XmStringExtent(3) on Linux
The text b2e0627a7cca, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A compound string function that determines the size of the smallest rectangle that will enclose the compound string "XmStringExtent" "compound string functions" "XmStringExtent"