code: func (w *Window) Focus() {
  	mode := byte(xproto.InputFocusPointerRoot)
  	err := xproto.SetInputFocusChecked(w.X.Conn(), mode, w.Id, 0).Check()
  	if err != nil {
  		xgbutil.Logger.Println(err)
  	}
  }
docs: // Focus tries to issue a SetInputFocus to get the focus.
  // If you're trying to change the top-level active window, please use
  // ewmh.ActiveWindowReq instead.