I think your FXAA might be too wide :) see https://github.com/mattdesl/glsl-fxaa/issues/3

[Texture offset too wide · Issue #3 · mattdesl/glsl-fxaa](https://github.com/mattdesl/glsl-fxaa/issues/3)
Not sure this repo is active / alive anymore, but I've spotted what I believe to be a bug that was introduced
 when @mitsuhiko ported FXAA from the Geeks3D version of the shader to WebGL, replacing ...
seems this copy/paste error spread quite a bit
If you have access to WebGL 2, you can use textureOffset as well. 
https://github.com/magcius/noclip.website/blob/master/src/gfx/helpers/GfxShaderLibrary.ts#L120-L135