  We study the entanglement dynamics of a one-dimensional hard-core quantum gas
initially confined in a box of size $L$ with saturated density $\rho=1$. The
gas is suddenly released into a region of size $2L$ by moving one of the box
edges. We show that the analytic prediction for the entanglement entropy
obtained from quantum fluctuating hydrodynamics holds quantitatively true even
after several reflections of the gas against the box edges. We further
investigate the long time limit $t/L\gg 1$ where a Floquet picture of the
non-equilibrium dynamics emerges and hydrodynamics eventually breaks down.
