Is your feature request related to a problem? Please describe.
Iris currently recalculates normals inside of GUIs. This messes with some mods' rendering. Notably, EMI's batched item renderering will have all 2d items turn dark. I have heard this also occurs in JEI though have not personally tested. There are no trivial solutions on EMI's end to resolve this issue.

Describe the solution you'd like
Iris should not recalculate normals during screen rendering. This can be non-trivial to determine, but a basic check of being on UI thread inside of screen rendering should be enough to capture a large chunk of screen rendering issues.

Describe alternatives you've considered
An API call to "trust normals" may be valuable for other developers, and is not necessarily mutually exclusive with this fix. However, it makes more sense to combine the two to capture a large subset of the normal recalculation issues natively and not require manual intervention.

Shown: blocks rendering correctly, enchanted books (unbatched) rendering correctly, various items rendering darkened.

This is still an issue over here as well. Iris and EMI for me.

My mod has the same issue. I am rendering all the hotbar items in one batch and they turn dark when shaders are enabled

Ran into this issue as well. Not a big issue but definitely one of the more noticeable ones.

This is still causing headaches for anyone trying to use a variety of mods like EMI and the mod described by RaphiMC. It's been noted by modpack devs like GamerPotion that this makes adding mods like above impossible until the issue is resolved.

Issue still effects EMI, also seems to effect Hearth & Home lattices despite being models.

This is also a issue for me

i use oculus and emi and its also an issue for me please find a fix :)