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import React from "react";
let on = false;
let rememberedReversePosition = getReverseScrollPosition();
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;
window.scrollTo(0, newPosition);
document.documentElement.style.scrollBehavior = old;
}
const scrollListener = (): void => {
rememberedReversePosition = getReverseScrollPosition();
};
const resizeObserver = new ResizeObserver(() => {
scrollToReverseScrollPosition(rememberedReversePosition);
});
export default function useReverseScrollPositionRemember(): void {
React.useEffect(() => {
if (on) return;
on = true;
window.addEventListener("scroll", scrollListener);
resizeObserver.observe(document.documentElement);
return () => {
resizeObserver.disconnect();
window.removeEventListener("scroll", scrollListener);
on = false;
};
}, []);
}
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