Shabbas Chazon Learning (when falls on 8 B'av)

The general minhag in Ashkenaz was not to learn after chatzous (and only what is mutor on tisho b'ov is mutar after chatzous). The reason is, as the Chassam Soufer says, because one thinks over in the night what he learns in the day, therefore one should not learn after chatzous dvorim ho'assurim beTisho B'ov, so that he won't think about it in Tisho B'ov. In the communities in Ashkenaz there were no Shiurim after Chatzous, and also most of the people didn't learn to themselves dvorim hoasurim after chatzous. See Chassam Soufer and Maharam Shif.
The same thing applies to Tisho B'ov that falls on Shabbos - after chatzous (before the 10th of Ov) one must learn only dvorim hamutorim beTisho B'ov. Before chatzous there is no problem to learn everything else.

Michael FRBSH