<div class="extra"></div>
<div id="wrapper">
  <div class="sticky">
    sticky
  </div>
  <div class='sticky sticky-subject'>sdjfnsa</div>
</div>
<div class="extra"></div>

.sticky {
  position: sticky;
  position: -webkit-sticky;
  background: #f83d23;
  width: 100px;
  height: 100px;
  top: 70px;
  display: flex;
  justify-content: center;
  align-items: center;
  box-shadow: 0 0 6px #000;
  color: #fff;
  z-index: 10;
}
  .sticky-subject{
    z-index: 0;
    top: 170px;
    height: 300px;
    overflow: hidden;
    background-color: green;
  }

.extra,
#wrapper {
  width: 75%;
  margin: auto;
  background-color: #ccc;
  margin-bottom: 60px;
}
#wrapper {
  height: auto;
}
.extra {
  height: 100px;
}
body {
  font-family: georgia;
  height: 1000px;
}
h4 {
  text-align: center;
}
@media (min-height: 768px) {
  #wrapper{
    height: 2000px;
  }
}