JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.list {
    position: absolute;
    list-style: none;
    background-color: gray;
    max-width: 100px;
    display: none;
}

JavaScript

var cur_x = 0;
var cur_y = 0;
$('body').mousemove(function(e){
    cur_x = e.pageX;
    cur_y = e.pageY;
});

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