Hi Steven,
Easy way is to use simpleMenubar widget instead of menubar.
If its not native mobile app and if you want to use menubar , then you can achieve this by adding custom styles to your theme. Below is the CSS code you that you need to add.
.custom-navbar .nav>li {display: inline-block; }
Apply .custom-navbar class to your menu bar widget.
Please see this forum post on how to add custom styles.
Hope this helps!
thank you - super helpful