Oh, I see how debugger becomes active. It is actually script debugger that is active, because the query in the script isn't actually being cancelled even though the script runner has cancelled.
It's a different symptom of this: (14.0.51.637) TOAD hangs when trying to cancel long running PL/SQL
Run your EXEC with F9 instead of F5 and it should cancel normally. We'll get that fixed in the betas after 14.0.