This is related to bug 97706. Basically we cannot postpone applying scroll changes for RenderBlocks with opposite writing mode.
A BT or RL layer needs to have its overflow, overflow in the opposite than the normal direction and it should thus be moved (scroll position change) to account for this. The flex box code does a 2-pass layout, one layout "without scrollbars" to find the actual size, and then one at the final size.
Postponing this initial "position correction" and applying it later as a scroll offset breaks layouts, as it is not really a scroll position offset.