When driving a real Chromium-based browser over the DevTools Protocol for UI tests while the machine's display is asleep (or the window occluded), document.visibilityState is "hidden". Pages rendered with React 19 streaming SSR (Next.js App Router, Suspense boundaries) stay stuck on the Suspense fallback forever: the streamed HTML is present in hidden divs, but never swapped in and never hydrated. No console error is shown.

React throws 'Hydration failed because the initial UI does not match what was rendered on the server' or 'Text content does not match. Server: X Client: Y' when using SSR frameworks (Next.js, Remix, Gatsby). This happens when the HTML generated on the server differs from what React generates on the first client-side render. Common triggers: (1) Using Date.now() or new Date() in render — different timestamps on server vs client. (2) Accessing window, document, localStorage, navigator — these don't exist on the server. (3) Conditional rendering based on browser state (window.innerWidth, matchMedia). (4) Random IDs or Math.random() in render output. (5) Browser extensions that modify the DOM before React hydrates. (6) Using typeof window !== 'undefined' incorrectly in render. (7) Different locale/timezone between server and client.