wp-includes/blocks/navigation-submenu/editor.css

.wp-block-navigation-submenu{
  display:block;
}
.wp-block-navigation-submenu .wp-block-navigation__submenu-container{
  z-index:28;
}
.wp-block-navigation-submenu.has-child-selected>.wp-block-navigation__submenu-container,.wp-block-navigation-submenu.is-selected>.wp-block-navigation__submenu-container{
  height:auto !important;
  left:-1px;
  min-width:200px !important;
  opacity:1 !important;
  position:absolute;
  top:100%;
  visibility:visible !important;
  width:auto !important;
}
@media (min-width:782px){
  .wp-block-navigation-submenu.has-child-selected>.wp-block-navigation__submenu-container .wp-block-navigation__submenu-container,.wp-block-navigation-submenu.is-selected>.wp-block-navigation__submenu-container .wp-block-navigation__submenu-container{
    left:100%;
    top:-1px;
  }
  .wp-block-navigation-submenu.has-child-selected>.wp-block-navigation__submenu-container .wp-block-navigation__submenu-container:before,.wp-block-navigation-submenu.is-selected>.wp-block-navigation__submenu-container .wp-block-navigation__submenu-container:before{
    background:#0000;
    content:"";
    display:block;
    height:100%;
    position:absolute;
    right:100%;
    width:.5em;
  }
}