This is a custom shader we are using right now to render flat shaded geometry into a deferred render buffer. Please ignore the custom made stuff (#pragma), it shouldn't be of relevance in this case.

Now, this shader works. Most of the time.

On **some particular systems**, namely a Surface Pro 4 and a Surface Pro 3 with Intel Iris 540 / HD 5000 GPUs, both running Windows 10, the following **"optimization"** breaks rendering.

- vertex shader:
  - line 13: remove unused *out* attribute, `out vec3 v_position;`
  - line 20: change into local var, `vec3 _position = a_position;`
  - line 23: change to use local var, `vec4 position = vec4(_position, 1.0);`
- fragment shader:
  - line 34: remove unused *in* attribute, `in vec3 v_position;`

Result is, that on this GPU, it now renders all black. Actually, judging by the output I see it looks like the alpha test fails all the time, discarding all fragments.

On other GPUs (NVidia on Win10/OS X, Intel on OS X) it works as intended.

It's been a horrible exercise to track the problem down this one *innocently looking* change. Would be nice to get an idea what's wrong about it...