Jim Graham pointed out that the current formula for margin corner radii was incorrect in my blog (http://hansmuller-webkit.blogspot.com/2013/02/padding-rounded-rectangle.html). It's also currently wrong in FloatRoundedRect::marginBounds():
float boundsRadiusX = std::max(rx(), margin);
float boundsRadiusY = std::max(ry(), margin);
float boundsRadiusX = rx() + margin;
float boundsRadiusY = ry() + margin;
I've corrected the blog :-).
Created attachment 195621 [details]
Created attachment 195623 [details]
Screenshot of the failing test case.
Created attachment 195772 [details]
Comment on attachment 195772 [details]
Clearing flags on attachment: 195772
Committed r147249: <http://trac.webkit.org/changeset/147249>
All reviewed patches have been landed. Closing bug.