JSFiddle - React, Tailwind, and code Playground

HTML

<div class="panel">
     <div class="arr"></div>
        <ul><li>Hello</li></ul>
    </div>

CSS

.panel { 
    width: 200px;
    min-height: 50px;
    background-color: #E0E9F9;
    position: relative;
    margin-top:30px;
    padding: 10px;
    border: 1px solid #AAA;
    box-shadow: 1px 1px 2px #AAA;
}
.arr { 
    height: 30px;
    width: 30px;
    transform: rotate(135deg);
    background-color: #E0E9F9;
    position: absolute;
    left: 45%;
    top: -16px;
    border-bottom: 1px solid #AAA;
    border-left: 1px solid #AAA;
    box-shadow: -1px 1px 1px #EEE;
}