It is not possible to use transitions with height:auto.
The trick with max-height is a pretty good solution but has some inconvenience, especially a weird delay if max-height is much higher than the real height.
Here is the trick I use : http://jsfiddle.net/g56jwux4/2/
Basically it is two imbricated DIVs translating in opposite directions. Take a look a the code at jsfiddle because my english is not good enough to explain it clearly.
HTML part:
<body>
<p>Technicaly this dropdown menu looks like a simple height transition.</p>
<p>But this pure css code also works this a variable number of choices in menu, working around the "height:auto" not taken into account by css transitions.</p>
<input type="checkbox" id="menuOpen"></input>
<label id="bouton" for="menuOpen"><span>Click on me !</span>
<div id="menu">
<div id="masque">
<div class="choix" id="choix1">Choix 1</div>
<div class="choix" id="choix2">Choix 2</div>
<div class="choix" id="choix3">Choix 3 très très long pour voir la taille finale du menu</div>
<div class="choix" id="choix4">Choix 4</div>
<div class="choix" id="choix5">Choix 5</div>
<div class="choix" id="choix6">Choix 6</div>
</div>
</div>
</label>
</body>
CSS Part :
body {
font-family: sans-serif;
}
#menuOpen {
display: none;
}
#bouton {
position: absolute;
left: 100px;
top: 100px;
width: 200px;
height: 30px;
background-color: lightgray;
cursor: pointer;
}
#bouton > span {
color: black;
padding: 6px;
line-height: 30px;
}
#menu {
position: absolute;
top: 100%;
overflow: hidden;
min-width: 100%;
transition: transform 0.3s linear 0s, visibility 0.3s linear 0s;
transform: translateY(-100%);
visibility: hidden;
color: white;
}
#menuOpen:checked + #bouton > #menu {
transform: translateY(0%);
visibility: visible;
transition: transform 0.3s linear 0s, visibility 0s linear 0s;
}
#menuOpen:checked + #bouton > #menu > #masque {
transform: translateY(0%);
}
#masque {
position: relative;
background-color: gray;
transform: translateY(100%);
transition: transform 0.3s linear 0s;
}
.choix {
white-space: nowrap;
padding: 3px 6px;
}
.choix:hover {
background-color: darkgray;
}