About [[MediaWiki:Vector-collapsiblenav-preference/en]]

It's a guess, but i'm quite sure that it's the sidebar. What else can be collapsed?

• Roan says: CollapsibleNav affects the sidebar (or parts of it? I forget).
• Trevor says: I think you are talking about the "namespace", "view" and "actions" menus? In vector these are rendered in 3 distinct places, while Monobook and Modern render them all in a single set of tabs.

Go figure.