Function XPGetWidgetExposedGeometry 

pub unsafe extern "C" fn XPGetWidgetExposedGeometry(
    inWidgetID: XPWidgetID,
    outLeft: *mut c_int,
    outTop: *mut c_int,
    outRight: *mut c_int,
    outBottom: *mut c_int,
)

XPGetWidgetExposedGeometry

This routine returns the bounds of the area of a widget that is completely within its parent widgets. Since a widget’s bounding box can be outside its parent, part of its area will not be eligible for mouse clicks and should not draw. Use XPGetWidgetGeometry to find out what area defines your widget’s shape, but use this routine to find out what area to actually draw into. Note that the widget library does not use OpenGL clipping to keep frame rates up, although you could use it internally.