@matcap Well I haven't seen a source on the whole editor making 774 change Nagatoro's personality. I know a popular explanation for 774 making Nagatoro nicer was due to NaruHina becoming canon and the doujins 774 made based on the couple. Even, then as far as I know, there's no real source for that. The closest to that is a Lost Pause video discussing Nagatoro. Regardless whether it was editorial mandate or not, I think a character acknowledging his/her actions as "messed up" (as Nagatoro stated in chapter 3) and then slowly change his/her behavior to rectify himself/herself counts "character development".
This is where I think you and I are having this difference in opinions over. Now correct me if I'm wrong, I don't want to misrepresent what you are saying, but I think when you say "development" you mean "kiss/confession/relationship upgrade of some sort". This is not something exclusive towards you, most anime fans seem to be of the mindset that any chapters before the couple gets together (or acknowledges their feelings) is just filler. And while that may hold true for some manga one thing 774 does to make sure the story does not fall into this trapping is by changing the dynamic of the main couple through gradual character growth.
Senpai starts out as a passive loner, focusing on his art/manga as he emotionally distanced himself from everyone he came into contact with to the point where he doesn't remember the faces of those he has met. After he meets Nagatoro, Senpai becomes more open. He hangs out with Nagatoro and her friends when he normally would stay at home, he gets more confident with how he interacts with others, ad he even sometimes calls out Nagatoro whenever he can find the opportunity.
Nagatoro was a mean person. She rarely interacts with guys, only giving halfhearted replies whenever she is shown talking to other guys. She even seems to show attitude to her friend group, if the way Nagatoro can easily intimidate her friends is any indicator. Her sadistic nature truly becomes apparent during her earlier interactions with Senpai. After spending time with Senpai, Nagatoro becomes gentler, relatively speaking. She rarely if ever bullies Senpai outside of light teasing and only seems to go all out if she's trying to motivate Senpai to be better. While she started off as confident and supposedly knowledgeable about dating, recently her interactions with Senpai are now showing that she has her own insecurities about relationships, and they are becoming more apparent as the manga goes on.
Just compare volume 1 and volume 4 of Nagatoro to see the difference in Senpai and Nagatoro's character.
The way the relationship between the two leads is changing the way Senpai and Nagatoro view themselves, each other, and other people around them is what 774 wants to focus on rather than having them date and have cute (and sadistic) moments together. Even if you don't count these personality/dynamic changes as actual development, you have to admit, this aspect makes the manga less repetitive than "Amano Megumi wa Suki Darake!" for example.
I feel weird talking about one manga while in a comment section for a different manga, I just wanted to clarify why I disagree with the whole "nothing is progressing in Nagatoro." argument. There is progress, it's just not as blatant a most manga would present it.