Fixed headers have been causing menus to slide up in Chrome even though there is adequate space to slide down. This is code written to force menus to slide down.
The only thing you would need to change is the top value, depending on the height of your primary navigation.
.RadMenu .rmSlide { top:0 !important; bottom: auto !important}
.RadMenu .rmSlide ul.rmLevel1 { top:53px !important; bottom:auto !important; max-height:0 !important; -webkit-transition: max-height 1s; -moz-transition: max-height 1s; transition: max-height 1s;}
.RadMenu .rmSlide ul.rmLevel1[aria-expanded="true"] { max-height:1000px !important; }