HTML+CSS+JS 自定义下拉菜单

效果演示

20-自定义下拉菜单.gif

实现了一个下拉菜单的动画效果，包括一个主按钮和一个下拉菜单列表。点击主按钮会展开下拉菜单列表，同时箭头会旋转，列表项会逐渐显示出来。鼠标悬停在列表上时，会出现一个浮动图标。整个下拉菜单的样式比较简洁，使用了黑色和灰色的背景色，以及白色和淡灰色的文字和图标颜色。动画效果使用了 CSS 的过渡和变换属性。

Code

HTML

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

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>自定义下拉菜单</title>
    <link rel="stylesheet" href="./20-自定义下拉菜单.css">
</head>

<body>
    <div class="dropdown-container">
        <button class="dropdown-button main-button">
            <span class="dropdown-title-icon">
                <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1024 1024">
                    <path
                        d="M937.4 423.9c-84 0-165.7-27.3-232.9-77.8v352.3c0 179.9-138.6 325.6-309.6 325.6S85.3 878.3 85.3 698.4c0-179.9 138.6-325.6 309.6-325.6 17.1 0 33.7 1.5 49.9 4.3v186.6c-15.5-6.1-32-9.2-48.6-9.2-76.3 0-138.2 65-138.2 145.3 0 80.2 61.9 145.3 138.2 145.3 76.2 0 138.1-65.1 138.1-145.3V0H707c0 134.5 103.7 243.5 231.6 243.5v180.3l-1.2 0.1"
                        p-id="1556"></path>
                </svg>
            </span>
            <span class="dropdown-title text-truncate">抖音</span>
            <span class="dropdown-arrow">
                <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
                    <path
                        d="M7.247 11.14 2.451 5.658C1.885 5.013 2.345 4 3.204 4h9.592a1 1 0 0 1 .753 1.659l-4.796 5.48a1 1 0 0 1-1.506 0z" />
                </svg>
            </span>
        </button>
        <div class="dropdown-list-container">
            <div class="dropdown-list-wrapper">
                <ul class="dropdown-list"></ul>
                <div class="floating-icon" aria-hidden="true"></div>
            </div>
        </div>
    </div>
</body>
<script src="./20-自定义下拉菜单.js"></script>
</html>

CSS

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --rotate-arrow: 0;
    --dropdown-height: 0;
    --list-opacity: 0;
    --translate-value: 0;
    --floating-icon-size: 26;
    --floating-icon-top: 0;
    --floating-icon-left: 0;
}

html {
    font-size: 80%;
}

body {
    height: 100%;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    background-color: #222429;
    padding: 1.5rem;
    line-height: 1.4rem;
}

button {
    border: none;
    background-color: transparent;
    cursor: pointer;
    outline: none;
}

svg {
    width: 1.6rem;
    height: 1.6rem;
}

.text-truncate {
    text-overflow: ellipsis;
    overflow: hidden;
    white-space: nowrap;
}

.dropdown-container {
    margin-top: 30vh;
    display: flex;
    flex-direction: column;
    width: 100%;
    max-width: 34rem;
}

.dropdown-title-icon,
.dropdown-arrow {
    display: inline-flex;
}

.dropdown-title {
    margin: 0 auto 0 1.8rem;
}

.dropdown-button {
    font-weight: 400;
    font-size: 1.7rem;
    display: flex;
    align-items: center;
    padding: 0 1.8rem;
}

.dropdown-button svg {
    fill: #b1b8ca;
    transition: all 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.dropdown-button svg,
.dropdown-button span {
    pointer-events: none;
}

.dropdown-button:hover,
.dropdown-button:focus {
    color: #fff;
}

.dropdown-button:hover svg,
.dropdown-button:focus svg {
    fill: #fff;
}

.main-button {
    height: 5.2rem;
    background-color: #333740;
    color: #b1b8ca;
    border-radius: 1.4rem;
    border: 0.1rem solid #494d59;
    transition: all 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.main-button:focus {
    border: 0.1rem solid #2c62f6;
    box-shadow: 0 0 0 0.2rem rgba(44, 98, 246, 0.4);
}

