Optional
During a live drag: flex frames are owned by flexLayout, so skip them here (avoids double moveChild). Omitted for one-shot drops (new nodes / paste), where we DO insert into flex at the main-axis index.
During a live drag: flex frames are owned by flexLayout, so skip them here (avoids double moveChild). Omitted for one-shot drops (new nodes / paste), where we DO insert into flex at the main-axis index.