Make unstable_useTransitions prop optional on Router component (#14646)

* Make unstable_useTransitions prop optional on Router component

* Update context

* Let undefined flow through even though behavior is the same

Author: brophdawg11

.changeset/four-sloths-notice.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix `unstable_useTransitions` prop on `<Router>` component to permit omission for backewards compatibility

packages/react-router/lib/components.tsx

@@ -740,7 +740,7 @@ export function RouterProvider({
                 location={state.location}
                 navigationType={state.historyAction}
                 navigator={navigator}
-                unstable_useTransitions={unstable_useTransitions === true}
+                unstable_useTransitions={unstable_useTransitions}
               >
                 <MemoizedDataRoutes
                   routes={router.routes}
@@ -895,7 +895,7 @@ export function MemoryRouter({
       location={state.location}
       navigationType={state.action}
       navigator={history}
-      unstable_useTransitions={unstable_useTransitions === true}
+      unstable_useTransitions={unstable_useTransitions}
     />
   );
 }
@@ -1310,9 +1310,20 @@ export interface RouterProps {
    */
   static?: boolean;
   /**
-   * Whether this router should wrap navigations in `React.startTransition()`
+   * Control whether router state updates are internally wrapped in
+   * [`React.startTransition`](https://react.dev/reference/react/startTransition).
+   *
+   * - When left `undefined`, all router state updates are wrapped in
+   *   `React.startTransition`
+   * - When set to `true`, {@link Link} and {@link Form} navigations will be wrapped
+   *   in `React.startTransition` and all router state updates are wrapped in
+   *   `React.startTransition`
+   * - When set to `false`, the router will not leverage `React.startTransition`
+   *   on any navigations or state changes.
+   *
+   * For more information, please see the [docs](https://reactrouter.com/explanation/react-transitions).
    */
-  unstable_useTransitions: boolean;
+  unstable_useTransitions?: boolean;
 }
 
 /**

packages/react-router/lib/context.ts

@@ -179,7 +179,7 @@ interface NavigationContextObject {
   basename: string;
   navigator: Navigator;
   static: boolean;
-  unstable_useTransitions: boolean;
+  unstable_useTransitions: boolean | undefined;
   // TODO: Re-introduce a singular `FutureConfig` once we land our first
   // future.unstable_ or future.v8_ flag
   future: {};

packages/react-router/lib/dom/lib.tsx

@@ -849,7 +849,7 @@ export function BrowserRouter({
       location={state.location}
       navigationType={state.action}
       navigator={history}
-      unstable_useTransitions={unstable_useTransitions === true}
+      unstable_useTransitions={unstable_useTransitions}
     />
   );
 }
@@ -940,7 +940,7 @@ export function HashRouter({
       location={state.location}
       navigationType={state.action}
       navigator={history}
-      unstable_useTransitions={unstable_useTransitions === true}
+      unstable_useTransitions={unstable_useTransitions}
     />
   );
 }
@@ -1027,7 +1027,7 @@ export function HistoryRouter({
       location={state.location}
       navigationType={state.action}
       navigator={history}
-      unstable_useTransitions={unstable_useTransitions === true}
+      unstable_useTransitions={unstable_useTransitions}
     />
   );
 }