No idea what you’ve done in your repo, I’m just working of the DDA master repo, which has game::multidrop in inventory_ui.cpp.
…Oh! I see what you mean. We want to edit game::multidrop, though, as opposed to game::drop or game::drop_in_direction, since game::multidrop is where the UI is defined.
Anyway, got a working implementation, if you wanna check it out.