The API implies that Components can only be glued to faces.
However, a component instance that is not in a group can be glued to a face inside that group.
(Also a component instance can be glued to a face inside a different component instance.)
So the method: componentInstance.glued_to can return either a face, a group, or another component.
What’s the easiest way to find the actual face inside the group (or other instance) that the instance is actually glued to?
Actually all I need is the face normal, so I could create a vertical vector, use it to create a vertical cline, transform the cline using the instance transform, then find the resulting cline direction. I think I answered my own question.