@ qBart and all plugin users:
This issue was detailed tested on this last days and the result if that the problem is a bug of WordPress Core (rejected from WordPress Team :lol) related to this class: wp_admin_bar_class !
The problem exist only on WordPress 3.8+ (trunk) 3.8.0 ~ 3.8.1 and 3.9+ (trunk) 3.9.0 due to bad MP6 interface integration CSS WordPress Core and “new” mobile responsivenes CSS layout.
To test it create a clean plugin or add on function.php this:
add_filter( 'wp_admin_bar_class', '__return_false' );
My plugin is compatible and tested up to WordPress 3.7+ (trunk)
WordPress 3.8+ and 3.9+ trunk was not “official” supported (only developer release support it) for now …
Before any my action, waiting the future of MP6 integration on WordPress 3.9+ and 4.0+ (trunk) sorry!
Read also this related thread
PLEASE NOTE THAT ANYONE HAVE SOLUTION IS BEEN ACCEPTED AND I HAPPY TO INSERT YOUR CODE ON MY PLUGIN