Detect Mouse leave stage while dragging in Actionscript 3

I encountered a similar problem in a PDF type viewer I had to build into a Flex application. I wanted the pan functions to still work even if the mouse left the stage or the browser window. Here is how I accomplished this, I've changed the code to remove references to Flex Framework classes so this should be applicable to any AS3 project. On mouseDown I would start tracking these values on a timer. _client can be any flash.display.DisplayObject in the target stage. In my case it was a Flex mx.controls.SWFLoader object, but in your case I suppose it would be the drag target:

private function get currentMouseX():Number
{
     return _client.stage.mouseX; 
}

private function get currentMouseY():Number
{
     return _client.stage.mouseY; 
}

The stage.mouseX and stage.mouseY values are defined relative to the stage whether the mouse is in the stage or even in the browser window (at least in Flash Player 10, I haven't tested this in earlier flash player versions). To see if the mouse is outside the stage just test and see if these values are within the stage, like so:

if (currentMouseY < 0 || 
    currentMouseY > _client.stage.height || 
    currentMouseX < 0 || 
    currentMouseX > _client.stage.width)
{
     // Do something here
}

EDIT: As to detecting a mouseUp event outside of the stage, if you register a listener on the stage, a mouseUp will be issued even if the event occurs outside of the stage or the browser. Here is the code for how I handle events function for reference. The _client object can be any flash.display.DisplayObject:

 // attach the events like so when you initialize
 _client.addEventListener(MouseEvent.MOUSE_DOWN  , handleMouse);   
 _client.addEventListener(MouseEvent.MOUSE_OUT   , handleMouse);
 _client.addEventListener(MouseEv