FYI: I reached out to Jetbrains support: if you have the grails plugin enabled in IntelliJ and try to use grails-core it will cause the source files to not be browseable. https://youtrack.jetbrains.com/issue/IDEA-366647 is the upstream ticket. I've updated our internal tracking ticket as well: https://github.com/apache/grails-core/issues/13910
I would suggest you disable the grails plugin while working on grails-core for a performant IDE. -James
