When the fast falls out during the week, the meal right before the fast is a mourning meal known as the Seuda Hamafsekes. Sitting on the floor, bread and ash, hard-boiled egg. When the fast begins on Shabbos, we do stop eating at shkia (sunset), but all the other elements of Tisha B’Av only begin when Shabbos ends. So that last meal before the fast is a Shabbos meal, Shalosh Seudos basically. And what kind of meal is it?
The Shulchan Aruch says it’s “K’Seudas Shlomo B’Shaato.” Like a meal at King Solomon’s table during the height of his power, while the first Beis Hamikdash stood in all its glory. The Halacha could have been phrased “There is no Seuda Hamafsekes on Shabbos. One eats his Shabbos meal as usual.”
This rather more extreme phrasing is saying something else: On Shabbos there is no Churban.
Ashkenazi practice has allowed for some minhagim (customs) of Aveilus (mourning) to be introduced to this Shabbos. But besides for the fact that many Ashkenazi poskim object to those introductions, even those who keep them make sure that none of them interfere with the Kedusha of Shabbos.
(I have presumed that Tisha B’Av is different from other fasts in this regard because it is not merely a fast day; it is a day of Aveilus, and Halacha has always recognized that Aveilus does exist in subdued form on Shabbos.)
When I was a Yeshiva Bochur in Montreal, I got to know Mr George Jacobovits, a very impressive and special Jew in his own right, who was also the brother of Rabbi Lord Jacobovits, then the Chief Rabbi of the United Kingdom. Mr George Jacobovits was kind enough to learn with me on Shabbos afternoons. We learned Gemara Beiya. That year, Tisha B’Av fell out on Shabbos, and I asked him if we would still be learning, as I had learned that we only learn topics appropriate to Tisha B’Av on the afternoon of Erev Tisha B’av. He look at me with narrowed eyes and said “Chas V’Shalom. My rebbe (Rav Elya Lopian ZT”L, a talmid of Kelm and one of the preeminent tsaddikim of that era, who lived first in England before moving to Israel) insisted that Shabbos is Shabbos, and that nothing of the nature of Tisha B’Av should be allowed to intrude into Shabbos. We will learn what we always learn.”