Works around a XenForo 2.3.x core lightbox defect that can rewrite the address bar to a URL ending in "/undefined", which then gets bookmarked, shared and crawled as a 404. Default OFF. When ON, it intercepts every XF.History.push call site-wide and skips only the ones whose url is undefined/falsy — it is NOT gated on whether a Fancybox toolbar is present, so enable only on sites confirmed to be affected.