When Tisha B'av begins after Shabbat, as it does this year, we begin fasting after sundown before Shabbat is over.
On the one hand, we postponed the full fast day of Tish'a B'av to Sunday, because you shouldn't fast on Shabbat. Yet, we're fasting while it is still Shabbat. Is this not a contradiction? Does beginning the fast during Shabbat not violate the concept of oneg Shabbat? Why is the fast allowed to begin before Shabbat is over?