fixe component goes over the screen code example

Example 1: css div went over top menu

#navbar {
  background-color:#990000;
  position:fixed;  
  z-index:1; /*Add this*/
  width:100%;
  height:50px;
  text-align:center;
  vertical-align:middle;
  line-height:50px;
  top:0px;
}

Example 2: middle div scrollable side div unscrollable below header

header {
position: absolute;
top: 0;
width: 100%
}

sideDiv {
padding-top: (height of header)
position: sticky;
top: 0;
height: 100vh
}
scrollableDiv {
padding-top: (height of header)
}