我有一个在桌面和移动设备上都很流畅的导航栏。但是，下拉项宽度不会根据父项的宽度调整大小。我想将鼠标悬停在下拉项上，它的宽度与我悬停的列表项的宽度相同。目前它有一个固定的宽度，但我似乎无法通过尝试其他选项来让它工作。我也无法将整个导航栏置于中心位置，因为目前它被粘在左侧。感谢您的帮助！
这是代码的代码笔：https://codepen.io/Macast/pen/rYQPNe
.HTML：
<nav>
<div id="logo">
<img src="images/J.-Freeman-&-Son-Landscape-Logo-White.png">
</div>
<label for="drop" class="toggle">Menu</label>
<input type="checkbox" id="drop" />
<ul class="menu">
<li><a href="#">Home</a></li>
<li>
<!-- First Tier Drop Down -->
<label for="drop-1" class="toggle">Short +</label>
<a href="#">Short</a>
<input type="checkbox" id="drop-1" />
<ul>
<li><a href="#">History</a></li>
<li><a href="#">Our Services</a></li>
<li><a href="#">Our Aim</a></li>
</ul>
</li>
<li>
<!-- First Tier Drop Down -->
<label for="drop-2" class="toggle">Dropdown Even Longer +</label>
<a href="#">Dropdown Even Longer</a>
<input type="checkbox" id="drop-2" />
<ul>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li>
<!-- Second Tier Drop Down -->
<label for="drop-3" class="toggle">More Options +</label>
<a href="#">More Options</a>
<input type="checkbox" id="drop-3" />
<ul>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
</ul>
</nav>
.CSS：
.toggle,
[id^=drop] {
display: none;
}
/* Giving a background-color to the nav container. */
nav {
margin: 0;
padding: 0;
background-color: #254441;
}
#logo {
display: block;
text-align: center;
/*padding: 0 30px;*/
/*float: left;*/
/*font-size: 20px;*/
/*line-height: 60px; */
}
#logo img {
width: 30%;
}
/* Since we'll have the "ul li" "float:left"
* we need to add a clear after the container. */
nav:after {
content: "";
display: table;
clear: both;
}
/* Removing padding, margin and "list-style" from the "ul",
* and adding "position:reltive" */
nav ul {
/*float: right;*/
padding: 0;
margin: 0;
list-style: none;
position: relative;
}
/* Positioning the navigation items inline */
nav ul li {
margin: 0px;
display: inline-block;
float: left;
background-color: #254441;
text-align: center;
}
/* Styling the links */
nav a {
display: block;
padding: 14px 20px;
color: #FFF;
font-size: 17px;
text-decoration: none;
text-align: center;
}
nav ul li ul li:hover {
background: #000000;
}
/* Background color change on Hover */
nav a:hover {
background-color: #000000;
}
/* Hide Dropdowns by Default
* and giving it a position of absolute */
nav ul ul {
display: none;
position: absolute;
/* has to be the same number as the "line-height" of "nav a" */
top: 60px;
}
/* Display Dropdowns on Hover */
nav ul li:hover > ul {
display: inherit;
}
/* Fisrt Tier Dropdown */
nav ul ul li {
width: 170px;
float: none;
display: list-item;
position: relative;
text-align: center;
}
/* Second, Third and more Tiers
* We move the 2nd and 3rd etc tier dropdowns to the left
* by the amount of the width of the first tier.
