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.

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.

makes sense.
And then what about the feat pr is raised for, it will be valid or invalid drop behaviour ?

I'd suggest iterating on this PR with the suggested change of approach. No need to open a separate PR.

I'd suggest irritating on this PR with the suggested change of approach. No need to open a separate PR.

Can you tell me more clearly what i need to implement? got little confused.

and what about the feat for which pr is raised ?

This PR. I'm suggesting you keep working on changes to it to implement my described updated approach (from my first comment in this discussion).

This PR. I'm suggesting you keep working on changes to it to implement my described updated approach (from my first comment in this discussion).

Yes but what about dropping on bottom, should I consider it as valid or invalid now?

I'm not understanding the question. All invalid drop locations shouldn't be valid white horizontal line placements, thus pre-filtering instead of post-rejecting.