styles.css

.header--fixed {
  position: absolute;
  top: 0;
  width: 100%;
}
.header--fixed.top {
  transition: none;
  transform: translateY(0);
}
.header--fixed.not-top {
  position: fixed;
  transform: translateY(-100%);
}
.header--fixed.slideDown.not-top {
  transition: transform 0.3s ease-in-out;
  transform: translateY(0);
}
.header--fixed.slideDown.top {
  transition: transform 0.3s ease-in-out;
  position: fixed;
}
.header--fixed.slideUp.not-top {
  transition: transform 0.3s ease-in-out;
  transform: translateY(-100%);
}
.header--fixed.slideUp.top {
  transform: translateY(-100%);
  position: absolute;
}

* {
  box-sizing: border-box;
}

body {
  background-color: #333;
  font-family: sans-serif;
  margin: 0;
  padding: 0;
}

.content {
  height: 100px;
  background-color: #333;
  color: white;
  display: flex;
  justify-items: center;
  align-items: center;
}

header {
  background: #9999ff;
  padding: 40px;
  margin-bottom: 100px;
}