My website (http://www.oia.co.za/schedules/) has a horizontal navigation menu with dropdowns on hover, and the darn thing shifts the neighbouring list-item the width of the dropdown on hover.
I can’t for the life of me figure out what the problem is, and my eyes are square from looking at the CSS all night.
I know it is something stupidly simple, and I’ve just looked at it for too long…
You could try updating the dropdown ul to be absolutely positioned: