XmCreateSeparatorGadget(3) on Linux
The text fc79d53c4e81, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
The SeparatorGadget creation function "XmCreateSeparatorGadget" "creation functions" "XmCreateSeparatorGadget"