With reference to /theme/boost/scss/moodle/drawer.scss there is the selector:

#page {
    margin-top: $navbar-height;
}

which is then defined further down as:

#page {
    margin-top: $fixed-header-y;
}

which overrides it.  However both $navbar-height and $fixed-header-y are the same value because of:

$fixed-header-y: $navbar-height;

at the top of the file.