Bootstrap collapsed menu not pushing content down when expanded

I don't know. This seems to work... (kind of a hack though).

.navbar-fixed-top {
  top: -70px; /* you'll have to figure out the exact number here */
}

.navbar-fixed-top, .navbar-fixed-bottom {
  position: relative; /* this can also be static */
}

If you are using a fixed navbar expanding the menu won't push down the content. For that you have to use static header itself. Check the bootstrap example link you gave as reference.

<nav class="navbar navbar-light bg-light navbar-expand-lg static-top">