Pascal, is not it bad to automatically shrink the surface since the user will lose the ability to untrim the original surface? What if an invisible copy of the surface is being temporarily created and shrunk, then the offset curve on surface is built on it, then that surface gets deleted automatically and the original surface remains?
Right, if the face is a lot smaller than the underlying surface, intersecting can get squirrely - but in that case … thinking… I guess shrinking to get the trimming curves might be OK - I’ll as a developer. (If you happen to have a case that shows this that would be helpful - I’ve seen it but don’t, I think, have a file)
A few years ago I created a topic somewhere here regarding an issue with trimming surfaces at a very small angle along the edge. I will try to find it. The solution I found was to temporarily bump the tolerance 10 or 100 times higher than the file tolerance, then after trimming I reverted to the original file tolerance.