In the beginning of the oneshot, it starts off with the main characters childhood (2 boys, one thought he was a girl) and the boy-who-thought-he-was-a-girl moved away and when he left, he held a grudge against the other boy for taking his toy truck or something.

In high school he moves back again (and now he knows that he's a guy) and then..... that's all I remember... is there anyone who knows the title of the oneshot 😕

It might be Hard Romantica.

[color=green]"Officially, this machine doesn't exist, you didn't get it from me,
and I don't know you. Make sure it doesn't leave the building."[/color]

Moved to Yaoi/Yuri~