MeshStandardMaterial.vertexTangents appears to have no effect

According to the Three.js docs (relevant link), I can precompute and enable vertex tangents in the standard material like so:

material.vertexTangents = true

Then, based on this line in WebGLProgram.js there should be a #define USE_TANGENT automatically added to the shader due to vertexTangents being true.

However, it’s not working in this JSFiddle where I’m overriding the fragment color:

The sphere should appear green based on the existence of USE_TANGENT, but instead it stays red. If instead I manually set material.defines.USE_TANGENT = true, then the sphere appears green as expected. In fact, it will keep working even if I set .vertexTangents = false.

This makes me confused – what is the expected behavior of .vertexTangents? It doesn’t appear to have any effect here.

Vertex tangents only have an effect if you are using a normal map:

In your fiddle, you are not applying a normal map to the material.

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