Thanks. Perhaps my bad suggesting that the default sub-menu is horizontal as I can’t find those examples now (definitely had them yesterday, I promise 🙂 ). Anyhow, I came to the conclusion that custom is the only way, so I implemented a quick proof-of-concept yesterday. It completely doesn’t work when in ‘mobile’ mode just yet, but the desktop CSS was dead straight forward – something along the lines of:
.dropdown-menu {
left: 0!important;
right: 0!important;
width: 100% !important;
max-width: none !important;
margin: 0 !important;
}
Again, this is just a proof-of-concept and not tested very widely at all but perhaps it might be a useful starting place if anyone else has this question in future.
Thanks again. 🙂