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Is there a keyboard shortcut for (un)commenting lines within an inline_script?
No, I'm afraid not. One of the things that I plan to do for the next major release (but that won't be for a while) is to separate the editor component from the OpenSesame source. This will make things easier to maintain, and the editor will also have more features, such as shortcuts for commenting and theming. I already have some of this working, and I'll drop you a line when I commit the first snapshots to GitHub.
I have received feedback on this from some of my colleagues here on this. They do miss more elaborate editing features for the inline_script item, so what you're working on sounds great!