#5923 and then #6295 updated css for the bottom sticker so it has

z-index: 999;
position: sticky

#top-sticker also has z-index:999 but no explicit positioning.

This causes elements in the top-sticker submenus to appear behind elements from the bottom sticker (see attached screenshot for an example).

Expected behaviour
Elements in the top-sticker should always be displayed above elements from the bottom-sticker.