隣 and 横: neighboring and beside
Compare adjacency with a position beside something.
A neighboring relationship
隣 (となり) refers to a neighbor in an order or arrangement. 「隣の家」 is the neighboring house and 「隣の席」 the adjacent seat. The relationship between items is central.
「隣の駅で降ります」 means getting off at an adjacent station. The direction of travel is needed to identify which neighboring station.
A position at the side
横 (よこ) means a side position or a horizontal direction. 「机の横に鞄を置きました」 means putting a bag beside the desk. The two objects need not be of the same kind.
「横を向いてください」 asks someone to face sideways. It requests a change in orientation, not a conversation with the neighboring person.
Where meanings overlap
Both 「私の隣に座ってください」 and 「私の横に座ってください」 can ask someone to sit beside the speaker. But the horizontal direction in 「横に線を引く」 cannot be replaced by 隣.
「隣に誰もいません」 says that nobody is next to the speaker. It does not say the entire place is empty; preserve the spatial scope of the negation.
Does 「横に長い窓」 mean the neighbor's window?
Show the answer and why
No. It is a window that is long horizontally. 横 indicates the direction of its length.