Per-face material causes face mesh to disappear after chamfer / shape rebuild

[tta602]

Description

When using the brush tool to apply a per-face material to a box, everything works correctly at first (only the selected face changes color).
However, after performing an editing command that rebuilds the shape (e.g. Chamfer / Fillet,... ), the entire object loses all faces and only the edges remain visible.

This happens consistently and seems to be caused by stale faceMaterialPair indices after the shape topology changes.

---

Steps to reproduce

1. Create a new Box.
2. Use the Brush Paint Tool to color one single face.
3. Perform Chamfer on any edge.
4. Observe the result.

Result:
The object loses all faces, only edges remain visible.

(See attached images)

---

Expected behavior

• After chamfering, the object should remain solid.
• Per-face material should either:
  • update correctly to the new face topology, or
  • be cleared/reset safely to prevent corrupted face data.

Actual behavior

• The entire face mesh becomes empty or partially missing.
• Only edges are rendered.
• Object becomes visually broken.

[xiangechen]

Let me think about the solution