Created attachment 191255 [details]
Ref-test highlighting the problem
If a region has rounded corners (via border-radius), it still receives mouse events as if it wouldn't (e.g. hit area is still rectangular).
In the attached test case, moving the mouse cursor over both the green and the red dot at the top left of the yellow circle should make it appear on a green background (via CSS :hover).
However, if the yellow circle is a region, the mouse events will propagate to its parent only over the would-be-rectangular border (green dot) and not inside the area between the rectangular border and the actual circular border (red dot).
On closer inspection, this doesn't look like a bug anymore - the same behavior happens without regions: children of an element with rounded borders will still receive mouse events even when the mouse is outside the rounded rectangle (but inside the "real" rectangle).
The lack of "reaction" when moving the mouse over the red dot in the actual file comes from the fact that the named flow contents "eat up" the mouse event and it never bubbles to the region (at least in the current event propagation model).
Closing as RESOLVED/INVALID :)