> Are there implicit yields in Squeak?

Yes. While it is true that Squeak's processes are non-preemptive if they run
at the same priority their behavior is somewhat out of the ordinary when
they get preempted by a higher priority process. Rather than being added as
the first process in the priority group (which would give you the expected
behavior) it is added as the last process thereby implicitly yielding to the
next process in this priority group.