When scrolling down, the sidebar gets the .aui-is-docked class, which adds position: fixed; to keep the sidebar in place. This works fine until another page is loaded (apparently via AJAX). If the page is too short to enable the scrollbar, or before scrolling, the sidebar will be over the top of the navbar, since it doesn't lose the .aui-is-docked class until it's removed by an onScroll event. See the attached screenshot for an example.
Issue #9581 resolved