* {
            margin: 0;
            padding: 0;
        }
        
        p {
            background: skyblue;
            text-align: center;
        }
        
        html,
        body {
            width: 100%;
            height: 100%;
        }
        
        .mask {
            width: 100%;
            height: 100%;
            position: fixed;
            left: 0;
            top: 0;
            background: rgba(0, 0, 0, .5);
            display: none;
        }
        
        .login {
            width: 400px;
            height: 300px;
            background: purple;
            position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50%, -50%);
            display: none;
            cursor: move;
        }
        
        .login>span {
            display: inline-block;
            width: 50px;
            height: 50px;
            background: red;
            position: absolute;
            top: 0;
            right: 0;
        }

<p>我是p标签</p>
    <a href="http://www.baidu.com" >官网</a>
    <div class="mask"></div>
    <div class="login">
        <span></span>
</div>

//1.拿到需要操作的元素
const oP = document.querySelector("p");
const oMask = document.querySelector(".mask");
const oLogin = document.querySelector(".login");
const oClose = oLogin.querySelector(".login>span");
// console.log(oClose);
 
//2.监听点击事件
oP.onclick = function() {
    oMask.style.display = "block";
    oLogin.style.display = "block";
        };
        oClose.onclick = function() {
            oMask.style.display = "none";
            oLogin.style.display = "none";
        };
 
//3.监听登录框的按下和移动事件
oLogin.onmousedown = function(e) {
            e = e || e.window;
 
            //1.计算固定不变的距离
            const x = e.pageX - oLogin.offsetLeft;
            const y = e.pageY - oLogin.offsetTop;
            // console.log(x);
 
            //2.监听移动事件
            oLogin.onmousemove = function(e) {
                e = e || e.window;
 
                //3.计算移动之后的偏移位
                let offsetX = e.pageX - x;
                let offsetY = e.pageY - y;
 
                //4.重新设置登录框的位置
                oLogin.style.left = offsetX + 'px';
                oLogin.style.top = offsetY + 'px';
            };
        };
 
        oLogin.onmouseup = function() {
            oLogin.onmousemove = null;
        };