  We investigate the maximum coding rate achievable over a two-user broadcast
channel for the scenario where a common message is transmitted using
variable-length stop-feedback codes. Specifically, upon decoding the common
message, each decoder sends a stop signal to the encoder, which transmits
continuously until it receives both stop signals. For the point-to-point case,
Polyanskiy, Poor, and Verd\'u (2011) recently demonstrated that variable-length
coding combined with stop feedback significantly increases the speed at which
the maximum coding rate converges to capacity. This speed-up manifests itself
in the absence of a square-root penalty in the asymptotic expansion of the
maximum coding rate for large blocklengths, a result a.k.a. zero dispersion. In
this paper, we show that this speed-up does not necessarily occur for the
broadcast channel with common message. Specifically, there exist scenarios for
which variable-length stop-feedback codes yield a positive dispersion.
