get viewport size in Real Time - from a div element (zindex -999 - in use).html

<p id="ViewPortSize"> </p>
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<!-- Load jQuery hosted by Google into your website -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>


<script>

$( "body" ).append( '<div id="Div_Browser_ViewPort" style="width:100%; height:100%; position:absolute; top:0px; left:0px; z-index:-999;"> </div>' );

function GetViewPortSize() {
$('#ViewPortSize').text("Width = " + $( "#Div_Browser_ViewPort" ).width() + " / Height = " + $( "#Div_Browser_ViewPort" ).height());
}

/* Execute "GetViewPortSize" function at page load */
$(GetViewPortSize);

/* Execute "GetViewPortSize" function everytime the browser viewport is resized */
$( window ).resize(GetViewPortSize);

</script>