New to the beta. Firstly, everything seems to run very smoothly on my iPad. However at some point the app stopped working completely: I couldn't open it anymore, I tried restarting my iPad but that didn't work. I had to reinstall it to use it again. It might've been caused by something in iOS 9 (beta 3) though. Also, I noticed that the MIDIPlayer doesn't seem to work properly in a UI action (even with ui.in_background, it's immediately stopped when the action function ends), although it can be fixed with an empty while loop looping until the end of the MIDI file is reached. Lastly, while it's not really a bug, it seems I have to open a new tab, tap Documentation and close the tab to open the documentation, which is a little bit unintuitive.
Anyway, nice work on the update, the tabbed editor and appex module are really useful!