Upon watching your video demo, I realized that the ideal behavior will be rejecting the placement opportunity in places that are siblings of artboards (generalized to be describable as "invalid placements"). This would need to be given to the frontend by the backend, or maybe better, answered by the backend in response to a query by the frontend. This way, we never show the white insertion line of a width that would indicate an invalid placement, which is a flaw of this "just correct it if it's invalid" approach that I realized now.