UI.X offers a sticky sidebar feature which allows users to scroll down the page while the sidebar stays in place. You can enable your sticky sidebar by going to Appearance > Styles > XenForo - UI.X > Style Property Groups > [UI.X] Sidebar. Check the box next to Sticky Sidebar.
iOS devices render offscreen elements that switch to position fixed during a scroll differently. To disable the sticky sidebar for iOS, check the Disable Sticky Sidebar for IOS in your [UI.X] Sidebar style properties.
You have the option of including a login form within your sidebar.
To enable this option, go to your [UI.X] Sidebar style properties and check the Show Login Form in Sidebar box.
Your sidebar can become collapsable so that it can toggled on pages that have a sidebar.
To enable this option, go to your [UI.X] Sidebar style properties and check the Enable Collapsible Sidebar box. You also have the option to click the Use Collapsible Sidebar Phrases box underneath if you would like to show phrases stating open and close next to your collapsable sidebar.