Normally, only mouse actions that occur within the application's virtual screen are processed by the runtime system. (The virtual screen is the drawing space allocated to the application.) Actions that occur outside of this space are handled by other applications or by the environment. The CAPTURE-MOUSE function causes the runtime to process all mouse messages, regardless of where they occur. This should be done only in special cases, because it prevents the user from using the mouse in any other application. Normally, you capture the mouse only when the user is marking or dragging some object on the screen. By capturing the mouse, you can control the action even if the user accidentally moves the mouse outside of the application. (Takes no additional parameters.)
When the mouse is captured, if the mouse is outside of the legal boundaries, the GET-MOUSE-STATUS and GET-MOUSE-SCREEN-STATUS functions no longer return zero in the row and column fields. Instead, the row and column fields contain the nearest legal coordinate to the mouse's actual position.