s/downdrop/dropdown/ in comment 0. :)

Tagging 'easy'. Just re-use the implementation for p-cactions.

r86861 (the fix for bug 24298) was followed-up by r99083 which made the feature more generic by applying it to any vectorMenu (instead of just p-cactions).

If this is not working on the language variants menu, perhaps it doesn't have the "vectorMenu" menu class. This would presumably cause other problems as well.

Checking SkinVector.php I see both p-varians and p-cations have class "vectorMenu" set. So it should be working.

It isn't working because I9a2ebd503e change the current-variant heading and the dropdown arrow heading (h4 and h5 respectively) to h3. And the script looks for the first h3 in a vectorMenu, which as of I9a2ebd503e is no longer the heading with the trigger in it.

Change 79847 had a related patch set uploaded by Hoo man:
Make variant selection menu toggleable by keyboard

https://gerrit.wikimedia.org/r/79847

Change 79847 merged by TheDJ:
Make variant selection menu toggleable by keyboard

https://gerrit.wikimedia.org/r/79847