If I cancel a long running query it appears that it is not really cancelled. The application can't be quit but has to be force-quit (Windows at least).
Please provide an option force-quite from with-in SQL Developer.
This is not new to v 3.2.20 but has been for quite a while.
We ask the database to cancel a query via the thin client JDBC interface.
It's then up to the database to cancel said query/process.
You may have a slightly better experience with canceling queries if you use the OCI/THICK option for your connections. This is enabled via the Preferences > Database > Advanced page. It will require you have an Oracle Client installed - although the 'lite' Instant Client will perfectly suffice.
I still think it would be useful to be able to force quit SQL Developer from within the application in such cases. It is a bit awkward to have to do through Task Manager for this.