Bingo!
Commenting out this chunk of my style.css now gives the desired effect
.main-navigation ul.nav-menu, .main-navigation div.nav-menu > ul {
border: none;
border-bottom:solid medium #606060;
/* display: inline-block !important;
text-align: left;
width: 100%;
*/
}
I'm not using media queries, mainly because I'm oldschool and don't own anything but a desktop system, but I'm guessing the display: line was the culprit since I'm not using media queries, so it would take effect regardless of screen width... So educational !
Once the site is made public I'll be sure to come back and tell you all about it. I've bent a lot of Wordpress themes, but this is my first with Tiny Forge. It's very powerful and extensive, but possibly it's top selling point is the fact you answered my question within an hour in English that even a caveman like me can comprehend.
You Sir truly rock! \o/