Run this new jtreg manual test with, well, jtreg on a system with 2 monitors attached.
It shows JOptionPane with instruction and after it is closed, shows something for a split second, then exits with success: no actual user interaction is possible.

If I compile and run the test separately, it does prove the fix to JDK-8138749 works properly.

NB: my version of jtreg is most recent.