Steven for the most part... lot's of the UYI material is pre-AFD. They where ready to record the songs but couldn't cause of the Steven situation, so then they decided to let Matt do the recordings. So the songs where done and they rehearsed for a few weeks and recorded the material. I'm sure Matt did it in his own style (with own fills) but the basic tracks are from Steven. And Duff aswell (YCBM intro for example).
Another example of Steven writing the drumtracks is to listen to them, all the UYI songs have distincive drums, different drum beats fitting the song. Not the same old standard beat which Matt is known for.. (Slither beat during the verses
)
also on Pretty Tied Up there's a cowbell played during the chorus which is typical Steven, ever heard Matt play a cowbell?