Perhaps in a parallel universe where the whole wizarding world is different. There are no enmities between the Slytherins and Gryffindors from the start. Draco and Harry are friends, and Draco sitting on Harry's lap could be a common occurrence during their friendly banter or while sharing secrets in a cozy corner of the Hogwarts library. It could be a sign of their deep - seated friendship and trust.
One possible scenario could be a moment of comfort. Maybe Harry is distraught over something like the latest threat from Voldemort and the professor offers a paternal or maternal kind of comfort. Another could be in a teaching moment, where the professor is trying to show Harry something unique, like a special spell movement, and having him sit on the lap makes it easier to demonstrate. It could also be a situation where Harry has been injured and the professor is providing some first aid or just trying to keep him calm while waiting for medical help.
It could also be a moment of affection during a celebration. For instance, after Gryffindor wins a Quidditch match. Ginny, being excited and proud, jumps on Harry's lap in the middle of the celebration in the common room. They would be laughing, sharing the joy of the victory, and perhaps stealing a quick kiss or two.
Maybe they are sitting by the Black Lake. They could be discussing their experiences in the wizarding world. Harry might be talking about his fight against Voldemort, and Luna would be offering her unique, otherworldly perspectives, perhaps comparing Voldemort to some dark magical entity she's heard of in old tales.
One possible scenario could be for a disguise. Maybe Harry needs to blend in among Draco's group for some sort of undercover mission at Hogwarts. Another could be as a result of a magical mishap, like a spell gone wrong in a transfiguration class that swaps their clothes. Or perhaps it's part of a bet or dare within the fanfiction's plot.
One possible scenario could be that Dumbledore is giving Harry some important advice in a very intimate and fatherly way. Harry might be feeling sad or confused and sitting on Dumbledore's lap makes him feel safe and listened to.
One possible plot could be that Harry accidentally falls onto Snape's lap during a chaotic Potions class. Snape is initially angry but then realizes Harry is hurt. So, instead of scolding him, he helps Harry get better while Harry is still on his lap in a rather awkward but kind moment. This would show a different side of Snape and a new dynamic between them.
Well, it could be a rather strange plot. Maybe it's a situation where Harry is injured and Snape has to hold him in this way for some kind of magical healing process. Harry might be in a weakened state and Snape, despite their past differences, has to take care of him. This could lead to an unexpected moment of understanding between them.
Well, it could be a story of a young girl sitting on her father's lap to show affection. Maybe she's just had a bad day at school and wants the warmth and security of her dad. Or perhaps it's a cultural thing where sitting on a father's lap is a common display of family closeness. There could also be a story where the father is teaching his daughter something while she sits on his lap, like how to tie her shoes or count numbers.
In some fanfictions, it could be a result of some magical influence. Maybe there was a spell or a charm that made Harry act this way. Or it could be an expression of deep - seated emotions that Harry has been suppressing for a long time. This action then becomes a way for him to break free from his self - imposed restraints and reach out to Draco in a very physical way.
A possible story could be that a young child had a bad day at school. When the child came home, the mom was sitting on the couch. The child ran to her and sat on her lap. The mom then gently stroked the child's hair and listened as the child told about the mean kids at school. She comforted the child with kind words and a warm hug, making the child feel better.