When I change settings for selection outline available in complementary shaders, it only works for single slab blocks. Single slab from double slab block targeted by KleeSlabs gets the default selection outline.

KleeSlabs is using the same ShapeRenderer.renderShape method that Vanilla Minecraft uses for its block outlines as well.

If the shader devs can point me towards what I'd have to do for their outline to work with KleeSlabs, I'd be happy to investigate.

Closing since this is more likely something that needs to be fixed within shader mods.