Unexpected Specular Highlights using Area Lights on Surfaces with strong Normals at Low Glancing Angles

A licensee is reporting shimmering specular highlights on the surface of a mesh with an Area Light while using MSAA, FXAA, or no AA settings. This occurs in the Forward and Deferred Renderer.

It was identified as a discrepancy between two different values used for the dot product between N and L within the Vis_SmithJointApprox() function. The specular shimmering occurs in regions where the the "N dot L" value used inside Vis_SmithJointApprox() is zero or close to zero, but the "N dot L" value which is attenuating the overall shading is not close to zero so it doesn't counteract it as it would if the two values were the same.

Open attached project.

Outcome Small shimmering specular highlights on the surface of the mesh.

Expected No shimmering and an even specular distribution across the surface.