Custom shaders don't work on instanced models

[j9liu]

While reviewing #10110, I discovered that custom shaders won't apply to instanced models, even if they're simple. I tested this with the sandcastle here.

What the custom shader is doing is applying a solid yellow color to the model:

However, this doesn't work for BoxInstanced.gltf:

[ptrgags]

I did some digging today and found that the model is rendering correctly based on PBR works.

If the specular value is high, then the reflections tend to overwhelm the diffuse color. By default, the glTF spec requires baseColor, metallic, and roughness to all be 1.0 (which corresponds to diffuse = 0.0, specular = vec3(1.0), roughness = 1.0. Since the Sandcastle in the description only changes the diffuse value, it is overwhelmed by the specular reflections.

However, if you reduce the specular value like in this sandcastle it does render yellow!

There's nothing actionable here so I'm closing this issue.