I'm not sure I'm able to explain this, as I've just learned all these things, but I'll try. The problem is that Chrome only applies the best font rendering to the layer it considers to be the primary content layer, probably for performance reasons. The other layers coming over it are considered to be sort of "flying animated objects", that shouldn't contain huge blocks of text. That said, not only the fixed layer is detached from the content layer (they don't move together when you scroll), but it's found under the content layer, which makes the content layer to be considered a "flying object". If you remove the negative z-index from the fixed layer, it still has z-index=0, and it's placed under the content layer. But if you apply a positive z-index like 2, for example, the fixed layer will appear over the content, and the text issue is gone. But this would also kill the purpose of the fixed layer in this case.
And I do agree with your idea about the visited wikis. This could even be used by non logged-in users.