  We diagram the behavior of 5-dimensional anti-de Sitter spacetime against
horizon formation in the gravitational collapse of a scalar field, treating the
scalar field mass and width of initial data as free parameters, which we call
the stability phase diagram. We find that the class of stable initial data
becomes larger and shifts to smaller widths as the field mass increases. In
addition to classifying initial data as stable or unstable, we identify two
other classes based on nonperturbative behavior. The class of metastable
initial data forms a horizon over longer time scales than suggested by the
lowest order perturbation theory at computationally accessible amplitudes, and
irregular initial data can exhibit non-monotonic and possibly chaotic behavior
in the horizon formation times. Our results include evidence for chaotic
behavior even in the collapse of a massless scalar field.
