Updatable MeshGeometry throws RangeError after a mesh shrinks

[Ram-Dawson]

Summary

MeshGeometry.applyMeshData can throw a RangeError when an updatable mesh is updated from a larger vertex count to a smaller one.

Reproduction

• Upstream baseline: 7ec6e96cb4b755dc244656a66e203b75622aceae
• Reproduction project: Ram-Dawson/flutter_scene_repros
• Failing baseline: repro/updatable-mesh-shrink-before
• Case documentation: Updatable Mesh Shrink
• Scenario: an updatable array-cube mesh performs 64 -> 1 -> 64

1. Check out repro/updatable-mesh-shrink-before from the linked reproduction repository.
2. Run fvm flutter pub get.
3. Run fvm flutter drive --driver=test_driver/integration_test.dart --target=integration_test/updatable_mesh_shrink_test.dart -d <device-id> --enable-flutter-gpu --enable-impeller.

Observed behavior

The shrink update throws from the GPU upload path:

RangeError (end): Invalid value: Not in inclusive range 0..4032: 4416
  ByteData.sublistView
  _RingBufferStream.writeRange
  MeshGeometry.applyMeshData

The stale range retained by a previously larger mesh extends beyond the smaller snapshot's CPU byte array. In a manual Flutter UI this exception escapes the slider callback, so the slider can appear to stop updating.

Expected behavior

Shrinking and then regrowing an updatable mesh should not attempt to upload bytes outside the current mesh snapshot.

Supporting evidence

The reproduction repository includes a focused failing tag, text evidence, and two supporting Android UI captures. The screenshots are not a same-frame visual comparison; the RangeError and focused integration test are the behavioral evidence.