This problem (or similar) also occurs on PLSQL developer 14.
One simple way to reproduce the similar problem (on plsql 14 or 15):
1 - Start executing a very slow query (that will take some minutes to start returning data);
2 - Previous than the results arrive, click on the panel where the query results should appear;
3 - Verify that PLSQL hang!

Thanks,
MAOR