The gemara (Ta'anis 30) quotes a braysa: 'kol she'hu machmas 9 Av asur l'echol basar, lishtos yayin, v'asur lirchotz' - In a meal due to 9 Av one cannot eat meat, dring wine, or bathe [afterwards], but in a meal not due to 9 Av these would be permitted. R' Yishmael b"r Yosi in the name of his father holds as long as one may eat, one may wash.
The Rosh (4:38) cites the Ramban who interprets the tana kamma of the braysa to mean that once one eats the seudah hamafseket before 9 Av, one is also prohibited from washing. Even though the seudah is eaten before dark and the laws of 9 Av are not yet in effect, since one has accepted some of the practices of aveilus (i.e. not eating meat or drinking wine), the prohibition of washing applies as well. This applies only to washing, where the enjoyment of feeling refreshed and clean from bathing before the fast would give one pleasure on the actual day of 9 Av itself, but not to wearing leather shoes or other aveilus practices.
The Rosh writes that this interpretation is a 'davar teima', a baffeling idea. How can it be that even after finishing the seudah hamafseket one is permitted to eat and drink right up until the very moment it gets dark and 9 Av begins, yet by eating the seudah hamafseket it becomes prohibited to wash? Clearly the aveilus practices of not eating meat or drinking wine during seudah hamafseket do not trigger the 9 Av prohibitions on eating and drinking to apply, so why should the aveilus practice trigger the prohibition of washing to apply?
Bli neder to be continued - comments welcome.