<div class="wrapper">
      <div class="container">
        <div class="content"></div>
      </div>
      <div class="full-sized"></div>
    </div>

body {
  margin: 0px;
}

.wrapper {
  height: 100vh;
  position: relative;
  overflow: auto;
}

.container {
  height: 100%;
  position: sticky;
  top: 0px;
}

.content {
  position: relative;
  height: 2500px;
  background-image: url(http://via.placeholder.com/640x70/);
}

.full-sized {
  position: absolute;
  width: 100%;
  height: 100%;
  background-color: green;
}