不确定您的实际实现。但是这段代码可以工作：

Not sure about your actual implementation. However this code works:

要使用你的代码，这里是代码。您的代码的一个明显问题是您需要在style.left和style.right中添加px。您还不知道如何从代码中实际处理事件。使用此代码，元素移动一个圆圈，你需要修复它，但你明白了。

To do it using your code, here is the code. One obvious issue with your code is you need to add 'px' to your style.left and style.right. Also it is unknown how you actually handle the event from your code. Using this code, the element moves in a circle, you need to fix it but you get the idea.

var divOverlay = document.getElementById ("overlay");
var isDown = false;
divOverlay.addEventListener('mousedown', function(e) {
    isDown = true;
}, true);

document.addEventListener('mouseup', function() {
  isDown = false;
}, true);

document.addEventListener('mousemove', function(event) {
   event.preventDefault();
   if (isDown) {
   var deltaX = event.movementX;
   var deltaY = event.movementY;
  var rect = divOverlay.getBoundingClientRect();
  divOverlay.style.left = rect.x + deltaX + 'px';
  divOverlay.style.top  = rect.x + deltaX + 'px';
 }
}, true);

以下是另一种方法。
Codepen示例

var offset = [0,0];
var divOverlay = document.getElementById ("overlay");
var isDown = false;

divOverlay.addEventListener('mousedown', function(e) {
isDown = true;
offset = [
    divOverlay.offsetLeft - e.clientX,
    divOverlay.offsetTop - e.clientY
 ];
}, true);

document.addEventListener('mouseup', function() {
   isDown = false;
}, true);

document.addEventListener('mousemove', function(e) {
    event.preventDefault();
    if (isDown) {
        divOverlay.style.left = (e.clientX + offset[0]) + 'px';
        divOverlay.style.top  = (e.clientY + offset[1]) + 'px';
   }
}, true);