Hi Rob,
Thanks for your email. I'm assuming you are seeing an "unexpected invocation" ExpectationError - is that correct?
I think there is a bug in mocha which I recently described [1] in this (unrelated) Github issue. I'm not sure my initial diagnosis (in that issue comment) was correct, but certainly the behaviour seems undesirable and I think is very closely related to the problem you are describing.
I'm pretty short of time to investigate this at the moment, but it would help if you could add a Github issue to report the problem and then we can keep track of progress in fixing it.
In the short term, while it may not be representative of the way the methods are actually invoked, you might be able to work around the problem by stubbing the method on the Child class instead.
Thanks, James.
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