The debug button is disabled for Grails 3, providing no easy way to issue a "grails run-app" command in debug mode. The UI provides no explanation.
The information for "Debugging Grails 3 Application" at https://www.jetbrains.com/help/idea/2016.1/getting-started-with-grails-3.html does not actually provide the same functionality.
After much investigation, I found a way to achieve the lost functionality. It requires creating 2 different entries in the Run/Debug Configurations pulldown menu:
(entry 1) invoke Grails run-app with a --debug-jvm flag
(entry 2) attach a debugger to it
I suspect this is exactly what happens behind the scenes when the debug button is clicked for a Grails 2.x app.