See bug 1174507 comment 3. When this bit related to height (because that was always the relevant axis), we could propagate it to ancestor frames in nsHTMLReflowState. With vertical writing mode support, when we reach an orthogonal-flow boundary it probably doesn't make sense to continue propagating the NS_FRAME_CONTAINS_RELATIVE_BSIZE flag; but do we need some other means to track the isize/bsize dependencies across this boundary?