Tested on two systems and builds:
Win8x64, GTX 680, 2.8 recent buildbots (Cycles & EEVEE)
Win7x64, Intel iGPU, 2.79 d59c2d1 (Cycles CPU PT & BPT)
Texture Coordinate > Window outputs only one value if used with an orthographic camera in the World material node tree.
Confirmed under both Cycles and EEVEE.
To reproduce render the two frame animation in the attached blend file.
Frame one shows the expected output as available under an Object material node tree.
Frame two shows the error where all pixels are homogenous and unsuitable for mapping anything really.