Granular hydrodynamics predicts symmetry-breaking instability in a two-dimensional ensemble of nearly elastically colliding smooth hard disks driven, at zero gravity, by a rapidly vibrating sidewall. Supercritical and subcritical symmetry-breaking bifurcations of the stripe state are identified, and the supercritical bifurcation curve is computed. The cluster dynamics proceed as a coarsening process mediated by the gas phase. Well above the bifurcation point the final steady state, selected by coarsening, represents a single strongly localized densely packed “droplet.”