The EndNote folk will know if it’s a bug but personally it doesn’t strike me as being a bug. There are two options for laying out the groups panel (Left, Off). Dragging the library window to the left (as shown in your attached screenshot) has a similar effect of hiding the group panel (i.e., designating the group panel “Off”). If the group panel is hidden by either method (dragging the library window or selecting “Off”) the group panel can be reinstated by clicking the Group Panel’s “Left” option. At least this works using EndNote X6.0.1 and Windows 7 Professional.