XmCascadeButton(3) on Linux
The text a7f71dbe56db, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
The CascadeButton widget class "XmCascadeButton" "widget class" "CascadeButton"