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HTML

<h1>Drag Handle</h1>

<div id="draggable">
    <div id="handle">
    </div>
    <textarea>This text is selectable</textarea>
</div>

CSS

#draggable {
    width: 100px;
    height: 100px;
    border: 1px solid black;
    display: flex;
    flex-direction: column;
}
#handle {
    width: 0;
    height: 0;
    border-right: 20px solid transparent;
    border-top: 20px solid black;
    cursor: move;
}
textarea {
  flex-grow: 1;
}

JavaScript

var draggable = document.getElementById('draggable');
var handle = document.getElementById('handle');
var target = false;
handle.onmousedown = function(e) {
    e.target.parentNode.setAttribute('draggable', 'true')
};

handle.onmouseup = function(e) {
    e.target.parentNode.setAttribute('draggable', 'false')
};

draggable.ondragstart = function(e) {
      e.dataTransfer.setData('text/plain', 'handle');
};

draggable.ondragend = function(e) {
      e.target.setAttribute('draggable', 'false')
};