code: func WmWindowOpacityGet(xu *xgbutil.XUtil, win xproto.Window) (float64, error) {
  	intOpacity, err := xprop.PropValNum(
  		xprop.GetProperty(xu, win, "_NET_WM_WINDOW_OPACITY"))
  	if err != nil {
  		return 0, err
  	}
  
  	return float64(uint(intOpacity)) / float64(0xffffffff), nil
  }
docs: // _NET_WM_WINDOW_OPACITY get
  // This isn't part of the EWMH spec, but is widely used by drop in
  // compositing managers (i.e., xcompmgr, cairo-compmgr, etc.).
  // This property is typically set not on a client window, but the *parent*
  // of a client window in reparenting window managers.
  // The float returned will be in the range [0.0, 1.0] where 0.0 is completely
  // transparent and 1.0 is completely opaque.