On Pesochim 105b, Tos DH Shema Mina (at the end), Tos brings the opinion of the BeHag, that one does not need to say Al HaGefen after the wine at the end of benching.
1) We were told that the Rosh is surprised at this, although I could not find the Rosh myself
2) We were also told that the Beis Yosef explains the BeHag as holding that the Benching is not a Hefsek between the meal before and the Kos Shel Brocho
Do you have any explanation on this BeHag. The comments of the Beis Yosef don't seem to help, in that Hefsek could explain whether a brocho Borei Peri Hagofen is required before drinking the Kos, but how could the benching serve as an after-brocho for the wine which is drunk afterwards!?
1) The Rosh is in Berachos Chapter 6 Siman 27.
2) The Beis Yosef in Siman 190 DH Veyvarech, quotes the Rosh. He does not
offer his own explanation but quotes Rabeinu Yonah in Berachos Ch. 6 (page
30b in Rif pages) DH Rav Ashi. Rabeinu Yonah understood that the Behag did
not require a Borei Pri ha'Gafen before the Kos either. The explanation is
that the Kos is a continuation of the wine drunk during the meal and Birkas
ha'Mazon does not constitute a Hefsek -- like the example of people who
interrupted their meal to greet a Chassan, who are not required to say a new
Berachah on their return provided that they left someone there. Since the Kos is part of the meal it is covered by Birkas ha'Mazon even though it is drunk after Birkas ha'Mazon.
The Bach suggests that there is a typographical error in the BeHaG. What he really wrote was that there is no need to recite a Brachah on wine *until*
after the Birkas ha'Mazon. This version in fact appears as a quotation from the Behag in the Rashbam on 103b DH Asur Lechu l'Mishti.