The other thing about the 'estrangement' is that--I don't know if people have dealt with this--but just because it seems like Draco isn't thinking about Harry at all doesn't mean he isn't. I mean, I don't think you can go too overboard because clearly his priorities are different, but he may very well have noticed Harry noticing and following him. Is he paranoid that Harry's going to see him fail? Does Harry's notice spur him on in anyway? Does he make him feel like he's willing him to fail? There's that moment where he's in total despair and who's right there but Harry and he attacks him as of course he would. And then what about after that?

I mean, you can wonder how he himself understood his new feelings about Harry, like after the nose-breaking where he might feel that put an end to it but then it didn't. Does he just happily let go of all the ways he used to want to rub Harry's face in it, or does he consciously think about it, even wistfully. Does he think about Harry's own dealings with the Dark Lord now that he's being kind of hunted by him himself? There's so many reasons for Harry and Draco to think about each other more and differently in HBP, even if canon avoids it by possibly having them repress it.