<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
.div {
display: none;
position: absolute;
width: 30px;
height: 30px;
margin: -15px 0 0 -15px;
background: rgba(37, 0, 255, 0.3);
-webkit-border-radius: 50%;
-moz-border-radius: 50%;
border-radius: 50%;
}
</style>
</head>
<body>
<div id="div" class="div"></div>
</body>
</html>
<script type="text/javascript">
window.onload = function() {
var a = 0
var oDiv = document.getElementById("div");
// window.onmousemove = function(ev) {
// var ev = ev || window.event;
// var ofLeft = document.documentElement.offsetLeft || document.body.offsetLeft;
// var ofTop = document.documentElement.offsetTop || document.body.offsetTop;
// var oLeft = ev.clientX + ofLeft;
// var oTop = ev.clientY + ofTop;
// oDiv.style.display = "block";
// oDiv.style.left = oLeft + "px";
// oDiv.style.top = oTop + "px";
// }
document.onmouseup = function(){
oDiv.style.display = "none";
a = 0;
}
document.onmousedown = function(){
a = 1
}
window.onmousemove = function(ev) {
if(a===1){
var ev = ev || window.event;
var ofLeft = document.documentElement.offsetLeft || document.body.offsetLeft;
var ofTop = document.documentElement.offsetTop || document.body.offsetTop;
var oLeft = ev.clientX + ofLeft;
var oTop = ev.clientY + ofTop;
oDiv.style.display = "block";
oDiv.style.left = oLeft + "px";
oDiv.style.top = oTop + "px";
}
}
}
</script>
原文：https://www.cnblogs.com/fxw1/p/14689265.html