https://bugs.freedesktop.org/show_bug.cgi?id=66067
--- Comment #12 from Nicholas Miell nmiell@gmail.com --- (In reply to comment #11)
ARB_fragment_program_shadow may leave it undefined, but the GL spec ...
Which spec exactly? GL specifications only cover GLSL shaders. ARB_fragment_program is separate from that and has its own rules. The extension specification clearly states that behaviour is undefined.
Section 8.23.1 of the GL 4.4 spec, which I paraphrased in comment #9.