I have seen a few people and had a customer ask how to detect when a flex app loses focus.
Here is a sample app that shows the technique and a practical application of this functionality.
When you click and drag a DividedBox separator, if you move the mouse outside the Flex application window, the app should realize this and “drop” the separator at that horizontal or vertical position (depending on the orientation of your divider). It doesn’t do this automatically (and maybe is should), but it is easily done and demonstrates the technique that can be used for other reasons.

Here is the simple app:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" creationComplete="addListeners(event)">
    <mx:Script>
        <![CDATA[
            private var lastX:Number;
            private var lastY:Number;          
           
            private function addListeners(event:Event):void{
                systemManager.stage.addEventListener(Event.MOUSE_LEAVE, mouseLeave);
                systemManager.stage.addEventListener(Event.DEACTIVATE,deactivate);
                systemManager.stage.addEventListener(MouseEvent.MOUSE_MOVE, mouseMove);                            
            }
           
            private function removeListeners(event:Event):void{
                systemManager.stage.removeEventListener(Event.MOUSE_LEAVE, mouseLeave);
                systemManager.stage.removeEventListener(flash.events.Event.DEACTIVATE,deactivate);
                systemManager.stage.removeEventListener(MouseEvent.MOUSE_MOVE, mouseMove);                                             
            }

            private function mouseMove(event:MouseEvent):void{
                //trace("move…"); 
                lastX=event.stageX;
                lastY=event.stageY;
            }          
           
            private function mouseLeave(event:Event):void{
                //trace("left");
                removeListeners(event);
                dispatchEvent(new MouseEvent(MouseEvent.MOUSE_UP,true,false,lastX,lastY)); 
            }
            private function deactivate(event:Event):void{
                //trace("deactivate");
                removeListeners(event);
                dispatchEvent(new MouseEvent(MouseEvent.MOUSE_UP,true,false,lastX,lastY)); 
            }
           
        ]]>
    </mx:Script>

    <mx:VDividedBox id="div1" dividerPress="addListeners(event)" dividerRelease="removeListeners(event)">
        <mx:VBox height="200" width="300" borderStyle="outset">
            <mx:Button label="Something"/>     
        </mx:VBox>
        <mx:VBox height="200" width="300" borderStyle="outset">
            <mx:TextArea width="100%"/>
        </mx:VBox >
    </mx:VDividedBox>
</mx:Application>
 

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This movie requires Flash Player 9