*/
nav ul ul ul li {
position: relative;
top: -60px;
/* has to be the same number as the "width" of "nav ul ul li" */
left: 170px;
}
/* Change ' +' in order to change the Dropdown symbol */
li > a:after {
content: ' +';
}
li > a:only-child:after {
content: '';
}
/* Media Queries
--------------------------------------------- */
@media all and (max-width: 768px) {
#logo {
display: block;
padding: 0;
width: 100%;
text-align: center;
float: none;
}
#logo img {
width: 100%;
box-sizing: border-box;
padding: 20px;
}
nav {
margin: 0;
}
/* Hide the navigation menu by default */
/* Also hide the */
.toggle + a,
.menu {
display: none;
}
/* Stylinf the toggle lable */
.toggle {
display: block;
background-color: #254441;
padding: 14px 20px;
color: #FFF;
font-size: 17px;
text-decoration: none;
border: none;
text-align: center;
}
.toggle:hover {
background-color: #000000;
}
/* Display Dropdown when clicked on Parent Lable */
[id^=drop]:checked + ul {
display: block;
}
/* Change menu item's width to 100% */
nav ul li {
display: block;
width: 100%;
}
nav ul ul .toggle,
nav ul ul a {
padding: 0 40px;
}
nav ul ul ul a {
padding: 0 80px;
}
nav a:hover,
nav ul ul ul a {
background-color: #000000;
}
nav ul li ul li .toggle,
nav ul ul a,
nav ul ul ul a {
padding: 14px 20px;
color: #FFF;
font-size: 17px;
}
nav ul li ul li .toggle,
nav ul ul a {
background-color: #212121;
}
/* Hide Dropdowns by Default */
nav ul ul {
float: none;
position: static;
color: #ffffff;
/* has to be the same number as the "line-height" of "nav a" */
}
/* Hide menus on hover */
nav ul ul li:hover > ul,
nav ul li:hover > ul {
display: none;
}
/* Fisrt Tier Dropdown */
nav ul ul li {
display: block;
width: 100%;
}
nav ul ul ul li {
position: static;
/* has to be the same number as the "width" of "nav ul ul li" */
}
}
@media all and (max-width: 330px) {
nav ul li {
display: block;
width: 94%;
}
}
要将下拉菜单大小调整为与包含列表项相同的大小，请在有问题的下拉元素 (nav ul ul上声明一个left并right属性值0，然后删除下拉菜单的嵌套列表项 (nav ul ul li) 上显式声明的width。
例：
nav ul ul {
display: none;
position: absolute;
top: 60px;
/* additional property values declared */
left: 0;
right: 0;
}
这是因为下拉菜单已定位absolute，并且包含父元素列表项 (nav ul li)已定位relative。
如果元素的父元素relative，则可以相对于其父元素定位absolute元素。
因此，我们在这里所做的只是定义下拉菜单宽度的范围，以从父宽度的"左到右"拉伸"。
代码片段演示：
.toggle,
[id^=drop] {
display: none;
}
/* Giving a background-color to the nav container. */
nav {
margin: 0;
padding: 0;
background-color: #254441;
}
#logo {
display: block;
text-align: center;
/*padding: 0 30px;*/
/*float: left;*/
/*font-size: 20px;*/
/*line-height: 60px; */
}
#logo img {
width: 30%;
}
/* Since we'll have the "ul li" "float:left"
* we need to add a clear after the container. */
nav:after {
content: "";
display: table;
clear: both;
}
/* Removing padding, margin and "list-style" from the "ul",
* and adding "position:reltive" */
nav ul {
text-align: center;
padding: 0;
margin: 0;
list-style: none;
position: relative;
}
/* Positioning the navigation items inline */
nav ul li {
margin: 0px;
display: inline-block;
background-color: #254441;
text-align: center;
position: relative;
}
/* Styling the links */
nav a {
display: block;
padding: 14px 20px;
color: #FFF;
font-size: 17px;
text-decoration: none;
text-align: center;
}
nav ul li ul li:hover {
background: #000000;
}
/* Background color change on Hover */
nav a:hover {
background-color: #000000;
}
/* Hide Dropdowns by Default
* and giving it a position of absolute */
nav ul ul {
display: none;
position: absolute;
/* has to be the same number as the "line-height" of "nav a" */
top: 60px;
left: 0;
right: 0;
}
/* Display Dropdowns on Hover */
nav ul li:hover > ul {
display: inherit;
}
/* Fisrt Tier Dropdown */
nav ul ul li {
/*width: 170px;*/
float: none;
display: list-item;
position: relative;
text-align: center;
}
/* Second, Third and more Tiers
* We move the 2nd and 3rd etc tier dropdowns to the left
* by the amount of the width of the first tier.
