Float fragments should be positioned leftmost/rightmost in each fragmentainer (region). Currently, the position for the float fragments seems to be based solely on the first region. (e.g. outside the second region if the first region is wider than the second one, or in the middle of the second region if the second region is wider than the first one). See attached ref-test for a visual example.
Created attachment 194008 [details] Ref-test highlighting the problem
Still repros on today's nightly (r151543)
CSS Regions were removed in Bug 174978.