Joseph

Well I gave it a go with @ArmanNisch 's approach, except with serve rather than browsersync, and I see no issue what so ever. - https://www.npmjs.com/package/serve Do yo use these...

Yeah, well you seemed to agree with @ArmanNisch 's approach, but you two are most likely looking at two wildly different issues, that manifest through the same error message. I...

Yeah like I've debugged it and stepping through I see a frame where the UI goes to light mode, and then the errors come in, and then it goes dark...

Here you go, but I am still not sure, exactly what breaks https://github.com/vercel/next.js/assets/21013447/e967d949-0cc2-40b4-b683-93f2132d328e

One more monitoring the HTML tag attributes. https://github.com/vercel/next.js/assets/21013447/10bfb075-bc57-413a-9172-a4201edea026

And btw, this could also be a coincidence, as if React is done doing a bunch of work, and paints, and also prints errors. Doesn't necessarily mean the next-themes is...

Mmm well, all the more reason to think something might be happening on Cloudflare. Do you know if you have HTML rewriter, or any setting activated in Cloudflare that could...

@DzTheRage japp date and locales can cause this issue easily. For example a call to `toLocaleUppercase` can break all hell loose... For these though, the supressHydrationWarning flag should suffice, https://react.dev/reference/react-dom/hydrate#suppressing-unavoidable-hydration-mismatch-errors

This is most likely an Undici error. - https://undici.nodejs.org/#/?id=network-address-family-autoselection Can you try to collect more data about the endpoints that are failing?

Hi, well, it's not a Next.js thing, rather Node.js' fetch implementation uses undici at its core. For a world pre- Node 18 (17 really) Next js did a polyfill with...