One may not open his windows
into a jointly owned courtyard, since he will be able to observe the
activities of his partners in the courtyard below without their
knowledge, thereby committing visual trespass.
The person opening the window could argue to his partner: “In any event
you need to conceal your actions in the courtyard from me, and windows
will not result in any additional visual trespass.”
However, his partner can reply, “When you were in your house and your
door was closed, I was able to use the courtyard for private
activities. But now, if you open a window to the courtyard, even when
you are in your home, I need to conceal myself from you.”