Comment 5 for bug 964192

in Ubuntu 13.04, changing the following in /usr/share/sushi/ui/mainWindow.js worked for me

    gravity: Gdk.Gravity.NORTH_WEST,
    windowPosition: Gtk.WindowPosition.CENTER_ALWAYS,

in

  this._gtkWindow = new Gtk.Window({ type: Gtk.WindowType.TOPLEVEL,
                                             focusOnMap: true,
                                             decorated: !this._clientDecorated,
                                             hasResizeGrip: false,
                                             skipPagerHint: true,
                                             skipTaskbarHint: true,
                                             gravity: Gdk.Gravity.NORTH_WEST,
                                             windowPosition: Gtk.WindowPosition.CENTER_ALWAYS,
                                             application: this._application });

For me the block started on line 77.

I don't know anything about gtk+, so I hope that someone that knows how to do this kind of things well finds the better solution. I believe that part of the problem might have to do with the window from were this one could be getting its relative "origin" position (hope that makes ANY sense...), since a little centered window seems to appear just before the actual sushi normal window shows, right when I hit space, and when playing with the different possible values I was able to find for "gravity" and "windowPosition", it appeared as if this little window was being the one to which the normal one related its positioning.

Hope that helps a little at least