The normal matrix should not be used to transform tangents

The normal is transformed by the normalMatrix, which is based on the inverse-transform.

The tangent should be transformed like any other direction vector so it remains in the tangent plane.

However, BufferGeometry.applyMatrix() transforms the tangent like so:

normalMatrix.applyToBufferAttribute( tangent );

This is not correct. It also leaves the tangent unnormalized.

Similarly, in the shader, defaultnormal_vertex.glsl.js does this:

vec3 transformedTangent = normalMatrix * objectTangent;

In the examples, TerrainShaderuses a similar pattern:

vTangent = normalize( normalMatrix * tangent.xyz );

[WestLangley]

TerrainShader has been removed.

Tangent transform in the shader has been fixed.

[donmccurdy]

That sounds like my mistake. 😓

https://github.com/mrdoob/three.js/issues/18006 looks good!