http://dev.w3.org/csswg/cssom-view/#element-scrolling-members

[[
If element's border box beginning edge and ending edge in the inline base direction are both outside box's beginning edge and ending edge in the inline base direction
]]

The element doesn't necessarily have the same writing mode as the scrolling box. The rules should use the scrolling box's writing mode and use the same physical sides on the element regardless of its writing mode, I think.