It isn't Zalgo (Zalgo is abusive use of Unicode symbols, not for their intended purpose). It's using a unicode symbol (combining overline) for its intended purpose, repeating decimals.[1][2] Is there a better, built-in Arc md way of displaying a repeating decimal?
It renders as-expected on NetBSD, Linux and Haiku. If you check the linked Overline Wikipedia page, «Overline (markup)» and «Overline (character)» look near-equivalent.
I see what may have been the problem, though. It looks like somewhere in the process the overline was duplicated. Probably an issue with my IME (I've been working on it for a while now, but it's still got a few quirks). Does this (6̅) look right to OS X users?
1. https://en.wikipedia.org/wiki/Repeating_decimal
2. https://en.wikipedia.org/wiki/Overline