Dockable (and, hence, undockable) edit windows.
Mostly, as mentioned previously elsewhere, I have two edit windows - one to do the actual edits and t'other to browse code. Sometimes I really need to look at two lots of code but my setup doesn't lend itself to 3 windows. What I'd like is to create a new edit window and then drag that out of the SE main window and drop it somewhere else on my desktop, as if it were a tool window.
So, thinking about that, why can't the normal edit windows be dragged hither and yon? SE Main would then end up as a kind of control panel, much like the old Delphi and C++Builder IDEs.