JSFiddle - React, Tailwind, and code Playground

HTML

<div id="debug"></div>
<div id="box">lots of content
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
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    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
    <div class="line">line</div>
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    <div class="line">line</div>
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    <div...

CSS

#box {
    position: absolute;
    top: 4em;
    left: 0;
    right: 0;
    bottom: 0;
    overflow-y: auto;
    border: 1px solid red;
}
#debug {
    position: fixed;
    top: 1em;
    left: 1em;
    padding: .5em;
    border: 1px solid blue;
}

JavaScript

var $box = $('#box'),
    $anchored = $('#anchored'),
    $debug = $('#debug');

$debug.text('position().top before scroll ' + ($anchored.position().top - $box.scrollTop()));

$box.animate({
    scrollTop: 100
}).promise().then(function () {
    $debug.text($debug.text() + ', after scroll ' + ($anchored.position().top + $box.scrollTop()));
});