Defining subregion physics in gauge theory is tricky due to the presence of gauge constraints that connect a subregion with its complement. This presents an obstruction to computing quantum information theoretic quantities like entanglement entropy, which require a well defined reduced density matrix.    
One way to deal with this obstruction involves the introduction of edge modes at the entangling surface, which breaks the problematic gauge constraints.    We revisit the analysis of these "entanglement" edge modes and explain their dynamical origin.  We identify a local and 'shrinkable'' boundary condition  that gives rise to such degrees of freedom. They can be interpreted as the Goldstone bosons of gauge transformations supported on the boundary, with the electric field component normal to the boundary as their symplectic conjugate.  We show that both the symplectic form and Hamiltonian exhibit a bulk-edge split, and that the thermal edge partition function is that of a codimension-two ghost scalar living on the horizon.  The corresponding thermal entropy gives an entanglement entropy across the horizon that is consistent with the conformal anomaly.