This is one of the new divi menu options. It has a hamburger icon and when you click that te menu slides in from the right and pushes the whole page to the left. With this modification the menu slides over the page.
Add this CSS snippet to the custom CSS field in the Divi settings:
.et_pb_slide_menu_active #page-container, .et_pb_slide_menu_active #main-header {
left: 0!important;
}
.et_pb_slide_menu_active #et-top-navigation {
margin-right: 320px;
}
Code language: CSS (css)