Board Thread:Support Requests - Designing Your Wiki/@comment-3225604-20140106062419/@comment-4018164-20140106124142

There's no way of doing this (in general). The container of the page content has a lower z-index than the wiki header (which means that the wiki header will always remain "closer" to you in the screen). Therefore, anything in the page's content will always remain below the header.

What you can do is to either:
 * 1) Add some CSS to your MediaWiki:Wikia.css and give both the page content and wiki header the same z-index.
 * 2) Use JavaScript (in MediaWiki:Wikia.js)to move your fluid box out of the page container, making it not affected by the wiki page's z-index.
 * Using CSS:

However, if you're using a CSS from this sort, you'd need to make sure that people are not abusing it for stuff like hiding the wiki rail, and so on.
 * Using JavaScript

Giving your fluid box container an id (on this example it's yourfluidbox) and then appending it to the body. Note that the default wikitext is applied on the usual page content container, and therefore, appending it to one of its parents will require adding further CSS: