how to show fixed possition drop down in up on div code example

Example: 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)
}