That template is the default “Simple Page” one and uses this snippet:
{{box cssClass="floatinginfobox" title="**Contents**"}}
{{toc/}}
{{/box}}
Which is equivalent to
(% class="box" %)
(((
**Contents**
{{toc/}}
)))
in the encyclopedia template.
I’d thought about this too, but the hovering TOC would get in the way of content further below.
Maybe this is possible by manipulating css? Since floatinginfobox is a cssclass too.
I thought about pinning the content in the left or right sidebar panels of xwiki instead. This would be nice to have independently of a TOC.
I have yet to find out how to display a TOC in a Panel which uses the currently viewed page as a reference 