[css-ui-3][css-backgrounds-3][css-box][css-shape] Is the border box rounded by border-radius?

I was writing tests for css-ui-3, and ran into an ambiguity.

About the cursor property, css-ui says this:

  "This property specifies the type of cursor to be
   displayed for the pointing device when within the
   element’s border-box."

I first assumed that the border-box was the rectangular bounding box that includes the borders, so applying border-radius would not change the area over which the cursor property applies.

However, that's not what browsers do, and they don't apply the cursor outside of the actual rounded border.

I haven't been able to find a spec defining border-box well enough to say if it is always a rectangle (in which case css-ui-3 should be changed to match what browsers do), or if it does follow the border-radius (in which case css-ui-3 is fine).

css21 doesn't know about border radius.

css-backgrounds-3 doesn't say.

css-box says the border box is always a rectangle even in the presence of border-radius, but that spec cannot be relied on.

css-shapes says that when used as a value of <shape-box>, border-box means following the border-radius. But I am not using it as a value of <shape-box>, just as an abstract notion, so that definition doesn't apply.

So, is css-ui-3 using a correct but unwritten definition of border-box, or should it switch to some other term?

 - Florian