Thank-you for this!  It makes it really easy to see that the normal mapping is working properly on faces that need translation to tangent space.

I think what I'm finding is that skybox textures don't seem to have a standard form.  In the attached image the dim red blob is at x=-7, the bright green at y=+7 and the dull blue at z=-7 (world).  I am not flipping the images on Y as I load them yet "cloudy skyboxes" on OGA work once the "up" and "down" textures were rotated 90° left and "park skyboxes" all work without any changes.  I don't think I'm doing anything funky in the shaders:

#version 150 core
uniform mat4  mvp_matrix;
in vec3  position;
out mediump vec3  texture_ordinates;
void  main ()
{
  texture_ordinates = normalize (position.xyz);
  gl_Position = mvp_matrix * vec4 (position, 1.0);
  gl_Position = gl_Position.xyww;
}

#version 150 core
#extension GL_NV_shadow_samplers_cube : enable
precision highp float;
uniform samplerCube  texture;   // The skybox texture
in mediump vec3  texture_ordinates;    // From the vertex shader
out vec4  color;
void  main ()
{
  color = textureCube (texture, texture_ordinates);
}