I decided to copy/paste the layout from 2c-r-fixed.css into my child theme style.css (to save overriding) and noticed that that the dropdown menu stopped displaying the sub-menus.
It turned out that as default.css is normally imported after 2c-r-fixed.css, the overflow:visible in the former usually overrides overflow:hidden in the latter, so there's usually no problem until that part is copied into style.css:
default.css
#access {
border-bottom:1px solid #ccc;
height:32px;
font-size:13px;
overflow:visible;
z-index:100;
}
2c-r-fixed.css
#access {
position:relative;
overflow:hidden;
}
I'm not sure if this is a feature or a bug but thought it might be worth posting about it.