.main-button .dropdown-arrow {
    margin-left: 1.8rem;
    transform: rotate(var(--rotate-arrow));
    transition: transform 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.list-button {
    height: 4.6rem;
    color: #b1b8ca;
    overflow: hidden;
    cursor: none;
    transition: color 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.dropdown-list-container {
    overflow: hidden;
    max-height: var(--dropdown-height);
    transition: max-height 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.dropdown-list-wrapper {
    margin-top: 1rem;
    padding: 1rem;
    background-color: #333740;
    border-radius: 1.4rem;
    border: 0.1rem solid #494d59;
    position: relative;
}

ul.dropdown-list {
    position: relative;
    list-style-type: none;
}

ul.dropdown-list::before {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    left: 0;
    z-index: 0;
    opacity: 0;
    height: 4.6rem;
    background-color: #2b2e34;
    border-radius: 1.4rem;
    pointer-events: none;
    transform: translateY(var(--translate-value));
    transition: all 0.4s linear;
}

li.dropdown-list-item {
    display: flex;
    flex-direction: column;
    position: relative;
    z-index: 1;
    opacity: var(--list-opacity);
    transition: opacity 0.8s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.floating-icon {
    width: calc(var(--floating-icon-size) * 1px);
    height: calc(var(--floating-icon-size) * 1px);
    position: absolute;
    left: var(--floating-icon-left);
    top: var(--floating-icon-top);
    background-color: #494d59;
    border-radius: 1rem;
    pointer-events: none;
    opacity: 0;
    z-index: 2;
    display: inline-flex;
    justify-content: center;
    align-items: center;
    transition: opacity 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

.floating-icon svg {
    fill: #fff;
}

ul.dropdown-list:hover::before,
ul.dropdown-list:hover ~ .floating-icon {
    opacity: 1;
}

JavaScript

const root=document.documentElement;
const dropdown_title_icon=document.querySelector(".dropdown-title-icon");
const dropdown_title=document.querySelector(".dropdown-title");
const dropdown_list=document.querySelector(".dropdown-list");
const main_button=document.querySelector(".main-button");
const floating_icon=document.querySelector(".floating-icon");

