Fixed it with adding z-index here:
#main-navigation > .sf-menu {
position: relative;
text-align: left;
z-index: 99999;
}
#main-navigation > .sf-menu > ul {
position: absolute;
top: 0;
left: 0;
width: 100%;
max-height: 0;
max-width: 16em;
overflow: hidden;
background: #333;
z-index: 99999;
