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

@@ -56,12 +56,10 @@ function Root() {
 				const fetchPromise = fetchContent(nextLocation)
 				const nextContentPromise = createFromFetch(fetchPromise)
 				contentCache.set(historyKey, nextContentPromise)
-				startTransition(() => setContentKey(historyKey)) // Fetch may suspend
+				startTransition(() => setContentKey(historyKey))
+			} else {
+				setContentKey(historyKey)
 			}
-			// Cache hit: Synchronous update
-			// ⚠️ No startTransition here:
-			// Resolved promises don’t suspend, and transitions add unnecessary scheduler work, risking freezes during rapid back/forward spamming.
-			setContentKey(historyKey)
 		}
 		window.addEventListener('popstate', handlePopState)
 		return () => window.removeEventListener('popstate', handlePopState)