I'm glad that you found out how to select a different syntax definition. Sorry that it wasn't more obvious.

With iOS 13 Apple changed the way external folders work. File provider apps need to be updated to support folder picking. So currently, this works with iCloud Drive, "On my iPad/iPhone", Working Copy (Git) and Secure ShellFish (SFTP).

I recommend to use Working Copy for Git.

Since this is implemented as an accessibility feature on the OS level I don't think apps can influence what happens when using the mouse on iOS 13 beyond what Apple already offers as configuration options.

Thanks, I'll look into it.

Just syntax highlighting, it does not run code. 

I'll consider it for a future update. Go to line is already implemented by the way.

Textastic has a syntax definition for R.

Yes, I have seen this happen on my machine, too. I'm not sure why it happens yet. I'll need to investigate, but it's not easy to debug.