I forgot that both the sets and the flow thread lay out before the containing RenderMultiColumnBlock has finished. That means it hasn't computed its overflow yet. Therefore there is no need to force it to compute its overflow from inside RenderMultiColumnFlowThread::layout, because the minute RenderMultiColumnFlowThread finishes its layout, the block will compute its overflow anyway.
Created attachment 189537 [details] Patch
Fixed in r143606.