Need Mobile Drop-Down Menu @ approx. 1280px breakpoint

This issue is now almost one month old.
Previously I requested help with a mobile drop-down menu to show at an approximate 1280px breakpoint and the code below was sent to me by an atheme team member:

@media (max-width: 1280px) {
  .header-mobile-menu {
    display: block;
@media (max-width: 1280px) {
  .main-navigation { 
     display: none; 
@media (max-width: 1280px) {
  .site-branding {
    margin-right: 10px;
    margin-left: 10px;
    -webkit-box-flex: 1;
    -ms-flex-positive: 1;
    flex-grow: 1;
    max-width: 100%;

Problem with this CSS: The mobile menu icon shows, but no drop-down menu appears. It’s empty so to speak.
Please fix the CSS so that I can actually have a working drop-down menu.

Hello Erika,

My answer from this topic is still actual:

Kind Regards, Roman.