Coordinates in Windows and viewBox

Formula: xm=evt.getClientX() and ym=evt.getClientY() give coordinates of pointer in window with (0;0) as origin at left top vertice of SVG zone.
This values don't depend of values in viewBox, you have to make some calculus to put object at mouse position by example.
Supposons que les paramètres sont les suivants: 
W is width and H is height (in HTML file for embedded object or in SVG file if it's alone called) 
viewBox="x0 y0 lo la" (in SVG file) 
Coordinates are  x0+xm*lo/W and y0+ym*la/H .....

Source 

<?xml version='1.0' encoding='iso-8859-1'?>
<svg width='400' height='400' viewBox="-200 -200 1800 1800" onmousemove='souris(evt)'>
<script><![CDATA[
function souris(evt)
{xm=evt.getClientX();ym=evt.getClientY();
xs=evt.getScreenX();ys=evt.getScreenY();
xsvg=-200+xm*5;ysvg=-200+ym*5;
root=evt.getTarget().getOwnerDocument();
root.getElementById('pos').getFirstChild().setData('Coordonnées fenêtre SVG: '+xm+' '+ym);
root.getElementById('pos2').getFirstChild().setData('Coordonnées fenêtre HTML: '+xs+' '+ys);
root.getElementById('pos3').getFirstChild().setData('Coordonnées viewBox: '+xsvg+' '+ysvg)}
]]></script>
<rect x='-200' y='-200' width='2000' height='2000' style='stroke-width:1;stroke:black;fill:yellow'/>
<text id='pos' x='-180' y='100' style='font-size:60'>Coordonnées fenêtre SVG: </text>
<text id='pos2' x='-180' y='200' style='font-size:60'>Coordonnées fenêtre HTML: </text>
<text id='pos3' x='-180' y='300' style='font-size:60'>Coordonnées viewBox: </text>
<text x='-180' y='400' style='font-size:60'>ViewBox: -200 -200 1800 1800</text>
</svg>