The layout of the widgetcontainer can be specified as vertical (similar to SFV) or horizontal (ditto but at the bottom) or float (the default where widgets can be moved around) or fixed (where end-users can not move the widgets.
See the Viewer Resource Center -> Samples -> Widget container layouts:
http://help.arcgis.com/en/webapps/flexviewer/samples/index.html#layouts.htm
I realize the viewer relies on config files, so maybe tab(s) for the config file in addition to, or instead of the source code tab.