The error Hydration failed because the initial UI does not match what was rendered on the server is arguably the most notorious stumbling block in the Next.js ecosystem. It usually comes paired with "Text content does not match server-rendered HTML" or "There was an error while hydrating." While often dismissed as a warning in development, these mismatches force React to discard the server-rendered HTML and perform a full client-side synchronous re-render. This negates the performance benefits of SSR/SSG and causes layout shifts. The Root Cause: The Reconciliation Gap Hydration is the process where React preserves the HTML generated by the server and simply attaches event listeners to it on the client. For this to work, the DOM tree generated by the server must be byte-for-byte identical to the DOM tree React generates during the initial client render. If they differ, React enters a "recovery" mode, treating the server HTML as invalid. This mismatc...
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