In the famous double slit experiment, a photon ( say) can behave as wave or particle depending on whether there is ( or how) an outside observer measuring the experiment.
Copenhagen interpretation interpret this wave-particle duality by saying that the photon behaves as wave when unobserved, and the act of interpretation "collapses" the wave function makes it behaves as particle.
How would Feynman sum over histories formulation interpret this duality? Or to put it the other way, how does Feynman sum over histories interpret Quantum decoherence? Or it is just a mathematical tool, nothing more, so it doesn't really care about whether a photon is a wave or particle and there is no underlying QM interpretation?