Unstashed messages enqueued in TestActorRef mailbox while processing a message from self are stuck until another message is received

In the following workflow, unstashed messages are enqueued in the mailbox but are not processed as expected:

1. Send message (messageOne) from TestKit to TestActorRef
2. TestActorRef underlying actor stashes messageOne and schedules a subsequent message to self (selfMessage)
3. selfMessage is sent and processed and messageOne is unstashed during processing of selfMessage
4. messageOne is not processed
5. Send another message (messageTwo) from TestKit to TestActorRef
6. Both messageOne and messageTwo are processed

akka.testkit.CallingThreadDispatcher#runQueue appears to process the provided MessageQueue at the time of invocation rather than using the active Mailbox MessageQueue.