Source: https://www.elegantthemes.com/blog/divi-resources/how-to-make-the-standard-and-vertical-navigation-overlap-on-your-divi-website

/* -- START fixed secondary menu -- */
.et_vertical_nav #top-header, .loaded.et_vertical_nav #top-header {
margin-left: 0 !important; 
z-index: 10000;
position: fixed;
width: 100%;
}
/* -- END fixed secondary menu -- */
Code language: CSS (css)