XmListPosSelected(3) on Linux
The text 65a338791f5a, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A List function that determines if the list item at a specified position is selected "XmListPosSelected" "List functions" "XmListPosSelected"