  Upon heating, ultrastable glassy films transform into liquids via a
propagating equilibration front, resembling the heterogeneous melting of
crystals. A microscopic understanding of this robust phenomenology is however
lacking because experimental resolution is limited. We simulate the
heterogeneous transformation kinetics of ultrastable configurations prepared
using the swap Monte Carlo algorithm, thus allowing direct comparison with
experiments. We resolve the liquid-glass interface both in space and time as
well as the underlying particle motion responsible for its propagation. We
perform a detailed statistical analysis of the interface geometry and kinetics
over a broad range of temperatures. We show that the dynamic heterogeneity of
the bulk liquid is passed on to the front which propagates heterogeneously in
space and intermittently in time. This observation allows us to relate the
averaged front velocity to the equilibrium diffusion coefficient of the liquid.
We suggest that an experimental characterisation of the interface geometry
during the heterogeneous devitrification of ultrastable glassy films would
provide direct experimental access to the long-sought characteristic
lengthscale of dynamic heterogeneity in bulk supercooled liquids.
