ok, now I get what you want to do....
You may try to dock something else there. You could open a unit in a 2nd source-editor (right click the editor tab of the unit). Then when you dock that 2nd editor window, you can close the unit, but keep the window => that can be used a spacer, I guess.
Possible (no idea if it works) you can then save this as a new "desktop setting" (see desktops in the toolbar). Then (if that works) you can quickly toggle between having it and not having it.