https://react.dev/reference/react/useLayoutEffect

https://react.dev/reference/react/useEffect

useEffect runs after the render whereas useLayoutEffect runs before the render.

I don't remember why anymore but we used to have useLayoutEffect in the _app.ts in Chubic to specifically run something that was supposed to load before the page rendered. I removed it a while back and don't really want hunt it down so my best guess is that it has something to do with loading something from local storage that affects something in the layout, probably toggling some kind of switch in the Header.