Add future.v7_fetcherPersist flag

.changeset/fetcher-cleanup.md

@@ -0,0 +1,11 @@
+---
+"@remix-run/router": minor
+---
+
+Add a new `future.v7_fetcherPersist` flag to the `@remix-run/router` to change the persistence behavior of fetchers when `router.deleteFetcher` is called. Instead of being immediately cleaned up, fetchers will persist until they return to an `idle` state([RFC](https://github.com/remix-run/remix/discussions/7698))
+
+- This is sort of a long-standing bug fix as the `useFetchers()` API was always supposed to only reflect **in-flight** fetcher information for pending/optimistic UI -- it was not intended to reflect fetcher data or hang onto fetchers after they returned to an `idle` state
+- With `v7_fetcherPersist`, the `router` only knows about in-flight fetchers - they do not exist in `state.fetchers` until a `fetch()` call is made, and they are removed as soon as it returns to `idle` (and the data is handed off to the React layer)
+- Keep an eye out for the following specific behavioral changes when opting into this flag and check your app for compatibility:
+  - Fetchers that complete _while still mounted_ will no longer appear in `useFetchers()`. They served effectively no purpose in there since you can access the data via `useFetcher().data`).
+  - Fetchers that previously unmounted _while in-flight_ will not be immediately aborted and will instead be cleaned up once they return to an `idle` state. They will remain exposed via `useFetchers` while in-flight so you can still access pending/optimistic data after unmount.

.changeset/fix-router-future-prop.md

@@ -0,0 +1,6 @@
+---
+"react-router": patch
+"react-router-dom": patch
+---
+
+Fix the `future`prop on `BrowserRouter`, `HashRouter` and `MemoryRouter` so that it accepts a `Partial<FutureConfig>` instead of requiring all flags to be included.

docs/guides/api-development-strategy.md

@@ -66,6 +66,7 @@ const router = createBrowserRouter(routes, {
 | Flag                     | Description                                                           |
 | ------------------------ | --------------------------------------------------------------------- |
 | `v7_normalizeFormMethod` | Normalize `useNavigation().formMethod` to be an uppercase HTTP Method |
+| `v7_fetcherPersist`      | Delay active fetcher cleanup until they return to an `idle` state     |
 
 ### React Router Future Flags