https://invisible-island.net/xterm/xtoolkit/
XtTranslateCoords - translate widget coordinates
#include <X11/Intrinsic.h> void XtTranslateCoords(Widget w, Position x, Position y, Position *rootx_return, Position *rooty_return);
rootx_return rooty_return Returns the root-relative x and y coordinates. x y Specify the widget-relative x and y coordinates. w Specifies the widget.
While XtTranslateCoords is similar to the Xlib XTranslateCoordinates function, it does not generate a server request because all the required information already is in the widget's data structures.
X Toolkit Intrinsics – C Language Interface Xlib - C Language X Interface X Version 11 libXt 1.1.5.2 XtTranslateCoords(3)