Hm, has just found one thing that may serve as a substitute for
..............const int xid = GDK_WINDOW_XID(window->window);
Guys, can you please check if this helps rxvt to have a tab?
Code:
Window focusWindow;
Display *focusDisplay = XOpenDisplay(NULL);
int param = 0;
int xid = XGetInputFocus(focusDisplay,&focusWindow,¶m);