Surfaces Intersect bug

Surfaces Intersect not right.
SSX_BUG.3dm (65.9 KB)

I think it fails because the surfaces are nearly coincident there. if you want that intersection you can cut the cylinder and make a Loft with the lower curve:

That, or close the intersection curve (CloseCrv), create a planar surface from that curve (PlanarSrf), scale that surface up a bit and use it to trim both objects.

yes, it works. And the red tangent line is still missing

Yeah, I do not know if that can be handled as a special case - I suspect the intersector will always have a hard time where surfaces come tangent to one another. You can get a better intersection by adding knots to the surfaces but it does not find the line.

Is this a demonstration of a failure case, or a practical problem for your modeling?