fix(popstate): skip unnecessary startTransition on cached navigations

Previously, every popstate update, including cached content, was wrapped in startTransition.
Rapid back/forward spam caused multiple transitions to enqueue in React's concurrent scheduler, leading to heavy reconciliation and freezing the browser tab.

Author: Ala-Ben-Aissia

exercises/04.router/05.solution.cache/ui/index.js

@@ -11,7 +11,7 @@ import {
 } from 'react'
 import { createRoot } from 'react-dom/client'
 import * as RSC from 'react-server-dom-esm/client'
-import { contentCache, useContentCache, generateKey } from './content-cache.js'
+import { contentCache, generateKey, useContentCache } from './content-cache.js'
 import { ErrorBoundary } from './error-boundary.js'
 import { shipFallbackSrc } from './img-utils.js'
 import { RouterContext, getGlobalLocation, useLinkHandler } from './router.js'
@@ -56,9 +56,10 @@ function Root() {
 				const fetchPromise = fetchContent(nextLocation)
 				const nextContentPromise = createFromFetch(fetchPromise)
 				contentCache.set(historyKey, nextContentPromise)
+				startTransition(() => setContentKey(historyKey))
+			} else {
+				setContentKey(historyKey)
 			}
-
-			startTransition(() => setContentKey(historyKey))
 		}
 		window.addEventListener('popstate', handlePopState)
 		return () => window.removeEventListener('popstate', handlePopState)