*/
nav ul ul ul li {
position: relative;
top: -60px;
/* has to be the same number as the "width" of "nav ul ul li" */
left: 170px;
}
/* Change ' +' in order to change the Dropdown symbol */
li > a:after {
content: ' +';
}
li > a:only-child:after {
content: '';
}
/* Media Queries
--------------------------------------------- */
@media all and (max-width: 768px) {
#logo {
display: block;
padding: 0;
width: 100%;
text-align: center;
float: none;
}
#logo img {
width: 100%;
box-sizing: border-box;
padding: 20px;
}
nav {
margin: 0;
}
/* Hide the navigation menu by default */
/* Also hide the */
.toggle + a,
.menu {
display: none;
}
/* Stylinf the toggle lable */
.toggle {
display: block;
background-color: #254441;
padding: 14px 20px;
color: #FFF;
font-size: 17px;
text-decoration: none;
border: none;
text-align: center;
}
.toggle:hover {
background-color: #000000;
}
/* Display Dropdown when clicked on Parent Lable */
[id^=drop]:checked + ul {
display: block;
}
/* Change menu item's width to 100% */
nav ul li {
display: block;
width: 100%;
}
nav ul ul .toggle,
nav ul ul a {
padding: 0 40px;
}
nav ul ul ul a {
padding: 0 80px;
}
nav a:hover,
nav ul ul ul a {
background-color: #000000;
}
nav ul li ul li .toggle,
nav ul ul a,
nav ul ul ul a {
padding: 14px 20px;
color: #FFF;
font-size: 17px;
}
nav ul li ul li .toggle,
nav ul ul a {
background-color: #212121;
}
/* Hide Dropdowns by Default */
nav ul ul {
float: none;
position: static;
color: #ffffff;
/* has to be the same number as the "line-height" of "nav a" */
}
/* Hide menus on hover */
nav ul ul li:hover > ul,
nav ul li:hover > ul {
display: none;
}
/* Fisrt Tier Dropdown */
nav ul ul li {
display: block;
width: 100%;
}
nav ul ul ul li {
position: static;
/* has to be the same number as the "width" of "nav ul ul li" */
}
}
@media all and (max-width: 330px) {
nav ul li {
display: block;
width: 94%;
}
}<nav>
<div id="logo">
<img src="images/J.-Freeman-&-Son-Landscape-Logo-White.png">
</div>
<label for="drop" class="toggle">Menu</label>
<input type="checkbox" id="drop" />
<ul class="menu">
<li><a href="#">Home</a></li>
<li>
<!-- First Tier Drop Down -->
<label for="drop-1" class="toggle">Short +</label>
<a href="#">Short</a>
<input type="checkbox" id="drop-1" />
<ul>
<li><a href="#">History</a></li>
<li><a href="#">Our Services</a></li>
<li><a href="#">Our Aim</a></li>
</ul>
</li>
<li>
<!-- First Tier Drop Down -->
<label for="drop-2" class="toggle">Dropdown Even Longer +</label>
<a href="#">Dropdown Even Longer</a>
<input type="checkbox" id="drop-2" />
<ul>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li>
<!-- Second Tier Drop Down -->
<label for="drop-3" class="toggle">More Options +</label>
<a href="#">More Options</a>
<input type="checkbox" id="drop-3" />
<ul>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
<li><a href="#">Option</a></li>
</ul>
</nav>此外：
您可以通过删除在嵌套列表项 (nav ul li上声明的float规则来水平居中导航菜单，因为这将否定任何对齐内容的尝试(除非您使用flex-box)，然后在包含的无序列表(.menu上声明text-align: center)，如下所示：
nav ul {
text-align: center;
padding: 0;
margin: 0;
list-style: none;
position: relative;
}