Eating After Finishing Seudas Shelishis

The Shulchan Aruch (553:1) writes: Even if one has finished his seudah hamafsekes, unless he explicitly accepted the fast upon himself, he may continue to eat. Nonetheless, the Mishnah Berurah (s.k. 2) brings from the Bach, that it’s best to stipulate verbally with one’s mouth, or at least think in his heart that one is not accepting upon himself the fast until bein hashmoshas. The Sha’ar HaTziyun (s.k. 7) adds: When Tisha B’Av falls on Sunday there is no need to make a stipulation, as it is forbidden to afflict oneself on Shabbos, therefore, we can assume that he never intended to bring upon himself the fast until after Shabbos.