Maintain scroll position on expanding page with JavaScript

Question

I am trying to maintain scroll position despite content being added to the top of the page, when you scroll to the top in a React app I am working on.

I have tried:

• Getting the position of the top element on the page before the content is added and then using something like window.scrollTo(0, elementPosition)
• Calculating the difference between the page height before and after the added content and scrolling back to the original position based on the difference.
• Determining where to scroll based on how far the Viewport was from the bottom before the added content was displayed as that position should not be changing.

None of it worked and I am at a loss. This is the code that is responsible for creating the content, rendering a portion of the content and then adding content once you scroll to the top:

//Creating the content:

useEffect(() => {
    fetch('/api/endpoint', {
        method: 'GET',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        }
    })
        .then(res => res.json())
        .then(res => {
            setLogUsers(res.users);
            setTotalAmount(res.log.length);
            setMessages(limitedRender(res.log));
        })
        .catch(err => console.log(err))
}, [renderAmount])

return(
    messages.map(e => {
        return(
            <Msg message = {e}/>
        );
    })
);

//Selecting a portion of the content and detecting when you scroll to the top:

const [totalAmount, setTotalAmount] = useState();
const [renderAmount, setRenderAmount] = useState(10);

const limitedRender = (arr) => {
  if (arr.length >= renderAmount) {
    return arr.slice(arr.length - (renderAmount), arr.length)
  }
  else {
    return arr
  }
}

const posTop = () => {
  return (
    typeof window.scrollY != 'undefined' ?  window.scrollY :
    document.documentElement && document.documentElement.scrollTop ?
    document.documentElement.scrollTop :
    document.body.scrollTop ? document.body.scrollTop : 0
  );
}

document.addEventListener('scroll', () => {
  if (posTop() === 0) {
    if (totalAmount - renderAmount >= 10) {
      setRenderAmount(renderAmount + 10);
    }
    else if (totalAmount - renderAmount < 10) {
      setRenderAmount(renderAmount + (totalAmount - renderAmount));
    }
  }
})

//Also may be important. I use this bit of code to ensure that the component stays scrolled to the bottom on initial render.

const btmUpView = () => {
    window.scrollTo(0, document.body.scrollHeight);
}

btmUpView();
return(
  <div>
    <div>
      <MsgFeed/>
    </div>
    <div>
      <input type="text"></input>
      <button onClick={() => sendMsg()}>Send</button>
    </div>
  </div>
);