This is a very minor issue, but still a usability issue. If you look at the bottom of this website, at the "My account" or "Wiki" menus that have subitems, when you click those menus they open/expand/unfold downwards, instead of upwards, which is obviously wrong when there is no available space below it but plenty of space above.
So maybe the fix is to make an exception for menus present in a footer, or if someone wants to create a "bulletproof" fix (to account for side menus or whatever situation), ideally maybe any menu should be evaluating its position on the currently visible area and determine which direction has the most free space to overlay the submenus on without causing scrolling.
This issue happens both on Tiki 21x and 24x, and affects more than one theme (it happens both on the default theme and on "tikipedia", for example).