Periodic electron spatial structures in gases occur within a window of voltages and pressures. Recent accurate solutions of Boltzmann’s equation portray this effect, but offer little physical insight into the causes of windowing. Here we show for the first time how such insight can be obtained using the fluid model established by Robson, White, and Petrović [Rev. Mod. Phys. 77, 1303 (2005)RMPHAT0034-686110.1103/RevModPhys.77.1303], with an appropriate generalization of the heat flux ansatz. Conversely, the success in portraying windowing itself becomes a stringent test of the integrity of this fluid model, which can then be applied to a wider range of problems.