UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.146 Safari/537.36

Steps to reproduce the problem:
1. go to https://www.shadertoy.com/view/4syyRh
2. compare on a intel dx9 chrome against a dx11
3. see it's not the same result

What is the expected behavior?
expected consistent behaviour between dx9 and dx11
(expected same behaviour that we get on other gpu like nvidia)

What went wrong?
seems a problem with "log2"

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 65.0.3325.146  Channel: stable
OS Version: 10.0
Flash Version: 

it comes from a user report having a problem with it on sketchfab on it's windows 7 intel chrome dx9 webgl1 on this model https://sketchfab.com/models/bf1bfc9a62f74acf92318fc177d71bd6 as user reported black band in the "refracted" parts

As we might fix it so it might not be a good way to check the bug, the shadertoy is the best simple illustration, and is the most "reduced test case" I could come with in shadertoy, the culprit code is in "GetCurve" glsl function.