Below is an example in Javascript of what I use across all browsers and all viewPorts. It uses svg matrix transforms. The events are attached to each element to be dragged. In the example I've included mouse position readout both for html and svg.
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<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>E - Universal Drag/Drop</title>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body style='padding:10px;font-family:arial'>
<center>
<h4>Universal Drag/Drop</h4>
<div style='width:90%;background-color:gainsboro;text-align:justify;padding:10px;border-radius:6px;'>
This example uses matrix transforms, with object methods, not strings. It can seamlessly drag/drop elements that have previously been transformed and reside it different viewPorts. It employs <b>getScreenCTM</b>, <b>createSVGTransform</b> and binds the element to a <b>transform List</b>
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<table>
<tr><td align=left>
Scenerio:<br />
A 400x400 DIV contains an SVG with viewBox=0 0 330 330.<br />
1.) The blue rect element is contained in a <g>.<br />
2.) The <g> element has been transformed.<br />
3.) The maroon rect resides in a different viewPort.<br />
4.) The orange circle has been transformed.<br />
5.) Drag/Drop the circles and rectangles.<br />
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<td align=left>
<div id="svgDiv" style='background-color:lightgreen;width:400px;height:400px;'>
<svg id="mySVG" width="100%" height="100%" viewBox="0 0 300 300" onmousemove="svgCursor(evt)">
<circle onmousedown=startDrag(evt) onmousemove=drag(evt) onmouseup=endDrag() id="redCircle" cx="120" cy="180" r="40" fill="red" stroke="black" stroke-width="2" />
<circle onmousedown=startDrag(evt) onmousemove=drag(evt) onmouseup=endDrag() id="orangeCircle" cx="200" cy="200" r="40" fill="orange" stroke="black" stroke-width="2" />
<svg viewBox="0 100 800 800">
<rect onmousedown=startDrag(evt) onmousemove=drag(evt) onmouseup=endDrag() id="maroonRect" x="220" y="250" width="60" height="60" fill="maroon" stroke="black" stroke-width="2" />
</svg>
<g id="myG" >
<rect onmousedown=startDrag(evt) onmousemove=drag(evt) onmouseup=endDrag() id="blueRect" x="220" y="250" width="60" height="60" fill="blue" stroke="black" stroke-width="2" />
</g>
</svg>
</div>
</td>
<td align=left>
<b>HTML Page Values:</b><br />
<input type=text id=htmlMouseXValue size=1 />: mouse X<br />
<input type=text id=htmlMouseYValue size=1 />: mouse Y<br />
<br />
<b>SVG Image Values:</b><br />
<input type=text id=svgXValue size=1 />: svg X<br />
<input type=text id=svgYValue size=1 />: svg Y<br />
</td>
</tr></table>
<br />SVG Source:<br />
<textarea id=svgSourceValue style='font-size:110%;font-family:lucida console;width:90%;height:200px'></textarea>
<br />Javascript:<br />
<textarea id=jsValue style='border-radius:26px;font-size:110%;font-weight:bold;color:midnightblue;padding:16px;background-color:beige;border-width:0px;font-size:100%;font-family:lucida console;width:90%;height:400px'></textarea>
</center>
<div id='browserDiv' style='padding:3px;position:absolute;top:5px;left:5px;background-color:gainsboro;'></div>
<script id=myScript>
var TransformRequestObj
var TransList
var DragTarget=null;
var Dragging = false;
var OffsetX = 0;
var OffsetY = 0;
//---mouse down over element---
function startDrag(evt)
{
if(!Dragging) //---prevents dragging conflicts on other draggable elements---
{
DragTarget = evt.target;
//---reference point to its respective viewport--
var pnt = DragTarget.ownerSVGElement.createSVGPoint();
pnt.x = evt.clientX;
pnt.y = evt.clientY;
//---elements transformed and/or in different(svg) viewports---
var sCTM = DragTarget.getScreenCTM();
var Pnt = pnt.matrixTransform(sCTM.inverse());
TransformRequestObj = DragTarget.ownerSVGElement.createSVGTransform()
//---attach new or existing transform to element, init its transform list---
var myTransListAnim=DragTarget.transform
TransList=myTransListAnim.baseVal
OffsetX = Pnt.x
OffsetY = Pnt.y
Dragging=true;
}
}
//---mouse move---
function drag(evt)
{
if(Dragging)
{
var pnt = DragTarget.ownerSVGElement.createSVGPoint();
pnt.x = evt.clientX;
pnt.y = evt.clientY;
//---elements in different(svg) viewports, and/or transformed ---
var sCTM = DragTarget.getScreenCTM();
var Pnt = pnt.matrixTransform(sCTM.inverse());
Pnt.x -= OffsetX;
Pnt.y -= OffsetY;
TransformRequestObj.setTranslate(Pnt.x,Pnt.y)
TransList.appendItem(TransformRequestObj)
TransList.consolidate()
}
}
//--mouse up---
function endDrag()
{
Dragging = false;
svgSourceValue.value=svgDiv.innerHTML
}
//---onload---
function initTransforms()
{
//---place some transforms on the elements---
//--- transform orange circle---
var transformRequestObj=mySVG.createSVGTransform()
var animTransformList=orangeCircle.transform
var transformList=animTransformList.baseVal
//---translate---
transformRequestObj.setTranslate(130,-300)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//----scale---
transformRequestObj.setScale(.5,.9)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//----skewY---
transformRequestObj.setSkewY(52)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//--init Transform on myG---
var transformRequestObj=mySVG.createSVGTransform()
var animTransformList=myG.transform
var transformList=animTransformList.baseVal
//---translate---
transformRequestObj.setTranslate(-50,-120)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//----skewX---
transformRequestObj.setSkewX(15)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//----skewY---
transformRequestObj.setSkewY(20)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
//---rotate---
transformRequestObj.setRotate(30,200,200)
transformList.appendItem(transformRequestObj)
transformList.consolidate()
}
document.onmousemove = htmCursor
//---event is the html event object---
function htmCursor(event)
{
var event = event || window.event;
myMouseX=event.clientX;
myMouseY=event.clientY;
myMouseX = myMouseX + document.documentElement.scrollLeft;
myMouseY = myMouseY + document.documentElement.scrollTop;
htmlMouseXValue.value=myMouseX
htmlMouseYValue.value=myMouseY
}
//---evt is the svg event object--
function svgCursor(evt)
{
var rect = svgDiv.getBoundingClientRect();
svgXValue.value=evt.clientX-rect.left
svgYValue.value=evt.clientY-rect.top
}
</script>
<script>
document.addEventListener("onload",init(),false)
function init()
{
initTransforms()
svgSourceValue.value=svgDiv.innerHTML
jsValue.value=myScript.text
}
</script>
</body>
</html>