const icons={
    "百度":
        "M226.8 535.7c96.8-20.8 83.6-136.4 80.6-161.7-4.8-39-50.6-107.2-112.8-101.8-78.3 7-89.8 120.2-89.8 120.2-10.5 52.3 25.4 164.1 122 143.3z m102.7 201.1c-2.9 8.2-9.1 28.9-3.7 47 10.8 40.6 46.1 42.4 46.1 42.4h50.7V702.4h-54.2c-24.5 7.3-36.3 26.3-38.9 34.4z m76.8-395c53.5 0 96.6-61.5 96.6-137.6 0-76-43.1-137.5-96.6-137.5-53.4 0-96.6 61.5-96.6 137.5 0 76.1 43.2 137.6 96.6 137.6z m230.2 9.1c71.4 9.3 117.4-67 126.4-124.8 9.3-57.6-36.8-124.7-87.3-136.2-50.7-11.6-113.9 69.5-119.7 122.4-6.8 64.8 9.3 129.4 80.6 138.6z m175 339.6S701 605 636.5 512.6c-87.4-136.3-211.7-80.8-253.2-11.6-41.4 69.3-105.8 113.1-115 124.7-9.3 11.5-133.5 78.5-105.9 200.9C189.9 949 286.8 946.8 286.8 946.8s71.3 7 154.2-11.5c82.8-18.4 154.1 4.6 154.1 4.6s193.4 64.7 246.4-60c52.8-124.8-30-189.4-30-189.4z m-331 185.6H354.8c-54.3-10.9-76-47.9-78.7-54.2-2.7-6.4-18.1-36.2-9.9-86.9 23.5-76 90.4-81.4 90.4-81.4h66.9v-82.3l57 0.9v303.9z m234.3-0.9H570c-56.1-14.4-58.7-54.3-58.7-54.3v-160l58.7-1v143.8c3.6 15.3 22.7 18.2 22.7 18.2h59.6v-161h62.5v214.3z m204.8-427.3c0-27.6-22.9-110.9-108.1-110.9-85.3 0-96.7 78.6-96.7 134.1 0 53 4.5 127.1 110.5 124.6 106-2.3 94.3-120 94.3-147.8z m0 0",
    "微信":
        "M308.73856 119.23456C23.65696 170.15296-71.37024 492.23936 155.392 639.66464c12.43392 7.99232 12.43392 7.104-6.21824 62.76096l-15.98464 47.65952 57.43104-30.784 57.43104-30.78656 30.49216 7.40096c31.96928 7.99232 72.82432 13.61664 100.0576 13.61664l16.28416 0-5.62688-21.61152c-44.70016-164.5952 109.82912-327.71072 310.8352-327.71072l27.2384 0-5.62432-19.53792C677.59616 186.43456 491.392 86.67136 308.73856 119.23456zM283.87072 263.40352c30.1952 20.4288 31.97184 64.5376 2.95936 83.48416-47.06816 30.78656-102.1312-23.38816-70.45632-69.57056C230.28736 256.59648 263.74144 249.78688 283.87072 263.40352zM526.62016 263.40352c49.73568 33.45408 12.43392 110.71744-43.22304 89.40288-40.25856-15.39328-44.99712-70.75072-7.40096-90.5856C490.79808 254.22848 513.88928 254.81984 526.62016 263.40352zM636.44928 385.37216c-141.2096 25.7536-239.19872 132.91776-233.57184 256.06656 7.40096 164.89472 200.71168 278.56896 386.32448 227.65312l21.90592-5.92128 46.1824 24.8704c25.4592 13.9136 46.77376 23.97696 47.36512 22.79168 0.59392-1.47968-4.43648-19.24352-10.95168-39.6672-14.79936-45.59104-15.09632-42.33472 4.73856-56.54272C1121.64864 654.464 925.67552 332.97408 636.44928 385.37216zM630.82496 518.28992c12.4288 8.28928 18.944 29.01248 13.61408 44.1088-11.24864 32.26624-59.49952 34.63424-72.52992 3.55328C557.10976 530.13248 597.9648 496.97536 630.82496 518.28992zM828.57472 521.84576c19.53792 18.64704 16.2816 50.32448-6.51264 62.16448-34.93376 17.76128-71.63904-17.76128-53.58336-51.80416C780.32128 510.2976 810.81344 504.97024 828.57472 521.84576z",
    "抖音":
        "M937.4 423.9c-84 0-165.7-27.3-232.9-77.8v352.3c0 179.9-138.6 325.6-309.6 325.6S85.3 878.3 85.3 698.4c0-179.9 138.6-325.6 309.6-325.6 17.1 0 33.7 1.5 49.9 4.3v186.6c-15.5-6.1-32-9.2-48.6-9.2-76.3 0-138.2 65-138.2 145.3 0 80.2 61.9 145.3 138.2 145.3 76.2 0 138.1-65.1 138.1-145.3V0H707c0 134.5 103.7 243.5 231.6 243.5v180.3l-1.2 0.1",
    "哔哩哔哩":
        "M306.005333 117.632L444.330667 256h135.296l138.368-138.325333a42.666667 42.666667 0 0 1 60.373333 60.373333L700.330667 256H789.333333A149.333333 149.333333 0 0 1 938.666667 405.333333v341.333334a149.333333 149.333333 0 0 1-149.333334 149.333333h-554.666666A149.333333 149.333333 0 0 1 85.333333 746.666667v-341.333334A149.333333 149.333333 0 0 1 234.666667 256h88.96L245.632 177.962667a42.666667 42.666667 0 0 1 60.373333-60.373334zM789.333333 341.333333h-554.666666a64 64 0 0 0-63.701334 57.856L170.666667 405.333333v341.333334a64 64 0 0 0 57.856 63.701333L234.666667 810.666667h554.666666a64 64 0 0 0 63.701334-57.856L853.333333 746.666667v-341.333334A64 64 0 0 0 789.333333 341.333333zM341.333333 469.333333a42.666667 42.666667 0 0 1 42.666667 42.666667v85.333333a42.666667 42.666667 0 0 1-85.333333 0v-85.333333a42.666667 42.666667 0 0 1 42.666666-42.666667z m341.333334 0a42.666667 42.666667 0 0 1 42.666666 42.666667v85.333333a42.666667 42.666667 0 0 1-85.333333 0v-85.333333a42.666667 42.666667 0 0 1 42.666667-42.666667z",
    "淘宝":
        "M168.5 273.7a68.7 68.7 0 1 0 137.4 0 68.7 68.7 0 1 0-137.4 0z m730 79.2s-23.7-184.4-426.9-70.1c17.3-30 25.6-49.5 25.6-49.5L396.4 205s-40.6 132.6-113 194.4c0 0 70.1 40.6 69.4 39.4 20.1-20.1 38.2-40.6 53.7-60.4 16.1-7 31.5-13.6 46.7-19.8-18.6 33.5-48.7 83.8-78.8 115.6l42.4 37s28.8-27.7 60.4-61.2h36v61.8H372.9v49.5h140.3v118.5c-1.7 0-3.6 0-5.4-0.2-15.4-0.7-39.5-3.3-49-18.2-11.5-18.1-3-51.5-2.4-71.9h-97l-3.4 1.8s-35.5 159.1 102.3 155.5c129.1 3.6 203-36 238.6-63.1l14.2 52.6 79.6-33.2-53.9-131.9-64.6 20.1 12.1 45.2c-16.6 12.4-35.6 21.7-56.2 28.4V561.3h137.1v-49.5H628.1V450h137.6v-49.5H521.3c17.6-21.4 31.5-41.1 35-53.6l-42.5-11.6c182.8-65.5 284.5-54.2 283.6 53.2v282.8s10.8 97.1-100.4 90.1l-60.2-12.9-14.2 57.1S882.5 880 903.7 680.2c21.3-200-5.2-327.3-5.2-327.3z m-707.4 18.3l-45.4 69.7 83.6 52.1s56 28.5 29.4 81.9C233.8 625.5 112 736.3 112 736.3l109 68.1c75.4-163.7 70.5-142 89.5-200.7 19.5-60.1 23.7-105.9-9.4-139.1-42.4-42.6-47-46.6-110-93.4z"
};

