Implement scroll restoration in the following conditions.

1. There is no cache for fetched data(i.e., no library like apollo/react-query)
2. There is no global state management, e.g. redux
3. No SSR. Just pure CSR.
4. Use react-router for client-side routing

https://scroll-restoration.netlify.app/

1. Scroll to a specific item on the current page
2. Click a link to go to another page
3. Repeat step 1., 2.
4. Try browser's Back/Forward button. Scroll restoration should work.

Firebase update the current page's state whenever an user scrolls. For example, if I scrollY to 2226 on the https://firebase.google.com/ page, the console.log(history.state) at that time will be

{
  path: 'https://firebase.google.com/'
  previousPath: undefined
  scrollX: 0
  scrollY: 2226 // the same value as the current `window.scrollY` value
}

I think on scroll event they repeatly call history.replaceState({ ..updated state data.. }, ''). And when an user navigate back, they do scroll restoration before adding scroll event listener.

Repo of the whole website.

I accidentally saw a working scroll restoration on this blog.

By the way, after reading their repo, I think their implementation works because there is not any async api call to fetch the content. It's a static website that their content already sit in the code. Therefore, the height of each page is always static.

1. [DONE] Make a custom hook and use functional component instead of class component

2. [DONE] Add async api call to make page's height dynamic

3. [DONE with pitfalls] Use sessionStorage to store scroll position of every page instead of the original implementation

4. [DONE but create bugs as 6., 7.] Fix bug that occurs because of the execution order as described below (DOM updated happens before clearing the last effect):

   render
   -> useEffect
   -> *re-render (update)
   -> *DOM updated
   -> *clear last effect
   -> useEffect
   
   render
   -> useEffect
   -> *render (mount)
   -> *clear last effect
   -> *DOM updated
   -> useEffect
   

5. [DONE] Use window's scroll event to save the scroll postion into sessionStorage instead.

6. Bug#1: Find '@BUG' keyword in the code to see more details

7. Bug#2: The window's scroll can be triggered by a browser when an user navigates to a new page which has different height from the previous page. Hence, the saved scroll position in sessionStorage can be replaced unexpectedly.

Because of bugs and many quirks, in the end, I decided to use implementation as explained in the Possible Solutions section instead.

1. Modify history to save scroll position: https://github.com/janpaul123/delayed-scroll-restoration-polyfill
2. https://stackoverflow.com/a/52978973/6568503
3. https://stackoverflow.com/a/58806940/6568503
4. https://stackoverflow.com/a/57432069/6568503

1. If an <a> tag was clicked, the returned function of useEffect() will not be called, i.e. componentWillUnmount() will not be called.
2. Because of 1., we cannot save the scroll position in sessionStorage or set history.scrollRestoration back to auto while an user navigating to an external link, i.e. clicking <a> tag.
3. Setting history.scrollRestoration = 'manual' affects all websites in the current tab, not just our own website.
4. On Firefox, if we don't set history.scrollRestoration = 'manual', the forward button will cause window.scrollY = 0 before saving to sessionStorage.
5. On Firefox, when we set history.scrollRestoration = 'manual', we also have to manually call scrollTo(0) on every PUSH page, not only on POP pages.