This is an interesting complication with offsetLeft/Top for columns and regions. This is an issue with the old columns as well as the new ones, so it's not a regression. Basically when testing the top left location of a box, in flipped block writing modes, this can result in you picking the wrong column. We need the ability to make different choices at boundaries if looking at the opposite edge of a box. I imagine this bug occurs with regions as well, and that it's not unique to columns.