MacOS X glFrustum generates -0.0 in W column of projection matrix; Windows glFrustum generates 0.0. This causes sign differences in the gl_ProjectionMatrixInverse between platforms. manually force the z coordinate sign in terrain sky projection to be negative like it ought to be

extra/terrain/shaders/shaders.factor

@@ -12,6 +12,7 @@ void main()
     gl_Position = v;
 
     vec4 p = gl_ProjectionMatrixInverse * v;
+    p.z = -abs(p.z);
     
     float s = sin(sky_theta), c = cos(sky_theta);
     direction = mat3(1, 0, 0,  0, c, s,  0, -s, c)