fix(navigation): allow useSelectedLayoutSegment(s) in Pages Router (#62584)

### What?

Do not fail when `useSelectedLayoutSegment` or
`useSelectedLayoutSegments` APIs are called in the Pages Router.

### Why?

This makes migration easier and creates consistency with our other App
Router-specific APIs that inherit the same behavior.

### How?

Similar to #47490, we return `null` if there is no Layout context
(indicating being in Pages Router)

Types are also overridden in the navigation compact module declaration
which kicks in during start to correct the types if we detect a `pages/`
directory.

Note to reviewer: #47490 didn't add a test, so I added one top-level,
let me know if you have a better suggestion for placing.

Closes NEXT-2506
Fixes #61464

packages/next/navigation-types/compat/navigation.d.ts

@@ -31,4 +31,20 @@ declare module 'next/navigation' {
       string | string[]
     >
   >(): T | null
+
+  /**
+   * A [Client Component](https://nextjs.org/docs/app/building-your-application/rendering/client-components) hook
+   * that lets you read the active route segments **below** the Layout it is called from.
+   *
+   * If used from `pages/`, the hook will return `null`.
+   */
+  export function useSelectedLayoutSegments(): string[] | null
+
+  /**
+   * A [Client Component](https://nextjs.org/docs/app/building-your-application/rendering/client-components) hook
+   * that lets you read the active route segment **one level below** the Layout it is called from.
+   *
+   * If used from `pages/`, the hook will return `null`.
+   */
+  export function useSelectedLayoutSegment(): string | null
 }

packages/next/src/client/components/navigation.ts

@@ -211,8 +211,11 @@ function useSelectedLayoutSegments(
   parallelRouteKey: string = 'children'
 ): string[] {
   clientHookInServerComponentError('useSelectedLayoutSegments')
-  const { tree } = useContext(LayoutRouterContext)
-  return getSelectedLayoutSegmentPath(tree, parallelRouteKey)
+  const context = useContext(LayoutRouterContext)
+  // @ts-expect-error This only happens in `pages`. Type is overwritten in navigation.d.ts
+  if (!context) return null
+
+  return getSelectedLayoutSegmentPath(context.tree, parallelRouteKey)
 }
 
 /**
@@ -238,7 +241,8 @@ function useSelectedLayoutSegment(
 ): string | null {
   clientHookInServerComponentError('useSelectedLayoutSegment')
   const selectedLayoutSegments = useSelectedLayoutSegments(parallelRouteKey)
-  if (selectedLayoutSegments.length === 0) {
+
+  if (!selectedLayoutSegments || selectedLayoutSegments.length === 0) {
     return null
   }
 

packages/next/src/shared/lib/app-router-context.shared-runtime.ts

@@ -148,7 +148,8 @@ export const LayoutRouterContext = React.createContext<{
   childNodes: CacheNode['parallelRoutes']
   tree: FlightRouterState
   url: string
-}>(null as any)
+} | null>(null)
+
 export const GlobalLayoutRouterContext = React.createContext<{
   buildId: string
   tree: FlightRouterState

test/e2e/useselectedlayoutsegment-s-in-pages-router/pages/index.tsx

@@ -0,0 +1,10 @@
+import {
+  useSelectedLayoutSegment,
+  useSelectedLayoutSegments,
+} from 'next/navigation'
+
+export default function Page() {
+  useSelectedLayoutSegment()
+  useSelectedLayoutSegments()
+  return <p id="hello-world">Hello World</p>
+}

test/e2e/useselectedlayoutsegment-s-in-pages-router/useselectedlayoutsegment-s-in-pages-router.test.ts

@@ -0,0 +1,14 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('useSelectedLayoutSegment(s) in Pages Router', () => {
+  const { next } = nextTestSetup({ files: __dirname })
+
+  it('Should render with `useSelectedLayoutSegment(s) hooks', async () => {
+    const browser = await next.browser('/')
+
+    await browser.waitForElementByCss('#hello-world')
+    expect(await browser.elementByCss('#hello-world').text()).toBe(
+      'Hello World'
+    )
+  })
+})