I am using the hack in the link below, but it’s a little awkward because it requires you to first scroll to the bottom of the navigation tree and grab the corner of the panel. So it’s not quite like Confluence where you can just grab anywhere on the panel edge and resize, and your panel width is not remembered if you navigate to a different page. A while ago, I tried to hack it further in order to allow the user to grab anywhere on the edge, but I ran into issues and haven’t revisited it yet.
I wish that this was a built-in XWiki feature because this is something that my users were (and still are) immediately frustrated by after migrating from Confluence to XWiki.