The @kinescope/react-kinescope-player library accesses browser APIs
(window/document) during server-side rendering. In Next.js App Router,
client components are SSR-rendered on full page loads (direct URL,
refresh) but not on RSC navigations. This caused a 500 error for all
lessons with a kinescopeId when accessed directly.
Fix: defer rendering KinescopeReactPlayer until after mount with
useEffect + useState(false), so it only runs in the browser.
admins
9a21c705b7