Manifolds with zero Chern term

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Has any study been made of manifolds where the integrand $e(\Omega)$ from the Chern-Gauss-Bonnet theorem (which I gather is called the Euler class) is identically zero? If so, is there any broader significance to such manifolds or umbrella statement that can be made about them? I have physics motivation so I'm especially interested in (3,1) Lorentzian manifolds if possible.