Whether to adjust the zIndex property of the element being dragged.
When set to false, no zIndex will be changed during dragging.
If true, the element will be put on top relative to its parent.
If a jQuery selector string is specified, the dragged element will be put on top of the elements identified by that selector.

Defaults to true.

<div id="div1">Hello DIV!</div>

<script type="text/javascript">
    jQuery(function ($) {
            stack: false