Add handling for back/forward (popstate) between old and new router (#38453)

Handles the case where you navigate between routes in `pages` and `app`.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

Author: timneutkens

packages/next/client/components/app-router.client.tsx

@@ -166,11 +166,16 @@ export default function AppRouter({
       return
     }
 
+    // Identifier is shortened intentionally.
+    // __NA is used to identify if the history entry can be handled by the app-router.
+    // __N is used to identify if the history entry can be handled by the old router.
+    const historyState = { __NA: true, tree }
     if (pushRef.pendingPush) {
       pushRef.pendingPush = false
-      window.history.pushState({ tree }, '', canonicalUrl)
+
+      window.history.pushState(historyState, '', canonicalUrl)
     } else {
-      window.history.replaceState({ tree }, '', canonicalUrl)
+      window.history.replaceState(historyState, '', canonicalUrl)
     }
   }, [tree, pushRef, canonicalUrl])
 
@@ -185,6 +190,13 @@ export default function AppRouter({
       return
     }
 
+    // TODO: this case happens when pushState/replaceState was called outside of Next.js or when the history entry was pushed by the old router.
+    // It reloads the page in this case but we might have to revisit this as the old router ignores it.
+    if (!state.__NA) {
+      window.location.reload()
+      return
+    }
+
     // @ts-ignore useTransition exists
     // TODO: Ideally the back button should not use startTransition as it should apply the updates synchronously
     // Without startTransition works if the cache is there for this path
@@ -193,7 +205,7 @@ export default function AppRouter({
         type: 'restore',
         payload: {
           url: new URL(window.location.href),
-          historyState: state,
+          tree: state.tree,
         },
       })
     })

packages/next/client/components/reducer.ts

@@ -247,8 +247,6 @@ type AppRouterState = {
   canonicalUrl: string
 }
 
-type HistoryState = { tree: FlightRouterState }
-
 export function reducer(
   state: AppRouterState,
   action:
@@ -265,7 +263,7 @@ export function reducer(
           }
         }
       }
-    | { type: 'restore'; payload: { url: URL; historyState: HistoryState } }
+    | { type: 'restore'; payload: { url: URL; tree: FlightRouterState } }
     | {
         type: 'server-patch'
         payload: {
@@ -276,14 +274,14 @@ export function reducer(
       }
 ): AppRouterState {
   if (action.type === 'restore') {
-    const { url, historyState } = action.payload
+    const { url, tree } = action.payload
     const href = url.pathname + url.search + url.hash
 
     return {
       canonicalUrl: href,
       pushRef: state.pushRef,
       cache: state.cache,
-      tree: historyState.tree,
+      tree: tree,
     }
   }
 

packages/next/shared/lib/router/router.ts

@@ -73,8 +73,9 @@ interface NextHistoryState {
 
 export type HistoryState =
   | null
-  | { __N: false }
-  | ({ __N: true; key: string } & NextHistoryState)
+  | { __NA: true; __N?: false }
+  | { __N: false; __NA?: false }
+  | ({ __NA?: false; __N: true; key: string } & NextHistoryState)
 
 function buildCancellationError() {
   return Object.assign(new Error('Route Cancelled'), {
@@ -840,6 +841,12 @@ export default class Router implements BaseRouter {
       return
     }
 
+    // __NA is used to identify if the history entry can be handled by the app-router.
+    if (state.__NA) {
+      window.location.reload()
+      return
+    }
+
     if (!state.__N) {
       return
     }