The app uses this library for USB serial communication: https://github.com/felHR85/UsbSerial
It comes in synchronous and asynchronous flavors . I've tried both but had better luck with the asynchronous overall so far. The downside is that flow control doesn't seem to be working so we have to have an artificial delay between commands or else sometimes they get spliced together on subsequent writes to the serial port. In the end I need to either work out the synchronous controller, or else figure out how to get flow control working, that would speed up, perhaps considerably, the time it takes to save code back to the board. Another option to explore is writing more than one line at a time. I had trouble with writing the full file at once, but I suspect chunks larger than a single line are possible.