Created attachment 128162 [details]
If a region has a :hover CSS pseudo-class it will not be applied consistently.
Content flowed into a region prevents the :hover pseudo-class from applying to the region.
If you hover over the region border, the hover class for the region is applied. At this moment, there is no easy way in which the content events are passed to the region, and to a certain extent to the fragmentainer.
Still repros on today's nightly (r151543)