Moreover, mousemove events are not triggered if you perform drag'n'drop over a browser window.
To track mouse coordinates during drag'n'drop you should attach handler for document.ondragover event and use it's originalEvent property.
Example:
var globalDragOver = function (e)
{
var original = e.originalEvent;
if (original)
{
window.x = original.pageX;
window.y = original.pageY;
}
}