixes #19893
Fixes #14369
Closes #14372

This example illustrates how to integrate mdbreact (material design bootstrap for react) with next.js

Adds an `isReady` field on `next/router` specifying whether the router fields are updated client-side and ready for use. Should only be used inside of `useEffect` methods and not for conditionally rendering on the server.

Closes: https://github.com/vercel/next.js/issues/8259
Closes: https://github.com/vercel/next.js/pull/9370

This pull request adjusts our experimental scroll restoration behavior to use `sessionStorage` as opposed to `History#replaceState` to track scroll position.

In addition, **it eliminates a scroll event listener** and only captures when a `pushState` event happens (thereby leaving state that needs snapshotted).

These merely adjusts implementation detail, and is covered by existing tests:
```
test/integration/scroll-back-restoration/
```

---

Fixes #16690
Fixes #17073
Fixes #20486

Building off [this Slack conversation](https://vercel.slack.com/archives/CGD3XGSD7/p1597329727013900), this PR adds a top-level section to the documentation on authentication patterns.

Please provide any and all comments! A few open thoughts I have:

- ~Should this include code snippets from the related providers or stay very high-level? At what point do we delegate to the examples folder?~ Keep things high level and delegate to examples folder 
- ~Should this include any related cards at the bottom?~ Added to the bottom
- ~Should other places in the documentation link back to here?~ Added link from routing
- Should it be a top-level route, or be underneath advanced?

This ensures we render the locale domain on the `href` when using `next/link` previously the provided `href` was stilling being rendered which differed from the resulting `href` that was navigated to. 

Fixes: https://github.com/vercel/next.js/issues/20612

This upgrades to the version of ncc with subbundling support.

* bump dependencies to latest versions

* use tailwindcss class names

* use next/image component

* pass in height & width. update tailwindcss classes

* update tailwindcss classes

* mention tailwindcss 2.0

partially fixes #20341

- Makes sure that font fallback does not get repeated for every request.
- Adds a test for the same.

Follow-up to https://github.com/vercel/next.js/pull/20594 this ensures non-notFound SSG data 404s do cause a hard navigation as this signals a new deployment has occurred and a hard navigation will load the new deployment's version of the page. 

Closes: https://github.com/vercel/next.js/issues/20623

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

The HTML Living Standard explicitly says `href` should be omitted to prevent the loading of an incorrectly sized image:
https://html.spec.whatwg.org/multipage/semantics.html#attr-link-imagesrcset

![image](https://user-images.githubusercontent.com/616428/103378205-8a013800-4aaf-11eb-9085-10f547263fed.png)

Since it's in the spec, I assume this is valid-enough HTML.

This also dedupes preloads which the old implementation did not.

---

Fixes #18756
x-ref #19118
Fixes #18720

* Update dbConnect.js

The previous logic was in my opinion prone to errors in situations when different calls to this function from different locations may happen at the same time ( like a race condition  ). If the readyState is anything but not 0, then we must not try to perform a new connection as this may cause consequent connections!

I hope this makes sense

* Update examples/with-mongodb-mongoose/utils/dbConnect.js

* Lint fix and minor changes
Co-authored-by: NLuis Alvarez D <luis@vercel.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

This pull request adds an `elements.delete` operation to the `useIntersection`'s cleanup function: `unobserve`.

Without this delete operation, next.js holds onto an unreachable reference of every observed element indefinitely (automatically every Link and Image is observed, so that means every rendered Link and Image element adds to the leak). I found this memory leak when building out an infinite feed in next.js with thousands of Link elements.

The final code block of the `unobserve` function body:

```tsx
  // Destroy observer when there's nothing left to watch:
  if (elements.size === 0) {
    observer.disconnect()
    observers.delete(id)
  }
```

Is effectively unreachable without this delete operation, as the `elements` map will never decrease in size as it is currently. This means that there will always be at least one IntersectionObserver instance in memory if useIntersection has been used once, regardless of if there are currently any components still using the hook.

https://blog.twitter.com/engineering/en_us/topics/infrastructure/2019/capping-image-fidelity-on-ultra-high-resolution-devices.html

> This means that most OLED screens that say they are 3x resolution, are actually 3x in the green color, but only 1.5x in the red and blue colors. Showing a 3x resolution image in the app vs a 2x resolution image will be visually the same, though the 3x image takes significantly more data. Even true 3x resolution screens are wasteful as the human eye cannot see that level of detail without something like a magnifying glass.

> ![image](https://user-images.githubusercontent.com/616428/103366340-61b61100-4a90-11eb-9c16-8467f8930247.png)

Even the iPhone doesn't have true 3x DPI!

related: #18756

This pull request makes `Router#push` and `Router#replace` function identically to `<Link />`, i.e. reset scroll when the new render is complete.

Users can opt out of this new behavior via:
```tsx
const path = '/my-page'
router.push(path, path, { scroll: false })
```

---

Fixes #3249

Fixes #19935

Closes https://github.com/vercel/next.js/issues/20036.

This ensures we detect domain specific locales and redirect them client-side. Tests have been added in the `i18n` suite to ensure the domain redirect is applied correctly during a client-side navigation

Fixes: https://github.com/vercel/next.js/issues/19174

This ensures we detect the `basePath` correctly for the index `basePath` route when either a `hash` or a `query` are present in the provided path. This also adds this specific test to our basePath test suite. 

Fixes: https://github.com/vercel/next.js/issues/19294
Fixes: https://github.com/vercel/next.js/issues/19437
Closes: https://github.com/vercel/next.js/pull/19444

This refactors to instead of throwing a specific error when a SSG data route 404s, we return it through props and render the 404 outside of the error handling flow. No additional tests have been added as existing tests should cover this. 

Closes: https://github.com/vercel/next.js/issues/19243

This moves the scroll reset behavior to happen synchronously with the DOM commit, instead of a few ticks after the render completes.

This is necessary for components that read scroll state on mount.

---

Fixes #6462