CSS vh and vw: Why does my div overlap with the div above it?

Question

In the following SSCCE, why does the #sliderArea div overlap the #topBar div above it, while div is a block-level element?

I expected it to appear below the #topBar?

JsFiddle here.

#left-div {
  float: left;
}
#right-div {
  float: right;
}
a {
  margin: 10px;
}
#topBar {
  width: 100%;
}
#sliderArea {
  background-color: yellow;
  width: 100vw;
  height: 100vh;
}
#four-cols {
  margin: 5px 100px 5px 100px;
}
#col-one {
  background-color: orange;
  height: 125px;
  width: 100px;
  display: inline-block;
}
#col-two {
  background-color: blue;
  height: 125px;
  width: 100px;
  display: inline-block;
}
#col-three {
  background-color: green;
  height: 125px;
  width: 100px;
  display: inline-block;
}
#col-four {
  background-color: red;
  height: 125px;
  width: 100px;
  display: inline-block;
}

<div id="topBar">
  <div id="left-div">
    <a href="#">Login</a>
  </div>
  <div id="right-div">
    <a href="#">Menu One</a>
    <a href="#">Menu Two</a>
  </div>
</div>
<div id="sliderArea"></div>
<div id="four-cols">
  <div id="col-one"></div>
  <div id="col-two"></div>
  <div id="col-three"></div>
  <div id="col-four"></div>
</div>

Answer 1

You need to clear the floats :

div{ clear:both; }

JSFiddle

Or just for #sliderArea element :

JSFiddle