The successor to Nvu has two sidebars, left and right of the editing area. Each sidebar can contain multiple sidebar items. Each sidebar item can be moved by simple drag-and-drop to the other sidebar, or even be detached from the main window. And of course reattached to the left or right sidebar if needed. Sidebar contents, positions and sizes of detached items, everything is persistent and will follow you across sessions. Writing an extension adding a new sidebar item is very simple, a doc is ready.