RenderFlowThreads should establish a block formatting context always. CSS Regions have named flow thread that they absolutely position, and so they got this for free by being absolutely positioned. RenderMultiColumnFlowThreads, however, are relative positioned, so we need to just add an isRenderFlowThread case specifically so we're not dependent on the positioning scheme used for the flow thread to establish the BFC.
Created attachment 189359 [details]
Fix landed in r143486.