XmTabStackXYToWidget(3) on Linux
The text 62f20d717e02, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch) shipped it.
A TabStack function that converts a pixel coordinate to the widget ID of the tab occupying that space "XmTabStackXYToWidget" "TabStack functions" "XmTabStackXYToWidget"