React Performance: Client — Event Listener Dedup. Keywords: event listener deduplicate global. Platform: React/Next.js. Description: Share global event listeners across component instances. Do: Use useSWRSubscription for shared listeners. Don't: Register listener per component instance. Good Example: useSWRSubscription('global-keydown', () => { window.addEventListener... }). Bad Example: useEffect(() => { window.addEventListener('keydown', handler) }, []). Severity: Low.