SidebarExample.css

html, body {
  width: 100%;
  height: 100%;
  margin: 0;
}

#sidebar, #content {
  height: 100%;
  overflow:auto;
  float:left;
  transition: width .35s;
}

#sidebar {
  background:white;
  width: 30%;
}

#sidebar.collapsed {
  width: 0;
}

#sidebar.collapsed + #content {
  width: 100%;
}

#content {
  background:gray;
  width: 70%;
}

button {
  width: 30px;
  height: 30px;
  background: black;
}