I tested and it's not supported in 2.79 so this maybe a corner case that has long been standing.
This happens also in final render.
- Open given file and switch to rendered view.
- Connect the 2nd checker color socket to another of the dysfunctional texco socket.
- Only glossy rays can now see the texco of the environment.
The issue seems to happen if the coordinate is plugged straight to the background or emission node. If the output is mixed it does work (default state of the file).
Not mixed, broken: