body{
  height: 400vh; /* pour pouvoir scroller */
}

#a{
 width:50px;
 height:50px;
 line-height: 50px;
 text-align: center;
 font-size: 40px;
 border:2px solid #000;
 border-radius: 50px;
 position: fixed;
 top:20px;
 left:20px;
}

#maDiv{
  margin-top: 100vh;
  transition: background-color 1s;
}

.rouge{
  background-color: red;
  position: sticky;
  top:50vh;
}