It looks like ReplicationCoordinatorImpl::_performElectionHandoff() is waiting for a remote command while holding the repl coord mutex. Or have I missed something?

In addition, _performElectionHandoff() is only called from inside ReplicationCoordinatorImpl::stepDown(), and my reading of that function is that the global lock will also be held while _performElectionHandoff is called (although earlier it gets released, it then gets reacquired).

This code was added in SERVER-35623.