memoize layout router context

[ztanner]

Since AppRouterState is promise-based (so we can use the values and suspend in render), the state itself isn't stable between renders. Each action corresponds with a new Promise object. When a link is hovered, a prefetch action is dispatched, even if the prefetch never happens (for example, if there's already a prefetch entry in the cache, and it doesn't need to prefetch again). In other words, the prefetch action will be dispatched but won't necessarily change the state.

This means that these no-op actions that don't actually change the state values will trigger a re-render. Most of the context providers in AppRouter are memoized with the exception of LayoutRouter context. This PR memoizes those values so that consumers are only notified of meaningful updates.

Fixes #63159

Closes NEXT-3127

[ztanner]

• memoize layout router context #64575 👈
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[github-actions]

All broken links are now fixed, thank you!

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
buildDuration	17.1s	17.3s	⚠️ +219ms
buildDurationCached	9.4s	8s	N/A
nodeModulesSize	199 MB	199 MB	⚠️ +1.37 kB
nextStartRea..uration (ms)	437ms	455ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
2453-HASH.js gzip	31.4 kB	31.4 kB	N/A
3304.HASH.js gzip	181 B	181 B	✓
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	✓
8299-HASH.js gzip	5.1 kB	5.1 kB	N/A
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main-app-HASH.js gzip	242 B	243 B	N/A
main-HASH.js gzip	29.6 kB	29.7 kB	N/A
webpack-HASH.js gzip	1.68 kB	1.68 kB	N/A
Overall change	99 kB	99 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

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_app-HASH.js gzip	196 B	197 B	N/A
_error-HASH.js gzip	184 B	184 B	✓
amp-HASH.js gzip	505 B	505 B	✓
css-HASH.js gzip	324 B	325 B	N/A
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edge-ssr-HASH.js gzip	258 B	258 B	✓
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hooks-HASH.js gzip	370 B	371 B	N/A
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withRouter-HASH.js gzip	309 B	309 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	6.63 kB	6.63 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
_buildManifest.js gzip	483 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
index.html gzip	529 B	529 B	✓
link.html gzip	542 B	542 B	✓
withRouter.html gzip	525 B	523 B	N/A
Overall change	1.07 kB	1.07 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
edge-ssr.js gzip	94.4 kB	94.4 kB	N/A
page.js gzip	3.05 kB	3.05 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
middleware-b..fest.js gzip	623 B	626 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	25.5 kB	25.5 kB	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	839 B	839 B	✓

Next Runtimes

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app-page-exp...dev.js gzip	171 kB	171 kB	✓
app-page-exp..prod.js gzip	97.5 kB	97.5 kB	✓
app-page-tur..prod.js gzip	99.2 kB	99.2 kB	✓
app-page-tur..prod.js gzip	93.5 kB	93.5 kB	✓
app-page.run...dev.js gzip	145 kB	145 kB	✓
app-page.run..prod.js gzip	92 kB	92 kB	✓
app-route-ex...dev.js gzip	21.5 kB	21.5 kB	✓
app-route-ex..prod.js gzip	15.2 kB	15.2 kB	✓
app-route-tu..prod.js gzip	15.2 kB	15.2 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route.ru...dev.js gzip	21.1 kB	21.1 kB	✓
app-route.ru..prod.js gzip	14.9 kB	14.9 kB	✓
pages-api-tu..prod.js gzip	9.55 kB	9.55 kB	✓
pages-api.ru...dev.js gzip	9.82 kB	9.82 kB	✓
pages-api.ru..prod.js gzip	9.55 kB	9.55 kB	✓
pages-turbo...prod.js gzip	21.4 kB	21.4 kB	✓
pages.runtim...dev.js gzip	22.1 kB	22.1 kB	✓
pages.runtim..prod.js gzip	21.4 kB	21.4 kB	✓
server.runti..prod.js gzip	51.5 kB	51.5 kB	✓
Overall change	945 kB	945 kB	✓

build cache

	vercel/next.js canary	vercel/next.js 04-16-memoize_layout_router_context	Change
0.pack gzip	1.59 MB	1.59 MB	N/A
index.pack gzip	107 kB	107 kB	N/A
Overall change	0 B	0 B	✓

Diff details

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Commit: 30c18e6