React Performance: Rendering — Activity Component. Keywords: activity show hide preserve. Platform: React/Next.js. Description: Use Activity component to preserve state/DOM for toggled components. Do: Use Activity for expensive toggle components. Don't: Unmount/remount on visibility toggle. Good Example: <Activity mode={isOpen ? 'visible' : 'hidden'}><Menu /></Activity>. Bad Example: {isOpen && <Menu />} // loses state. Severity: Medium.