Next.js beginner here: Why does Server-Side Rendering data sometimes appear on client-side only?

I totally get this, I've seen this exact flicker happen on some of my landing pages and it can be a real headache, especially when you're trying to nail that first impression for user experience. When you're dealing with Next.js data fetching, especially with getServerSideProps, the perceived inconsistency often stems from understanding the difference between an initial page load and subsequent client-side navigations.

While getServerSideProps is designed to pre-render your page with data on the server for every request, which is excellent for SEO and initial load performance, the behavior changes slightly during client-side transitions. Here are the common reasons you might be observing this "client-side only" appearance:

• Client-Side Navigations: This is the most frequent cause. When you navigate to a page using <Link> or router.push within your Next.js application, Next.js doesn't fetch a whole new HTML document. Instead, it makes an API request to the server to fetch *only the data* (as JSON) that getServerSideProps would provide. Your React component then re-renders client-side with this new data. This is by design for a snappier single-page application feel, but if the component isn't prepared for this transition (e.g., no skeleton loaders), you might see a flicker.
• Hydration Mismatch: After the server sends the initial HTML (with data from getServerSideProps), React on the client-side "hydrates" this static HTML, attaching event listeners and making it interactive. If there's a mismatch between what the server rendered and what React expects to render on the client (e.g., due to client-only code running before hydration, browser extensions, or incorrect conditional rendering based on client-specific variables), React will re-render the component. This re-render can cause a noticeable flicker or layout shift. Check for useEffect hooks that modify the DOM right on mount without proper server-side safeguards.
• Asynchronous Client-Side Effects: Even if getServerSideProps provides initial data, components might still have their own useEffect hooks that fetch additional data or perform other asynchronous operations client-side. If these operations cause a visual change after the initial render, it can look like delayed data. Ensure your components gracefully handle the initial data from props before triggering further client-side fetches.

To mitigate this and improve perceived performance, ensure you're using <Link> correctly for internal navigations. For a smoother user experience during client-side data fetching, consider implementing skeleton loaders or CSS placeholders for your content areas. This provides immediate visual feedback, reducing the perceived delay even when data is loaded client-side after initial hydration. Hope this helps optimize your site's performance and user experience!

Yeah, that totally clicks with the client-side navigation explanation – I think that's def what I'm seeing most often when I click around. It explains why a hard refresh acts differently too. I'm gonna go back and implement some skeleton loaders for those data sections to improve the perceived performance during those transitions.