  We use numerical simulations of circumplanetary disks to determine the
boundary between disks that are radially truncated by the tidal potential, and
those where gas escapes the Hill sphere. We consider a model problem, in which
a coplanar circumplanetary disk is resupplied with gas at an injection radius
smaller than the Hill radius. We evolve the disk using the PHANTOM Smoothed
Particle Hydrodynamics code until a steady-state is reached. We find that the
most significant dependence of the truncation boundary is on the disk aspect
ratio $H/R$. Circumplanetary disks are efficiently truncated for $H/R \lesssim
0.2$. For $H/R \simeq 0.3$, up to about half of the injected mass, depending on
the injection radius, flows outwards through the decretion disk and escapes. As
expected from analytic arguments, the conditions ($H/R$ and Shakura-Sunyaev
$\alpha$) required for tidal truncation are independent of planet mass. A
simulation with larger $\alpha=0.1$ shows stronger outflow than one with
$\alpha=0.01$, but the dependence on transport efficiency is less important
than variations of $H/R$. Our results suggest two distinct classes of
circumplanetary disks: tidally truncated thin disks with dust-poor outer
regions, and thicker actively decreting disks with enhanced dust-to-gas ratios.
Applying our results to the PDS 70c system, we predict a largely truncated
circumplanetary disk, but it is possible that enough mass escapes to support an
outward flow of dust that could explain the observed disk size.
