The icons used in the navigation bar (that come from Font-Awesome) are not showing properly. I would guess they need a "font-weight:900" but I'm not sure.
When a navbar item is hovered over, it gets a 1px border added which shifts everything around in the navbar both horizontally and expands everything up vertically. That should be removed, or all navbar items should have an invisible border to start with so their size doesn't change.
An issue we have in other themes may appear here too, when you fix the icons - the "closed" and "open" icons need a set width so they don't also shift things around in the navbar on state change.
Two issues:
An issue we have in other themes may appear here too, when you fix the icons - the "closed" and "open" icons need a set width so they don't also shift things around in the navbar on state change.