Drag and drop isn't really a window management feature, right? It's a kind of uniform IPC for GUI apps, mostly a way to talk about files and clipboard contents

Drag and drop isn't really a window management feature, right? It's a kind of uniform IPC for GUI apps, mostly a way to talk about files and clipboard contents (temporary files, I guess). At any rate, it has nothing to do with the display server. That said, I do agree that I would generally expect even older Apple codebases to be less crufty than stuff like Xorg. Apple rips out legacy APIs and rewrites things pretty ruthlessly on basically every macOS release.