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import React from "react";
let on = false;
let reverseScrollPosition = getReverseScrollPosition();
let reverseScrollToPosition: number | null = null;
let lastScrollPosition = window.scrollY;
export function getReverseScrollPosition(): number {
if (document.documentElement.scrollHeight <= window.innerHeight) {
return 0;
} else {
return (
document.documentElement.scrollHeight -
document.documentElement.scrollTop -
window.innerHeight
);
}
}
export function scrollToReverseScrollPosition(reversePosition: number): void {
if (document.documentElement.scrollHeight <= window.innerHeight) return;
const old = document.documentElement.style.scrollBehavior;
document.documentElement.style.scrollBehavior = "auto";
const newPosition =
document.documentElement.scrollHeight -
window.innerHeight -
reversePosition;
reverseScrollToPosition = newPosition;
window.scrollTo(0, newPosition);
document.documentElement.style.scrollBehavior = old;
}
const scrollListener = (): void => {
if (
reverseScrollToPosition != null &&
Math.abs(window.scrollY - reverseScrollToPosition) > 50
) {
console.log(
`Reverse scroll position coerce. Required: ${reverseScrollToPosition}. Actual: ${window.scrollY}.`
);
scrollToReverseScrollPosition(reverseScrollPosition);
return;
}
if (
reverseScrollToPosition == null &&
Math.abs(window.scrollY - lastScrollPosition) > 1000
) {
console.log(
`Scroll jump detected. New: ${window.scrollY}. Old: ${lastScrollPosition}.`
);
scrollToReverseScrollPosition(reverseScrollPosition);
return;
}
reverseScrollToPosition = null;
lastScrollPosition = window.scrollY;
reverseScrollPosition = getReverseScrollPosition();
};
const resizeObserver = new ResizeObserver(() => {
scrollToReverseScrollPosition(reverseScrollPosition);
});
export default function useReverseScrollPositionRemember(): void {
React.useEffect(() => {
if (on) return;
on = true;
window.addEventListener("scroll", scrollListener);
resizeObserver.observe(document.documentElement);
return () => {
window.removeEventListener("scroll", scrollListener);
resizeObserver.disconnect();
on = false;
};
}, []);
}
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