mix with each other to form the mass eigenstates photon, $\gamma$, and $Z$. Higher-order contributions lead to a mixing between these states, giving rise to a $\gamma-Z$ propagator matrix. In a renormalisable gauge, the (would-be) Goldstone boson as an unphysical degree of freedom does not vanish, and mixing contributions with this unphysical scalar particle need to be taken into account as well. In the quark sector of the SM the relation between the interaction and the mass eigenstates is encoded in terms of the CKM matrix [@Cabibbo:1963yz; @Kobayashi:1973fv]. The additional states in models of physics beyond the SM (BSM) usually follow the same pattern, i.e. interaction eigenstates mix with each other to form mass eigenstates, and the propagators of the latter are of matrix-type as a consequence of mixing contributions at higher orders (in some cases it can also be convenient to derive loop-corrected mass eigenstates directly from lowest-order interaction eigenstates, avoiding the introduction of lowest-order mass eigenstates). In particular, extended Higgs sectors usually contain several neutral Higgs states that can mix with each other. If ${\mathcal{CP}}$-conservation is assumed, ${\mathcal{CP}}$-even and ${\mathcal{CP}}$-odd states can only mix among themselves. In the general case where the possibility of ${\mathcal{CP}}$-violating interactions affecting the 