<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
    font-size: 200px;
}

#offscreen {
    position: absolute;
    top: -500vh;
    left: -500vw;
}
</style>
</head>
</body>
<div id="onscreen">DRAG ME</div>
<div id="offscreen">FAR OFFSCREEN</div>
<script>
function selectChildNodesOfElement(element)
{
    let range = document.createRange();
    range.setStartBefore(element.childNodes[0])
    range.setEndAfter(element.childNodes[element.childNodes.length - 1]);
    getSelection().removeAllRanges();
    getSelection().addRange(range);
}
selectChildNodesOfElement(onscreen);
document.body.addEventListener("dragstart", () => selectChildNodesOfElement(offscreen));
</script>
</body>
</html>