Hello,
Removing this line fixed the menu dropping down on our demo. Your results may vary based on how many menu items you have on the same line. Only option is really to reduce the font size, padding or amount of menu items.
s5_flex_menu.css - line 141
#s5_nav li.mainParentBtn .s5_level1_span2 {
padding-right: 16px;
}
You could also swap to the mobile menu sooner by modifying the first line of s5_responsive_bars.css
Regards,