Merge branch 'dev' into callsite-revalidation-optout

Author: brophdawg11

.changeset/big-rings-protect.md

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+---
+"react-router": patch
+---
+
+Fix a Framework Mode bug where the `defaultShouldRevalidate` parameter to `shouldRevalidate` would not be correct after `action` returned a 4xx/5xx response (`true` when it should have been `false`)
+
+- If your `shouldRevalidate` function relied on that parameter, you may have seen unintended revalidations

.changeset/bright-brooms-hammer.md

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+---
+"@react-router/dev": patch
+---
+
+Load environment variables before evaluating `routes.ts`
+
+For example, you can now compute your routes based on [`VITE_`-prefixed environment variables](https://vite.dev/guide/env-and-mode#env-variables):
+
+```txt
+# .env
+VITE_ENV_ROUTE=my-route
+```
+
+```ts
+// app/routes.ts
+import { type RouteConfig, route } from "@react-router/dev/routes";
+
+const routes: RouteConfig = [];
+if (import.meta.env.VITE_ENV_ROUTE === "my-route") {
+  routes.push(route("my-route", "routes/my-route.tsx"));
+}
+
+export default routes;
+```

.changeset/curly-balloons-fry.md

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+---
+"react-router": patch
+---
+
+Fix `fetcher.submit` failing with plain objects containing a `tagName` property

.changeset/dull-moons-share.md

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+---
+"react-router": patch
+---
+
+[UNSTABLE] Add `unstable_pattern` to the parameters for client side `unstable_onError`, refactor how it's called by `RouterProvider` to avoid potential strict mode issues

.changeset/fluffy-walls-add.md

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+---
+"react-router": patch
+---
+
+Add new `unstable_useTransitions` flag to routers to give users control over the usage of [`React.startTransition`](https://react.dev/reference/react/startTransition) and [`React.useOptimistic`](https://react.dev/reference/react/useOptimistic).
+
+- Framework Mode + Data Mode:
+  - `<HydratedRouter unstable_transition>`/`<RouterProvider unstable_transition>`
+  - When left unset (current default behavior)
+    - Router state updates are wrapped in `React.startTransition`
+    - ⚠️ This can lead to buggy behaviors if you are wrapping your own navigations/fetchers in `React.startTransition`
+    - You should set the flag to `true` if you run into this scenario to get the enhanced `useOptimistic` behavior (requires React 19)
+  - When set to `true`
+    - Router state updates remain wrapped in `React.startTransition` (as they are without the flag)
+    - `Link`/`Form` navigations will be wrapped in `React.startTransition`
+    - A subset of router state info will be surfaced to the UI _during_ navigations via `React.useOptimistic` (i.e., `useNavigation()`, `useFetchers()`, etc.)
+      - ⚠️ This is a React 19 API so you must also be React 19 to opt into this flag for Framework/Data Mode
+  - When set to `false`
+    - The router will not leverage `React.startTransition` or `React.useOptimistic` on any navigations or state changes
+- Declarative Mode
+  - `<BrowserRouter unstable_useTransitions>`
+  - When left unset
+    - Router state updates are wrapped in `React.startTransition`
+  - When set to `true`
+    - Router state updates remain wrapped in `React.startTransition` (as they are without the flag)
+    - `Link`/`Form` navigations will be wrapped in `React.startTransition`
+  - When set to `false`
+    - the router will not leverage `React.startTransition` on any navigations or state changes

.changeset/forty-bugs-turn.md

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+---
+"react-router": patch
+---
+
+Fix the promise returned from `useNavigate` in Framework/Data Mode so that it properly tracks the duration of `popstate` navigations (i.e., `navigate(-1)`)