Gemara

2b (Inference): Our Mishnah says that if partners build a wall in the middle if (both) wanted. If not, one could not force the other. This teaches that Hezek Re'iyah (invasion of privacy) is not considered damage.

23a: Rav Yosef had date trees. Bloodletters sat under them. Ravens came to eat the blood of their Kelim, and damaged the dates. Rav Yosef ordered the bloodletters to leave. Abaye said that they are Muchzak to sit there.

Rav Yosef: Rav Nachman taught that there is no Chazakah for something that damages.

Abaye: Rav Mari taught that that refers to smoke, and Rav Zevid taught that it refers to a privy. (It does not apply to other things!)

Rav Yosef: I am very sensitive. The ravens bother me like smoke and a Beis ha'Kisei.

41a: Rav Anan's land was flooded. The boundaries were washed away. He rebuilt the wall on his neighbor's property. Rav Nachman ruled that he must return to the original border.

Rav Anan: I made a Chazakah!

Rav Nachman: The Halachah does not follow R. Yehudah and R. Yishmael, who says that in front of the original owner, one gets a Chazakah immediately!

Rav Anan: I can prove that he helped me to build the wall. (This shows that he conceded to me all the property on my side.)

Rav Nachman: He erred. He thought that it really was yours. You yourself would not have built the wall on his side unless you erred!

58b (Mishnah): One gets a Chazakah for a Mitzri window, but not for a Tzuri window. A Mitzri window is one which a person's head does not fit.

59a (Shmuel): A window made for light (surely, it is meant to be permanent) gets a Chazakah regardless of its size.

59b (Mishnah): One may not open a window into a jointly owned Chatzer.

(Beraisa): A case occurred in which Reuven opened a window into a joint Chatzer. R. Yishmael b'Rebbi Yosi said that he has a Chazakah. R. Chiya ordered him to close the window.

(Rav Nachman): If Shimon built a wall blocking light from entering Levi's window, Shimon is immediately Muchzak. If Levi was truly Muchzak in his window, he would never allow someone to block it.

Rishonim

Ramban (2b DH Alma): In a Teshuvah, the Rif asked what is the proof (on 2b) that Hezek Re'iyah is not Hezek. Perhaps it is Hezek, and here the partners are Muchzak to live without a wall, with Hezek Re'iyah! We say so about two roofs (without walls) on opposite sides of Reshus ha'Rabim. The Rif answers that if Hezek Re'iyah is Hezek, there is no Chazakah, like we say about smoke or a privy. The Ri mi'Gash asked that there is a Chazakah for a Tzuri window! We must say that this is when there is no Hezek Re'iyah. Whenever there is Hezek Re'iyah, one can say 'I thought that I could bear it, but I see that I cannot.'

Nimukei Yosef (2b DH Ta'ama): The Halachah is, Hezek Re'iyah is Hezek.

Rosh (1:2): Chazakah is when one side damages and is not harmed. When both sides are damaged, one can say 'why should I protest? I thought that you would request a wall! Also, Hezek Re'iyah of a window is not constant, for it is for light, not for looking into the neighbor's Chatzer. One may not look at what his neighbor does! People go in and out of a Chatzer. One cannot avoid seeing what his neighbor does. If witnesses say that he pardoned, he cannot retract.

Rambam (Hilchos Shechenim 2:14): If partners split a Chatzer or one may force the other to split, one can force the other to build a wall in the middle, lest the other see him using his share. Hezek Re'iyah is Hezek, and there is no Chazakah for it in a Chatzer. Even if they were without a wall for many years, one can force the other.

Magid Mishneh: The Rambam holds like his Rebbi (the Ri mi'Gash), that there is no Chazakah only in a Chatzer, for they damage each other without an action. Therefore, even after three years one can force the other. They hold that if one did an act, e.g. opened a window, there is a Chazakah.

Rambam (7:6): If Reuven opened a window to Shimon's Chatzer and Shimon pardoned this, or showed that he allows this, e.g. he helped Reuven or knew about the damage (Hezek Re'iyah), Reuven has a Chazakah for the window. Shimon cannot later demand that he close it.

Rambam (11:4): One can make a Chazakah in Nezek, except for four things: a privy, dust, shaking the land, and Hezek Re'iyah. One can force his neighbor to make a wall at any time.

Ra'avad: One must bring a proof for all of these. Chazakah does not Mevatel (the right to force one's neighbor to help make) a wall, because each damages the other. Each can say 'this is why I didn't protest.' Also, he can say 'I pardoned due to the expenditure that I would incur.'

Rosh (3:73): Some say that 'does not have a Chazakah ' means that David (his neighbor) can build opposite it, but David cannot force him to close the window due to Hezek Re'iyah, for he has a Chazakah. This is unreasonable. Since he does not have a Chazakah, David can force him to close it, for sometimes David is harmed by it!

Poskim

Shulchan Aruch (CM 154:7): If Reuven opened a window to Shimon's Chatzer and Shimon pardoned this, or showed that he allows this, e.g. he helped Reuven or knew about the damage, Reuven has a Chazakah for the window. Shimon cannot later demand that he close it.

Beis Yosef (DH ul'Divrei): According to those who say that there is a Chazakah for Hezek Re'iyah, immediately or after three years, the Gemara is straightforward. We discuss a window to which Hezek Re'iyah applies. The Rashbam, Rosh and Tur hold like this. It seems that the Rambam agrees. The Rif and Ra'avad hold that there is no Chazakah to Hezek Re'iyah. They must say that we discuss a window without Hezek Re'iyah, e.g. it opens to a place that is not used. If so, what is R. Zeira's Chidush that he cannot protest?

Be'er ha'Golah (6): The Magid Mishneh explained that the Rambam holds that there is Chazakah for Hezek Re'iyah, except for a Chatzer. Also they explain that 58b discusses a widow with Hezek Re'iyah!

SMA (19): The Rambam and Shulchan Aruch hold that it suffices that Shimon knew and did not protest. The Rosh (1:2) and others require three years even if Shimon helped; the Shulchan Aruch brings his opinion at the end of Siman 142. If he explicitly pardoned, all agree that he has a Chazakah immediately.

Gra (39): The Rambam and Shulchan Aruch hold that we discuss whether one may build opposite the window, but in any case Shimon cannot force him to close the window after he built it.

Gra (40): Bava Basra 41a shows that if Shimon helped, Reuven has a Chazakah, and all the more so if Shimon explicitly pardoned.

Rema: It is proper to rule like this, since he made a Chazakah on the window, even though many argue and say that Chazakah does not help for Hezek Re'iyah.

Gra (43,44): The Rema agrees that there is a Chazakah for Hezek Re'iyah. However, he holds that when there is no Chazakah, Shimon can force Reuven to close the window. We learn from R. Chiya.

Gra (45): The opposing opinion learns from 2b. 41a proves that helping gives a Chazakah, but not for Hezek Re'iyah.

Rema (ibid.): Even the opinion that holds that it helps, this means that Shimon cannot close the window or protest, but Reuven may not stand in the window and look at the Chatzer, lest he damage through Hezek Re'iyah. Chazakah does not help for this. Shimon can protest against it. Before Reuven has a Chazakah, Shimon can protest and Reuven must close it, even though Shimon does not want to build near it now.