Hi PaQ, on closer examination it looks like there is a bug in here.

It actually should not be trying to align to the surface normal in this case at all, because the center point of that trim edge is not actually on the surface.

This case of an edge being an exact arc but the surface not being planar is not being handled correctly right now. It assumes that the center point of the edge is on the surface as it would be on a planar surface like a disc, and it is getting the surface normal at the closest point on the surface to the edge's center point.

It should not be aligning to the surface here at all, it should either not try to align it or it could align to the plane of the trim edge instead which probably makes sense.

I got a little confused from your initial image because I thought there was a filled in planar disc there instead of just a hole. And actually if you put a planar disc in there the snaps on a disc like that will work as you were expecting.

I'll see if I can fix this up, I think once it is fixed it should behave in this case as you originally expected if I can add snap plane alignment to a trim edge on center snap.

So thanks for reporting it, you can see an even more striking instance of this same bug if you draw a cylinder, rotate it around to an odd angle, then delete the planar end caps and draw a circle snapped on to the edge's center in that case. Since the closest point to the cylinder is way far off it is a more noticeable effect. It was kind of masked previously because it was handled fine if the cylinder caps were in place.

- Michael