The sidebar on my forum SOMETIMES jumps down below the the node list leaving an empty bar on the right side of the page. The problem is that it only happens from time to time, and not always
This is a function of the width your browser window or device (viewport).
You can play around with the size of the three breakpoints and/or the width of the sidebar but this is part of what makes Xenforo responsive: at a certain point in screen/window size there won't be enough room for everything unless the sidebar drops down below the content.
