https://invisible-island.net/xterm/xtoolkit/
XtRegisterDrawable - register a drawable with the Intrinsics event dis-
patcher
#include <X11/Intrinsic.h>
void XtRegisterDrawable(Display* display, Drawable drawable, Widget
widget);
void XtUnregisterDrawable(Display* display, Drawable drawable);
display Specifies the drawable's display.
drawable Specifies the drawable to register.
widget Specifies the widget to register the drawable for.
XtRegisterDrawable associates the specified drawable with the specified
widget so that future calls to XtWindowToWidget with the drawable will
return the widget. The default event dispatcher will dispatch future
core events that arrive with the drawable to the widget as though the
event contained the widget's window, but the event itself will not be
changed in any way when being passed to even handler or action proce-
dures.
If the drawable is already registered with another widget, or if the
drawable is the window of a widget in the client's widget tree, the
results of calling XtRegisterDrawable are undefined.
XtUnregisterDrawable removes an association created with XtRegister-
Drawable. If the drawable is the window of a widget in the client's
widget tree the results of calling XtUnregisterDrawable are undefined.
X Toolkit Intrinsics – C Language Interface
Xlib - C Language X Interface
X Version 11 libXt 1.1.5.2 XtRegisterDrawable(3)