The Magen Avraham writes that if one starts Shaalos Seudos before the
Time of Mincha, but the meal stretches and goes into the time of Shaalos
Seudos, he is Yoitze Shaalos Seudos. The Elya Rabba, however, quotes the Sefer
Chasidim which holds that a person cannot split a meal so that half of it
should be consideredShaalos Seudos. He holds that a full meal needs to
be eaten at the time when Shaalos Seudos may be eaten.
The Magen Avraham writes that one is not allowed to start a meal if it is
known that the meal will stretch past Chatzos (refer to the Halacha in
Simon 471). The Machtzes Hashekel explains this is the same Halacha as we see
On Erev Pesach.On Erev Pesach a person cannot eat a meal if he knows it
Will stretch past the tenth hour. Therefore, in our case one cannot eat if he
knows the meal will stretch past Chatzos (i.e. one should rather wait and
eat the meal after Mincha). The Shaar Hatzion says that this applies only according to the Shita of the Shulchon Orech that forbids even a small meal after Chatzos. However, we follow the Rama who permits small meals after Chatzos and classifies a Shabbos meal as a small meal.