Called when Slate detects that a widget started to be dragged. Usage: A widget can ask Slate to detect a drag. OnMouseDown() reply withFReply::Handled().DetectDrag( SharedThis(this) ). Slate will either send anOnDragDetected()event or do nothing. If the user releases a mouse button or leaves the widget before a drag is triggered (maybe user started at the very edge) then no event will be sent.