Created attachment 162740 [details] Test case In some cases the region range for floats is not computed correctly. This makes the region style for the float to not be correctly applied. See the attached test.
Created attachment 162741 [details] Test case reference
After some investigations this may be a layout bug, not a regions one. I'm not very familiar (yet) with the floats spec but what I'm seeing is that the containing block of the float doesn't get the chance to call setLogicalHeight using the height of the float because this happens in layoutBlockChild but not in positionNewFloats.
It seems that the region range for a box is not necessarily contained inside the region range of the parent block. We should make sure the region styling algorithm doesn't make this assumption.
Still repros on today's nightly (r151543)
CSS Regions were removed in Bug 174978.