Preview:
//Por tanto, deberías usar document.documentElement en lugar de document.firstChild para obtener el //elemento raíz.

           var box1=document.getElementById("box1");
           document.addEventListener("mousemove", function(e){
           var st=document.documentElement.scrollTop;
               //console.log(st);
               // Obtener las coordenadas del mouse
               var left= e.clientX;
               var top= e.clientY;
               box1.style.left=left+st+"px";
               box1.style.top=top+st+"px";
           });
        };

//   <box id="box1"></box>
/*
     #box1{
            width: 100px;
            height: 100px;
            background-color: blueviolet;
            position: absolute;
        }
*/
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