I did some test by doing random modification in GLSL code like multiplying red component by 2 and I can tell the GLSL shader is indeed contributing to the display, and that before the early per-pixel discard/return for irrelevant pixel, it properly selects the pixels to operate on, but in the end, the FXAA effect is not seen.
I did some test by doing random modification in GLSL code like multiplying red component by 2 and I can tell the GLSL shader is indeed contributing to the display, and that before the early per-pixel discard/return for irrelevant pixel, it properly selects the pixels to operate on, but in the end, the FXAA effect is not seen.