// 下拉列表项
const list_items=["百度","微信","抖音","哔哩哔哩","淘宝"];

const iconTemplate=(path)=>{
    return `
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1024 1024">
            <path d="${path}"></path>
        </svg>
    `;
}

const listItemTemplate=(text,translate_value)=>{
    return `
        <li class="dropdown-list-item">
            <button class="dropdown-button list-button" data-translate-value="${translate_value}%">
                <span class="text-truncate">${text}</span>
            </button>
        </li>
    `;
}

const renderListItems=()=>{
    dropdown_list.innerHTML+=list_items.map((item,index)=>{
        return listItemTemplate(item,100*index);
    }).join("");
}

window.addEventListener("load",()=>{
    renderListItems();
})

const setDropdownProps=(deg,ht,opacity)=>{
    root.style.setProperty("--rotate-arrow",deg!==0?deg+"deg":0);
    root.style.setProperty("--dropdown-height",ht!==0?ht+"rem":0);
    root.style.setProperty("--list-opacity",opacity);
}

main_button.addEventListener("click",()=>{
    const list_wrapper_sizes=3.5;
    const dropdown_open_height=4.6*list_items.length+list_wrapper_sizes;
    const curr_dropdown_height=root.style.getPropertyValue("--dropdown-height")||0;
    curr_dropdown_height==="0"?setDropdownProps(180,dropdown_open_height,1):setDropdownProps(0,0,0);
})

