The gemara in Taanis 29b comments that you are allowed to eat meat and wine at your seuda shlishis (even though it is ever tisha b'av). Rashi comments there that there is no seuda hamafsekes on shabbos. RYBS explains that Rashi holds that the seuda hamafsekes is a din in the fast (taanis) and the dinim of the fast don't apply on shabbos. In other words, the fast is mechayev a person to be mafsik. Therefore on Shabbos there is no seuda hamafsekes. The MA in Siman תקנ"ב comments that a person should not eat the seuda with friends and should be sad. There are 2 ways to understand this.
1. He agrees with Rashi's principle that there is no seuda hamafsekes, however, this din of friends is based on the regular din of aveilus on erev tisha b'av
2. He disagrees with rashi and holds that the seuda hamafsekes is a din in the aveilus and therefore applies on Shabbos as well.
The Gra has a famous comment about shabbos Chazon. The shulchan aruch baed on the Rishonim paskens that a person should not change their clothes for shabbos chazon, but rather wear weekday clothes. The Gra disagrees and says that Aveilus B'pharhesya is assur and wearing week day clothes is aveilus b'pharhesya. The minhag nowadays seems to be like the Gra.
Recently, I heard that a number of Charedi rabbanim are going back to the psak of shulchan aruch for the following reason. Anyone who knows the (non-Chassidish) Charedi worlds knows that there is very little difference between weekday clothes and Shabbos clothes, especially for Rabbanim, Roshei Yeshiva, etc. They wear a dark suit and a hat all the time. When I see the Rabbanim in my neighborhood I can never tell the difference between weekday clothes and shabbos, many of them wear a long black frock 7 days a week, and those who don't wear dark suits. Therefore, for them, it is not aveilus b'pharhesya to wear week day clothes, very few people if anyone will be able to tell the difference.
The same sevara applies to the more modern world in America where many people wear suits all week to work.