Moving a circular or chromosomal backbone splits the decorator

Moving an empty Circular or Chromosomal backbone on the canvas splits it visually, with the the right-hand decorator moving to the right and leaving a gap. Only happens to the last moved backbone, and moving a different one fixes the previous one.

<img width="281" height="178" alt="Image" src="https://github.com/user-attachments/assets/6f0ca5c4-b8e0-4d79-9aee-cb987857acb4" />