Sleep in Spock's Mock waits when run by CompletableFuture - testing

Question

When I separately run the runAsyncWithMock test, it waits for 3 seconds until the mock's execution is finalised, rather than get terminated like the other 2 tests.
I was not able to figure out why.
It is interesting that:
When multiple Runnables are executed by CompletableFuture.runAsync in a row in the runAsyncWithMock test, only the first one waits, the others not.
When having multiple duplicated runAsyncWithMock tests, each and every of them runs for 3s when the whole specification is executed.
When using Class instance rather than a Mock, the test is finalised immediately.
Any idea what I got wrong?
My configuration:
macOS Mojave 10.14.6
Spock 1.3-groovy-2.4
Groovy 2.4.15
JDK 1.8.0_201
The repo containing the whole Gradle project for reproduction:
https://github.com/lobodpav/CompletableFutureMisbehavingTestInSpock
The problematic test's code:
#Stepwise
class SpockCompletableFutureTest extends Specification {
def runnable = Stub(Runnable) {
run() >> {
println "${Date.newInstance()} BEGIN1 in thread ${Thread.currentThread()}"
sleep(3000)
println "${Date.newInstance()} END1 in thread ${Thread.currentThread()}"
}
}
def "runAsyncWithMock"() {
when:
CompletableFuture.runAsync(runnable)
then:
true
}
def "runAsyncWithMockAndClosure"() {
when:
CompletableFuture.runAsync({ runnable.run() })
then:
true
}
def "runAsyncWithClass"() {
when:
CompletableFuture.runAsync(new Runnable() {
void run() {
println "${Date.newInstance()} BEGIN2 in thread ${Thread.currentThread()}"
sleep(3000)
println "${Date.newInstance()} END2 in thread ${Thread.currentThread()}"
}
})
then:
true
}
}

Answer 1

This is caused by the synchronized methods in https://github.com/spockframework/spock/blob/master/spock-core/src/main/java/org/spockframework/mock/runtime/MockController.java when a mock is executed it delegates through the handle method. The Specification also uses the synchronized methods, in this case probably leaveScope, and is thus blocked by the sleeping Stub method.
Since this is a thread interleaving problem I guess that additional closure in runAsyncWithMockAndClosure moves the execution of the stub method behind the leaveScope and thus changes the ordering/blocking.

Answer 2

Oh, just now after writing my last comment I saw a difference:
You use #Stepwise (I didn't when I tried at first), an annotation I almost never use because it creates dependencies between feature methods (bad, bad testing practice). While I cannot say why this has the effect described by you only when running the first method, I can tell you that removing the annotation fixes it.
P.S.: With #Stepwise you cannot even execute the second or third method separately because the runner will always run the preceding one(s) first, because - well, the specification is said to be executed step-wise. ;-)
Update: I could briefly reproduce the problem with #Stepwise, but after recompilation now it does not happen anymore, neither with or without that annotation.