I am not sure and I have not been by the Litvacks for a few days, so I have not looked at the Gemara on pg 23 in Bava Kama nor the Tosphot nor the Shulchan Aruch where the Gra bring his idea. But from what I vaguely recall i think the Shulchan Aruch says that if one lights a fire in his own courtyard and it caused damage there to his neighbor’s property who live in the same courtyard, then he is liable both for his arrows and his property but if there a wall and the fire jumped over it then the liability IS only for damage by mean of hi property. On the courtyard the liability would be for both open and hidden thing but outside the courtyard only for open thing. But this would not be like the gemara where Rava says if there is a wall then there is no liability for damage outside the wall since it was not negligence,---------------------------I am not sure and I have not been by the Litvacks for a few days, so I have not looked at the גמרא on pg. כ''ג in בבא קמא nor the תוספות nor the שלחן ערוך where the גר''א bring his idea. But from what I vaguely recall I think the שלחן ערוך says that if one lights a fire in his own courtyard and it caused damage there to his neighbor’s property who lives in the same courtyard, then he is liable both for damage caused by his arrows and caused by his property. But if there a wall and the fire jumped over it, then the liability IS only for damage by means of his property. In the courtyard the liability would be for both open and hidden things, but outside the courtyard, only for open things. But this would not be like the גמרא where רבא says if there is a wall then there is no liability for damage outside the wall since it was not negligence