fix(react-router): carry viewTransition on preferBack back-nav; memoize hook options

- preferBack: when the back path is taken, forward the link's `viewTransition`
  intent to the popstate-driven navigation (set `router.shouldViewTransition`
  before `history.back()`), so a `viewTransition` link behaves consistently on
  both its push and pop paths instead of only honoring `defaultViewTransition`.
- useIsBackNavigation: memoize the navigation options on their primitive fields
  (mirroring `useLinkProps`) so `buildLocation` no longer re-runs on every render
  when callers pass a fresh inline options object.
- Add a test asserting the link's `viewTransition` is set when going back.

Author: gkurt

packages/react-router/src/link.tsx

@@ -642,6 +642,11 @@ export function useLinkProps<
       // Target is the previous entry — pop instead of pushing to preserve forward
       // history and native scroll restoration (handled via the router's popstate).
       if (isBackNavigation) {
+        // Carry the link's view-transition intent onto the popstate-driven back
+        // navigation, which otherwise only honors `defaultViewTransition`.
+        if (viewTransition !== undefined) {
+          router.shouldViewTransition = viewTransition
+        }
         router.history.back({ ignoreBlocker })
         return
       }

packages/react-router/src/useIsBackNavigation.ts

@@ -74,12 +74,33 @@ export function useIsBackNavigation<
     (prev, next) => prev.href === next.href,
   )
 
+  // Memoize on the primitive option fields (callers usually pass a fresh inline
+  // object) so `buildLocation` below doesn't re-run every render. Mirrors the
+  // `_options` memo in `useLinkProps`.
+  // eslint-disable-next-line react-hooks/rules-of-hooks
+  const _options = React.useMemo(
+    () => options,
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+    [
+      router,
+      options.from,
+      options._fromLocation,
+      options.hash,
+      options.to,
+      options.search,
+      options.params,
+      options.state,
+      options.mask,
+      options.unsafeRelative,
+    ],
+  )
+
   // eslint-disable-next-line react-hooks/rules-of-hooks
   return React.useMemo(() => {
     const next = router.buildLocation({
       _fromLocation: currentLocation,
-      ...options,
+      ..._options,
     } as any)
     return resolveIsBackNavigation(router, currentLocation, next, match)
-  }, [router, currentLocation, options, match])
+  }, [router, currentLocation, _options, match])
 }

packages/react-router/tests/useIsBackNavigation.test.tsx

@@ -43,6 +43,9 @@ function RootComponent() {
       <Link to="/contact" preferBack data-testid="preferback-contact">
         Prefer contact
       </Link>
+      <Link to="/" preferBack viewTransition data-testid="preferback-home-vt">
+        Back home (view transition)
+      </Link>
       <HookProbe />
       <Outlet />
     </>
@@ -170,6 +173,27 @@ describe('Link preferBack', () => {
     expect(router.history.location.state.__TSR_index).toBe(2)
   })
 
+  test('carries the link\'s viewTransition intent onto the back navigation', async () => {
+    const { router } = setup(['/'])
+    await screen.findByText('IndexTitle')
+    await act(() => fireEvent.click(screen.getByText('About')))
+    await screen.findByText('AboutTitle')
+
+    // Capture `router.shouldViewTransition` at the moment `back()` is invoked —
+    // the popstate-driven load consumes and resets it shortly after.
+    const origBack = router.history.back.bind(router.history)
+    let vtAtBack: unknown = 'unset'
+    vi.spyOn(router.history, 'back').mockImplementation((opts) => {
+      vtAtBack = router.shouldViewTransition
+      return origBack(opts)
+    })
+
+    await act(() => fireEvent.click(screen.getByTestId('preferback-home-vt')))
+    await screen.findByText('IndexTitle')
+
+    expect(vtAtBack).toBe(true)
+  })
+
   describe('match modes (pathname vs exact)', () => {
     function ModeProbe() {
       const exactMatch = useIsBackNavigation({ to: '/about', search: { page: 3 } }, 'exact')