XmMainWindow(3) on Linux
The text d8f8830128fc, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
The MainWindow widget class "XmMainWindow" "widget class" "MainWindow"