How can I transition height:0;height:auto; using CSS?

Question

I'm trying to use CSS transitions to make

slide down.




Starts at height: 0;. On hover, the height is set to height:auto;. However, this results in it just appearing, without transitioning,




If I go from height: 40px; to height: auto; then it slides up to height: 0;< ;/code> and then suddenly jump to the correct height.

How else can I do this without using JavaScript?




#child0 {
  height: 0;
  overflow: hidden;
  background-color: #dedede;
  -moz-transition: height 1s ease;
  -webkit-transition: height 1s ease;
  -o-transition: height 1s ease;
  transition: height 1s ease;
}
#parent0:hover #child0 {
  height: auto;
}
#child40 {
  height: 40px;
  overflow: hidden;
  background-color: #dedede;
  -moz-transition: height 1s ease;
  -webkit-transition: height 1s ease;
  -o-transition: height 1s ease;
  transition: height 1s ease;
}
#parent40:hover #child40 {
  height: auto;
}
h1 {
  font-weight: bold;
}

The only difference between the two snippets of CSS is one has height: 0, the other height: 40.
<hr>
<div id="parent0">
  <h1>Hover me (height: 0)</h1>
  <div id="child0">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>
<hr>
<div id="parent40">
  <h1>Hover me (height: 40)</h1>
  <div id="child40">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>

Answer 1

You should use scaleY.

ul {
  background-color: #eee;
  transform: scaleY(0);    
  transform-origin: top;
  transition: transform 0.26s ease;
}
p:hover ~ ul {
  transform: scaleY(1);
}

<p>Hover This</p>
<ul>
  <li>Coffee</li>
  <li>Tea</li>
  <li>Milk</li>
</ul>

I made a vendor-prefixed version of the above code on jsfiddle and changed your jsfiddle to use scaleY instead of height.

edit Some people don't like the way scaleY converts content. If this is a problem then I'd recommend using clip instead.

ul {
  clip: rect(auto, auto, 0, auto);
  position: absolute;
  margin: -1rem 0;
  padding: .5rem;

  color: white;

  background-color: rgba(0, 0, 0, 0.8);

  transition-property: clip;
  transition-duration: 0.5s;
  transition-timing-function: cubic-bezier(0.175, 0.885, 0.32, 1.275);
}
h3:hover ~ ul,
h3:active ~ ul,
ul:hover {
  clip: rect(auto, auto, 10rem, auto);
}

<h3>Hover here</h3>
<ul>
  <li>This list</li>
  <li>is clipped.</li>
  <li>A clip transition</li>
  <li>will show it</li>
</ul>
<p>
  Some text...
</p>

Answer 2

Use max-height in transitions instead of height. And set the value of max-height to something larger than your box.

See Chris Jordan's answer to here in another JSFiddle demo /stackoverflow.com/a/20226830/18706">

#menu #list {
    max-height: 0;
    transition: max-height 0.15s ease-out;
    overflow: hidden;
    background: #d5d5d5;
}

#menu:hover #list {
    max-height: 500px;
    transition: max-height 0.25s ease-in;
}

<div id="menu">
    <a>hover me</a>
    <ul id="list">
        <!-- Create a bunch, or not a bunch, of li's to see the timing. -->
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>