XmGetPostedFromWidget(3) on Linux
The text 06961cb5ceb4, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A RowColumn function that returns the widget from which a menu was posted "XmGetPostedFromWidget" "RowColumn functions" "XmGetPostedFromWidget"