Import: "Merge Vertices" option damages custom attributes

[scurest]

Describe the bug
When importing with "Merge Vertices", the importer does not consider _custom_attributes when determining vertex uniqueness, causing custom attribute values to be imported incorrectly.

To Reproduce

1. Import CustomAttributeTestGLB.zip.
2. This is a cube with a custom attribute named _Q. Using the spreadsheet, confirm _Q is a constant 0-5 on each of the six sides of the cube.
3. Import it again with "Merge Vertices".
4. Using the spreadsheet, confirm that the range of _Q is wrong, and the value at many corners is now incorrect.

Expected behavior
Either vertices with distinct custom attributes should not be merged, or custom attributes should be imported as CORNER domain attributes when using "Merge Vertices".

Version

• OS: Linux
• Blender Version: 4.3.1

[julienduroure]

Hello,
I can confirm.
It is a know limitation that I didn't implemented
Let's set it as bug

See

glTF-Blender-IO/addons/io_scene_gltf2/blender/imp/mesh.py

Line 858 in 9b3176b

# We don't split vertices only because of custom attribute