Following tests currently affected: fast/regions/selection/selecting-text-ignoring-region-horiz-bt.html fast/regions/selection/selecting-text-ignoring-region-vert-lr.html fast/regions/selection/selecting-text-in-2-regions-horiz-bt.html fast/regions/selection/selecting-text-in-2-regions-vert-lr.html
BTW, fast/regions/region-content-flown-into-region.html has a very similar issue and it was flagged in TestExpectations file on bug #115437 related to subpixel layout.
CSS Regions were removed in Bug 174978.