Fade-in effect on a drop-down menu doesn't happen always

Question

I want my drop-down menu to have a fade-in effect when mouse-hovered. I've written the following code for that, but the fade-in effect can be observed sometimes only, not always. Code:

* {
  margin: 0;
  padding: 0;
}

.header {
  position: fixed;
  list-style-type: none;
  height: 35px;
  width: 100%;
  background: #333333;
}

.header > li {
  display: inline-block;
  padding: 8px;
  position: relative;
  color: #FFF;
}

.header > li:hover {
  background: #000000;
}

.dropdown ul {
  list-style-type: none;
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  color: red;
  width: 100%;
  opacity: 0;
  background: yellow;
  border: 1px solid black;
  transition: opacity 1s;
}

.dropdown:hover ul {
  opacity: 1;
  display: block;
}

<ul class="header">
  <li class="home">Test</li>
  <li class="dropdown">Dropdown &#10097;
    <ul>
      <li>item 1</li>
      <li>item 2</li>
      <li>item 3</li>
    </ul>
  </li>
</ul>

I think the effect doesn't occur when I unhover and hover over "Dropdown ❱" within a second. I'm trying to do this using HTML and CSS only.

How can I achieve what I want?

BTW, How do I create a fade-out effect for the menu?

Answer 1

display is not an animatable property so you can use visibility instead of display.

Here is the list of animatable-properties.

* {
  margin: 0;
  padding: 0;
}
.header {
  position: fixed;
  list-style-type: none;
  height: 35px;
  width: 100%;
  background: #333333;
}
.header > li {
  display: inline-block;
  padding: 8px;
  position: relative;
  color: #FFF;
}
.header > li:hover {
  background: #000000;
}
.dropdown ul {
  list-style-type: none;
  /*display: none;*/
  visibility: hidden;
  position: absolute;
  top: 100%;
  left: 0;
  color: red;
  width: 100%;
  opacity: 0;
  background: yellow;
  border: 1px solid black;
  transition: visibility 1s, opacity 1s;
}
.dropdown:hover ul {
  opacity: 1;
  visibility: visible;
}

<ul class="header">
  <li class="home">Test</li>
  <li class="dropdown">Dropdown &#10097;
    <ul>
      <li>item 1</li>
      <li>item 2</li>
      <li>item 3</li>
    </ul>
  </li>
</ul>