Primary Navigation Bar alternates between stacked and horizontal display...

Site URL: https://www.sitis.be/

Hi there,

I have some troubles to fix the display of my Primary nav bar on Moksha Template (Brine family). I don't understand why it's sometimes aligned horizontally (which is the expecting result) and sometimes stacked as on the 2nd screenshot below. (Same screen size for both). I noticed this issue with Chrome and Firefox engines.

I tried these following codes but I'm obviously not a boss  and of course it didn't work  > in Custom CSS :

.Header-nav-inner { 
 display: inline-block !important;}

and

.header-nav-folder-item {text-align: inline block !important;}

Any clues to fix that ? Many thanks in advance !

Aurélie