原生JS手风琴组件：过渡效果未生效的排查

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>手风琴</title>
    <style>
        body,
        ul {
            padding: 0;
            margin: 0;
        }

        li {
            list-style: none;
        }

        .nav {
            width: 150px;
            height: 310px;
            margin-top: 30px;
            margin-left: 50px;
            font-size: 20px;
            border: 1px solid #ccc;
        }

        .nav>ul>li:nth-child(2n+1) {
            background-color: cadetblue;
        }

        .nav>ul>li:nth-child(2n+2) {
            height: 160px;
            display: none;
            transition: all 1s;
        }

        .nav>ul>#selected {
            background-color: rgb(46, 115, 117);
        }

        .nav>ul>li:nth-child(2) {
            display: block;
        }
    </style>
</head>

<body>
    <div class="nav">
        <ul>
            <li id="selected">标题1</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
            <li>标题2</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
            <li>标题3</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
            <li>标题4</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
            <li>标题5</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
            <li>标题6</li>
            <li>
                <ul>
                    <li>1</li>
                    <li>2</li>
                    <li>3</li>
                    <li>4</li>
                </ul>
            </li>
        </ul>
    </div>
    <script>
        var title = document.querySelectorAll(".nav>ul>li:nth-child(2n+1)");
        for (var i = 0; i < title.length; i++) {
            title[i].onmouseover = function () {
                for (var j = 0; j < title.length; j++) {
                    title[j].nextElementSibling.style.display = "none";
                    title[j].id = "";
                }
                this.id = "selected";
                this.nextElementSibling.style.display = "block";
            }
        }
    </script>
</body>

</html>