The first load of a Next.js app is a document — HTML down the wire, parsed, hydrated. But once the app is interactive, open your Network tab and you'll notice it never asks for a document again. Instead you see a trickle of odd little requests: some are GETs that return something that isn't HTML, and some are POSTs to the page you're already on, streaming a response back. Those POSTs are the part that looks weird, and they're the reason this post exists.