!DavidBiesack has reported on mediawiki.org:
if we use CategoryTree for a navigation box on a set of pages within a category with multiple subcategories, is there any way to preserve the expanded/collapsed state across page views? The desired effect is similar to using an HTML frameset, with the category tree in one frame on the left, and the navigated content in a frame on the right, but ideally done within the standard MediaWiki model

@JackPotte would like to use this feature on this French Wiktionary page.