We’ve recently been gathering assorted open-source projects that exist in our company under a single roof — the JetBrains organization on GitHub. One of the latest additions to the organization is an official mirror of the IntelliJ IDEA Community Edition codebase.
You’re welcome to fork the codebase and send us pull requests with your contributions. We’ll review them and merge them if they’re accepted. Please don’t forget to follow our code conventions and sign the contributor agreement if your contributions are non-trivial.