dropdown_list.addEventListener("mouseover",(e)=>{
    const translate_value=e.target.dataset.translateValue;
    root.style.setProperty("--translate-value",translate_value);
})

dropdown_list.addEventListener("click",(e)=>{
    const clicked_item_text=e.target.innerText.toLowerCase().trim();
    const clicked_item_icon=icons[clicked_item_text];
    dropdown_title_icon.innerHTML=iconTemplate(clicked_item_icon);
    dropdown_title.innerHTML=clicked_item_text;
    setDropdownProps(0,0,0);
})

dropdown_list.addEventListener("mousemove",(e)=>{
    const icon_size=root.style.getPropertyValue("--floating-icon-size")||0;
    const x=e.clientX-dropdown_list.getBoundingClientRect().x;
    const y=e.clientY-dropdown_list.getBoundingClientRect().y;
    const targetText=e.target.innerText.toLowerCase().trim();
    const hover_item_text=icons[targetText];
    floating_icon.innerHTML=iconTemplate(hover_item_text);
    root.style.setProperty("--floating-icon-left",x-icon_size/2+"px");
    root.style.setProperty("--floating-icon-top",y-icon_size/2+"px");
})

实现思路拆分

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

这段代码是设置所有元素的外边距和内边距为0，并且使用border-box盒模型。

:root {
    --rotate-arrow: 0;
    --dropdown-height: 0;
    --list-opacity: 0;
    --translate-value: 0;
    --floating-icon-size: 26;
    --floating-icon-top: 0;
    --floating-icon-left: 0;
}

这段代码是定义了一些CSS变量，用于后面的样式中。

html {
    font-size: 80%;
}

这段代码是设置HTML元素的字体大小为80%。

body {
    height: 100%;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    background-color: #222429;
    padding: 1.5rem;
    line-height: 1.4rem;
}

这段代码是设置body元素的高度为100%，并且使用flex布局，使其垂直居中。同时设置了背景色、内边距和行高。

button {
    border: none;
    background-color: transparent;
    cursor: pointer;
    outline: none;
}

这段代码是设置按钮元素的样式，去掉边框、背景色和轮廓线，并且设置光标为手型。

svg {
    width: 1.6rem;
    height: 1.6rem;
}

这段代码是设置SVG元素的宽度和高度为1.6rem。

.text-truncate {
    text-overflow: ellipsis;
    overflow: hidden;
    white-space: nowrap;
}

这段代码是定义了一个CSS类，用于截断文本并显示省略号。

.dropdown-container {
    margin-top: 30vh;
    display: flex;
    flex-direction: column;
    width: 100%;
    max-width: 34rem;
}

这段代码是设置下拉菜单容器的样式，包括垂直方向的外边距、使用flex布局、宽度和最大宽度。

.dropdown-title-icon,
.dropdown-arrow {
    display: inline-flex;
}

这段代码是设置下拉菜单标题图标和箭头的样式，使用inline-flex布局。

.dropdown-title {
    margin: 0 auto 0 1.8rem;
}

这段代码是设置下拉菜单标题的样式，包括左外边距和居中对齐。

.dropdown-button {
    font-weight: 400;
    font-size: 1.7rem;
    display: flex;
    align-items: center;
    padding: 0 1.8rem;
}

这段代码是设置下拉菜单按钮的样式，包括字体粗细、字体大小、使用flex布局、垂直居中和内边距。

.dropdown-button svg,
.dropdown-button span {
    pointer-events: none;
}

这段代码是设置下拉菜单按钮中的SVG图标和文本不响应鼠标事件。

.dropdown-button svg {
    fill: #b1b8ca;
    transition: all 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单按钮中的SVG图标的填充颜色，并且设置过渡效果。

