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/** OVERLAY **/
#overlay {
    position: fixed;
    left: 0;
    top: 0;
    bottom: 0;
    right: 0;
    background: #000;
    opacity: 0.8;
    filter: alpha(opacity=80);
}
#loading {
    width: 50px;
    height: 57px;
    position: absolute;
    top: 50%;
    left: 50%;
    margin: -28px 0 0 -25px;
}

<div id="overlay" style="display:none">
  <div id="loading">
  	<img src="<?php echo get_stylesheet_directory_uri() ?>/img/svg-loaders/tail-spin.svg" alt="Loading, please wait...">
  </div>
</div>

(function($){
    /** New Overlay EventHandler **/
    $('button').click(function(e){
        $('#overlay').show();
        $(document).ajaxComplete(function(e){
            $('#overlay').hide();
        });
    });
}(jQuery));