XmTabStackGetSelectedTab(3) on Linux
The text 7abe768efa91, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A TabStack function that returns the widget ID of the currently selectedtab "XmTabStackGetSelectedTab" "TabStack functions" "XmTabStackGetSelectedTab"