Had few years break from developing IDEA plugins and now decided to implement several ideas and hopefully update my old stuff.
Anyhow upgraded to latest version 10, setuped JDK and IntelliJ SDK. What I'm missing is OpenApi sources. After reading "Developing IDEA plugins" I pulled sources of Community edition. 700Mb (I'm impressed!). Guide says I need to compile it and then I will be able to run IDEA. That's a bit too much to my taste :) Thanks for sources but basic package with OpenAPI would be more than enough. I will start compilation now, but still would like to ask if there is a smaller package which could contain openapi sources?