.dropdown-button:hover,
.dropdown-button:focus {
    color: #fff;
}

这段代码是设置下拉菜单按钮在鼠标悬停或获得焦点时的文本颜色。

.dropdown-button:hover svg,
.dropdown-button:focus svg {
    fill: #fff;
}

这段代码是设置下拉菜单按钮在鼠标悬停或获得焦点时的SVG图标填充颜色。

.main-button {
    height: 5.2rem;
    background-color: #333740;
    color: #b1b8ca;
    border-radius: 1.4rem;
    border: 0.1rem solid #494d59;
    transition: all 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单主按钮的样式，包括高度、背景色、文本颜色、圆角和边框，并且设置过渡效果。

.main-button:focus {
    border: 0.1rem solid #2c62f6;
    box-shadow: 0 0 0 0.2rem rgba(44, 98, 246, 0.4);
}

这段代码是设置下拉菜单主按钮在获得焦点时的边框和阴影效果。

.main-button .dropdown-arrow {
    margin-left: 1.8rem;
    transform: rotate(var(--rotate-arrow));
    transition: transform 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单主按钮中的箭头的样式，包括左外边距和旋转效果，并且设置过渡效果。

.list-button {
    height: 4.6rem;
    color: #b1b8ca;
    overflow: hidden;
    cursor: none;
    transition: color 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单列表项的样式，包括高度、文本颜色、隐藏溢出内容和禁用鼠标光标。

.dropdown-list-container {
    overflow: hidden;
    max-height: var(--dropdown-height);
    transition: max-height 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单列表容器的样式，包括隐藏溢出内容和设置最大高度，并且设置过渡效果。

.dropdown-list-wrapper {
    margin-top: 1rem;
    padding: 1rem;
    background-color: #333740;
    border-radius: 1.4rem;
    border: 0.1rem solid #494d59;
    position: relative;
}

这段代码是设置下拉菜单列表的样式，包括上外边距、内边距、背景色、圆角和边框，并且设置相对定位。

ul.dropdown-list {
    position: relative;
    list-style-type: none;
}

这段代码是设置下拉菜单列表的样式，包括相对定位和去掉列表项的默认样式。

ul.dropdown-list::before {
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    left: 0;
    z-index: 0;
    opacity: 0;
    height: 4.6rem;
    background-color: #2b2e34;
    border-radius: 1.4rem;
    pointer-events: none;
    transform: translateY(var(--translate-value));
    transition: all 0.4s linear;
}

这段代码是设置下拉菜单列表的伪元素样式，包括绝对定位、透明度、高度、背景色、圆角、不响应鼠标事件和垂直方向的平移效果，并且设置过渡效果。

li.dropdown-list-item {
    display: flex;
    flex-direction: column;
    position: relative;
    z-index: 1;
    opacity: var(--list-opacity);
    transition: opacity 0.8s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单列表项的样式，包括使用flex布局、相对定位、层级、透明度和过渡效果。

.floating-icon {
    width: calc(var(--floating-icon-size) * 1px);
    height: calc(var(--floating-icon-size) * 1px);
    position: absolute;
    left: var(--floating-icon-left);
    top: var(--floating-icon-top);
    background-color: #494d59;
    border-radius: 1rem;
    pointer-events: none;
    opacity: 0;
    z-index: 2;
    display: inline-flex;
    justify-content: center;
    align-items: center;
    transition: opacity 0.4s cubic-bezier(0.25, 0.46, 0.45, 0.94);
}

这段代码是设置下拉菜单列表项中的浮动图标的样式，包括宽度、高度、绝对定位、左上角位置、背景色、圆角、不响应鼠标事件、透明度、层级、使用inline-flex布局、垂直居中和过渡效果。

.floating-icon svg {
    fill: #fff;
}

这段代码是设置下拉菜单列表项中的浮动图标中的SVG图标的填充颜色。

ul.dropdown-list:hover::before,
ul.dropdown-list:hover ~ .floating-icon {
    opacity: 1;
}

这段代码是设置下拉菜单列表在鼠标悬停时的伪元素和浮动图标的透明度为1，以显示出来。