JSFiddle - React, Tailwind, and code Playground

HTML

<div class="spacer"></div>
<div class="list-container">
    <textarea class="textarea"></textarea>
    <ul class="list">
        <li>item</li>
    </ul>
</div>

CSS

textarea
{
    width: 300px;
    height: 70px;
    padding: 3px;
}

.list {
    list-style: none;
    background-color: gray;
    display: none;
    
    position: absolute;
    top: 76px;
    left: 0;
    
    margin: 0;
    padding: 0;
    border: 0;
}

.list-container
{
    position: relative;
}

.list li
{
    padding: 5px;
    width: 294px;
}

JavaScript

$(".textarea").bind("keypress", function (e) {
    if (String.fromCharCode(e.keyCode) == '@') {
         $(".list").show();
    } else{
         $(".list").hide();
    }
});