Divi 4 Theme Builder Hack for Fixed Navigation Bars

functions.php

/**
 * Custom Body Class for Child Theme
 */
function divi_child_body_class( $classes ) {
  $classes[] = 'child';
  return $classes;
}
add_action( 'body_class', 'divi_child_body_class' );

/**
 * Fixed Body Class for Theme Builder Header
 */
function divi_child_tb_fixed_body_class( $classes ) {
  $has_tb_header = in_array( 'et-tb-has-header', $classes );
  $is_fixed_header = 'on' === et_get_option( 'divi_fixed_nav', 'on' );

  if ($has_tb_header) {
    if ($is_fixed_header) {
      $classes[] = 'et_fixed_nav';
    } else {
      $classes[] = 'et_non_fixed_nav';
    }
    $classes[] = 'et_show_nav';
    unset($classes[array_search('et-tb-has-header', $classes)]);
  }
  return $classes;
}
add_filter( 'body_class', 'divi_child_tb_fixed_body_class');

/**
 * Set Layout ID for the Main Header
 */
function divi_child_set_layout_id( $layout_id, $post_type ) {
  if ($post_type === 'et_header_layout') {
    $layout_id = 'main-header';
  }
  return $layout_id;
}
add_filter( 'et_builder_layout_id', 'divi_child_set_layout_id', 10, 2);

style.css

/**
 * THEME BUILDER HEADER FIX
 */
.child.et_fixed_nav.et_show_nav #main-header .et_pb_fullwidth_menu .et_pb_row,
.child.et_non_fixed_nav.et_show_nav #main-header .et_pb_fullwidth_menu .et_pb_row {
  margin-top: 0;
  margin-bottom: 0;
}

@media screen and (max-width: 980px) {
  .child.et_fixed_nav #main-header {
    position: fixed; /* Delete this if you don't want a fixed menu on mobile devices! */
  }
}

.child .et_pb_fullwidth_menu .et_pb_menu__search {
  padding: 0 2rem;
  background-color: #fff;
}

/* Fix the upper margin in the Template Builder */
.child header#main-header.et-fb-root-ancestor {
  top: 0 !important;
}

/* Fix the padding at the bottom in the Template Builder */
.child header#main-header #et_pb_root {
  padding-bottom: 0;
}