Edit in JSFiddle

<div class="wrap">
    <div class="pull-tab"></div>
    <textarea placeholder="drag the cyan triangle..."></textarea>
</div>
textarea {
    position: relative;
    margin: 20px 0 0 20px;
    z-index: 1;
}
.wrap {
    position: relative;
    display: inline-block;
}
.wrap:after {
    content:"";
    border-top: 20px solid black;
    border-left: 20px solid transparent;
    border-right: 20px solid transparent;
    -webkit-transform: rotate(-45deg);
    z-index: 1;
    opacity: 0.5;
    position: absolute;
    right: -18px;
    bottom: -3px;
    pointer-events: none;
}
.pull-tab {
    height: 0px;
    width: 0px;
    border-top: 20px solid cyan;
    border-left: 20px solid transparent;
    border-right: 20px solid transparent;
    -webkit-transform: rotate(-45deg);
    position: absolute;
    bottom: 0px;
    right: -15px;
    pointer-events: none;
    z-index: 2;
}