As I understand the OS X hierarchy of windows versus panels, a panel modifies the object on the main window, and a window is its own, separate, data. This makes sense with Features, Font Info, Sort and Mark. Space Center is its own window because it displays the same data in a different manner, which is also logical. The exception to this is Kerning and Groups. These two panels currently block any interaction with the rest of the font, and would also make more sense as pop-out windows – like Space Center. It would also make it possible to keep an eye on the groups, and modify Space Center while kerning.