I made a fatal mistake in the design of this program.
I used a single TextArea rather than having multiple gadgets. When source tabs are switched, the text area text is just changed to match whatever source is selected. Unfortunately, formatting the source code takes a significant amount of time, and there is a very annoying pause when switching sources. The solution is to use multiple text area gadgets, but this is not terribly easy, due to the way the program is written.
I used a single TextArea rather than having multiple gadgets. When source tabs are switched, the text area text is just changed to match whatever source is selected. Unfortunately, formatting the source code takes a significant amount of time, and there is a very annoying pause when switching sources. The solution is to use multiple text area gadgets, but this is not terribly easy, due to the way the program is written.