JSFiddle - React, Tailwind, and code Playground

HTML

<!doctype html>
<html>
    <head>
        <title>Scrollbar Issue Example</title>
    </head>
    <body>
        <form>
            <fieldset>
                <legend>Flags:</legend>
                <div>
                    <ul>
                        <li><input type="checkbox" id="c1"> <label for="c1">Flag 1</label></li>
                        <li><input type="checkbox" id="c2"> <label for="c2">Flag 2</label></li>
                        <li><input type="checkbox" id="c3"> <label for="c3">Flag 3</label></li>
                        <li><input type="checkbox" id="c4"> <label for="c4">Flag 4</label></li>
                        <li><input type="checkbox" id="c5"> <label for="c5">Flag 5</label></li>
                        <li><input type="checkbox" id="c6"> <label for="c6">Flag 6</label></li>
                        <li><input type="checkbox" id="c7"> <label for="c7">Flag 7</label></li>
                        <li><input type="checkbox" id="c8"> <label for="c8">Flag 8</label></li>
                        <li><input type="checkbox" id="c9"> <label for="c9">Flag 9</label></li>
                        <li><input type="checkbox" id="c10"> <label for="c10">Flag 10</label></li>
                        <li><input type="checkbox" id="c11"> <label for="c11">Flag 11</label></li>
                        <li><input type="checkbox" id="c12"> <label for="c12">Flag 12</label></li>
                    </ul>
                </div>
            </fieldset>
            
            <fieldset class="scroll">
                <legend>Scroll:</legend>
                <div>
                    <ul>
                        <li><input type="checkbox" id="c1"> <label for="c1">Flag 1</label></li>
                        <li><input type="checkbox" id="c2"> <label for="c2">Flag 2</label></li>
                        <li><input type="checkbox" id="c3"> <label for="c3">Flag 3</label></li>
                        <li><input type="checkbox" id="c4"> <label for="c4">Flag 4</label></li>
           ...

CSS

fieldset
{
    border: 1px solid #DEDDDD;
    border-radius: 5px 5px 5px 5px;
    float: left;
    position: relative;
    white-space: nowrap;
    margin: 20px;
}

legend
{
    font-weight: bold;
    margin-left: 5px;
}

div
{
    max-height: 180px;
    overflow: auto;
    padding-right: 10px;
}

ul
{
    padding: 0 10px 5px;
}

input[type="checkbox"]
{
    margin: 3px 0;
}

label
{
    cursor: pointer;
}


fieldset.scroll div
{
    position: relative;
    top: 0px;
}

fieldset.scroll ul
{
    padding-top: 9px;
}