I don't think it was Saegye's desk? I'm pretty sure Saegye went to Jinsol's desk because he missed him, and found the diary because it was sticking out.
Its chapter 31 and I re-read, I think it is ambiguous, I can not say for sure if it was Saegye's own desk or if it was Jinsol's
Putting it in Saegye's desk wouldn't make much sense. Jinsol said he was deliberately cruel to Saegye when he ended things so that Saegye would be able to move on more quickly and forget about him. Putting the diary in his desk would defeat the purpose.
Even if he left it in his own desk, still emotional manipulation, maybe he couldn’t predict Saegye’s complex emotions that “might” have led him to Jinsol’s desk, but he could have predicted that Saegye would thoroughly look through his things to find out the reason why, OR the author conveniently left the book there as a plot device, OR Jinsol “forgot” his book, but it clearly shows him looking at his diary and contemplating something before leaving school, so you tell me
It was updating across several weeks that I forgot these flashbacks are literally of Jinsol's own words from his diary, that fucking manipulative bastard left his dumb diary on purpose under Saegye's desk, if you said that excuses do not matter anymore than why did you leave your diary in his desk??? Because that entire diary is a big fucking excuse that would make him pity you and forgive you for thrashing his love and taking his choices away from him, you have no right Jinsol, no right at all, you deserve nothing from Saegye and he doesn't have to give it to you. "Let's say everything was a lie," you mainipulative bastard, trying to make him think you had no choice to do what you did to him and you regret it you and you fell in love with him, you didn't want to do it but had to, that hurts for you as much as it hurts for him? YOUR ENTIRE DIARY IS FUCKING MANIPULATION, YOUO SHOULD HAVE BURNT IT IN THE FIRE YOU DUMBASS, I just hope the ending is Saegye happily married and completely in love with someone else and Jinsol looking from afar and regretting EVERY SINGLE FUCKING CHOICE HE MADE, I WILL GODDAMN THROW A TABLE IF SAEGYE FORGIVES HIM I SWEARE.