Well, I can't answer for @x42 (Robin Gareus) but for me personally the refactoring of the Editor code so that we could have multiple "editors" was both interesting and hugely challenging.

I didn't want to replicate the code we already had for the Editor, and figuring out to refactor this took a lot of time and experimentation and failures. Although there are still some rough spots, in general I'm very happy with how things turned out.

Clip recording was also a bit of a challenge. It uses an entirely different mechanism than timeline recording, and as usual I got the basics working in a couple of days, followed by months of polishing (and likely, quite a few more to go as we get feedback from users).