Div disappears during CSS transition

Question

I have two divs beside one another. With the click of a button, the div on the left collapses to the left leaving only the div on the right. Since the div on the right is width:100%;, it fills the space left behind by the collapsed left div. The only problem is that while the div is collapsing/the CSS transition is in progress, the right div disappears for a moment and I can't figure out why.

HTML

<div>
  <div id="demo" class="collapse show width">
    <aside>
    </aside>
  </div><!--COLLAPSE-->

  <main>
    <button role="button" class="btn btn-danger" data-toggle="collapse" data-target="#demo">
      simple horizontal collapsible
    </button>
    <div class="sample">
      Some TExt
    </div>
  </main>
</div>

CSS

html {
  height: 100%
}

body {
  height: 100%;
  font: 14px "Lucida Grande", Helvetica, Arial, sans-serif;
  background: gray;
}

a {
  color: #00B7FF;
}

.collapse {
  visibility: hidden;
}

.collapse.show {
  visibility: visible;
  display: block;
}

.collapsing {
  position: relative;
  height: 0;
  overflow: hidden;
  -webkit-transition-property: height, visibility;
  transition-property: height, visibility;
  -webkit-transition-duration: 0.45s;
  transition-duration: 0.45s;
  -webkit-transition-timing-function: ease;
  transition-timing-function: ease;
}

.collapsing.width {
  -webkit-transition-property: width, visibility;
  transition-property: width, visibility;
  width: 0;
  height: auto;
}

aside {
  height: 100vh;
  width: 250px;
  background-color: white;
  /*border-right:1px solid black;*/
  display: block;
  float: left;
}

.sample {
  margin-top: 100px;
  text-align: center;
}

main {
  width: 100%;
  height: 100vh;
  background-color: pink;
  display: inline;
}

LIVE EXAMPLE

https://jsfiddle.net/djjvqtop/1/

This occurs in all browsers. Is there a way to prevent this?

Answer 1

The problem is caused by a fact that adding overflow: hidden changes the height of #demo. Initially it is 0, because it contains nothing but the float element. Class .collapsing adds overflow: hidden, which makes it expand to contain the float. So #demo.collapsing has the height of <aside> and the <main> element is below it and out of sight.

One solution is to make #demo also float left.