I've just pulled master then I got rid of all idea config files and build direcotries to start with a clean state:
find . -name "*.iml" -type f | xargs rm -f
Then I run the same command as you:
./gradlew jar idea --rerun-tasks
Finally I opened the project in IntelliJ and tried rebuilding it (Build > Rebuild project). The fan is spinning like crazy, CPU usage is high and it seems to be stuck on "Groovy stub generator: initialization [tests of groovy-xml and 8 more]". After then killing the IDE and trying to run DownUpStepTest I'm seeing the same symptoms as you are - stuck in "Groovyc: semantic analysis [tests of groovy-xml and 8 more]" with CPU is not busy.
For reference, I'm using IntelliJ 2018.1 181.4203.550.