I want a dropdown menu, but when I set the position “fixed” it doesn't work

问题

I need the position to be fixed, I can't figure out how to align the menu otherwise with the top of the screen, I always get gaps between the edge of the screen and the menu.

html:

<ul>
  <li class="dropdown">
    <a href="javascript:void(0)" class="dropbtn">Monitor</a>
    <div class="dropdown-content">
      <a href="Index.html">Tube 1</a>
      <a href="#">Tube 2</a>
    </div>
  </li>
  <li><a href="Developer.html">Input</a></li>
  <li><a href="Information.html">Information</a></li>
  <li style="float:right" id="Credit"><a>Text</a></li>
</ul>

css:

ul {
  z-index: 1;
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  border: 1px solid #e7e7e7;
  background-color: #f3f3f3;
  top: 0;
  right: 0;
  width: 100%;
  font-family: "Arial Black";
  font-size: 16px;
}

li {
  float: left;
}

li a, .dropbtn {
  display: inline-block;
  color: #666;
  text-align: center;
  padding: 10px 16px;
  text-decoration: none;
}

li a:hover:not(.active), .dropdown:hover .dropbtn {
  background-color: #ddd;
}

li a.active {
  color: white;
  background-color: #008CBA;
}

li.dropdown {
  display: inline-block;
}

.dropdown-content {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
  text-align: left;
}

.dropdown-content a:hover {background-color: #f1f1f1}

.dropdown:hover .dropdown-content {
  display: block;
}

When I add position: fixed; to ul, the dropdown menu doesn't work anymore, but when I don't add it, I get ugly gaps between the edge of the screen and the menu.

回答1:

Your body have margin: 8px. Just remove margin. Add in CSS:

body{
    margin: 0;
}

回答2:

You should also add "position: fixed;" to dropdown-content.

ul {
  z-index: 1;
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  border: 1px solid #e7e7e7;
  background-color: #f3f3f3;
  top: 0;
  right: 0;
  width: 100%;
  font-family: "Arial Black";
  font-size: 16px;
  position: fixed;
}

li {
  float: left;
}

li a, .dropbtn {
  display: inline-block;
  color: #666;
  text-align: center;
  padding: 10px 16px;
  text-decoration: none;
}

li a:hover:not(.active), .dropdown:hover .dropbtn {
  background-color: #ddd;
}

li a.active {
  color: white;
  background-color: #008CBA;
}

li.dropdown {
  display: inline-block;
}

.dropdown-content {
  display: none;
  position: fixed;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-content a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
  text-align: left;
}

.dropdown-content a:hover {background-color: #f1f1f1}

.dropdown:hover .dropdown-content {
  display: block;
}

<ul>
  <li class="dropdown">
    <a href="javascript:void(0)" class="dropbtn">Monitor</a>
    <div class="dropdown-content">
      <a href="Index.html">Tube 1</a>
      <a href="#">Tube 2</a>
    </div>
  </li>
  <li><a href="Developer.html">Input</a></li>
  <li><a href="Information.html">Information</a></li>
  <li style="float:right" id="Credit"><a>Text</a></li>
</ul>
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来源：https://stackoverflow.com/questions/54135333/i-want-a-dropdown-menu-but-when-i-set-the-position-fixed-it-doesnt-work