A slider for use in a track having an end stop therein, the slider having a detent therein and including a releasable restrictor having a rib thereon, wherein the releasable restrictor can be held in an active position by an interaction between the rib and the detent, or an inactive position, and wherein the releasable restrictor is automatically reset in its active position when the slider reaches the end stop in the track. More controlled closing of a window sash is thereby possible.