A media-sheet stacking system (10) for stacking media sheets on a device output tray (OT) comprises a kicker (25) to advance a media sheet along the output tray by pushing a trailing edge of the media sheet as it exits the device, and a movable element (32a) arranged to face the output tray across a space that receives media sheets exiting the device. A drive mechanism (33) is provided to drive the movable element in the direction of advance of the media sheet along the output tray.