This is eventually coming to browsers in a clean way:
http://caniuse.com/#feat=css-sticky
It is not yet finalised, it'll be in the next version of one of the CSS specifications:
http://dev.w3.org/csswg/css-position-3/


However of course we don't need this to implement this. I'll give it a little thought.

Really pleased with this. I used the stuck_nav CSS class that our JavaScript handles. It was designed for the panel menus, but worked with the top ones with some tweaks.

Added class, moved overflow to inner element instead:

{+START,IF_NON_EMPTY,{CONTENT}}
<nav class="menu_type__dropdown stuck_nav" role="navigation" style="overflow: visible">
<ul onmouseout="return deset_active_menu()" class="nl" id="r_{MENU|*}_d" style="overflow: hidden">
{CONTENT}
</ul>

{$REQUIRE_JAVASCRIPT,javascript_menu_popup}
</nav>
{+END}

and had to add a higher z-index into the JavaScript.

wow, wonderful, thanks so much! Very pleased indeed.