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https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API shared by @jackymaxj
developer.mozilla.org
Intersection Observer API - Web APIs | MDN
The Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport.The Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport.
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