MouseEvent.MOUSE_WHEEL to move object on stage issue

I'm using the MouseEvent.MOUSE_WHEEL event to shift the .y position of a MC on stage.  This is working BUT I'm getting an bright yellow box that outlines the MC when the mosue wheel is activated. 

Note:  I'm publishing as a projector and I've tried on both OSX and Windows - the same result.  

Can anyone advise as to why my MC is outlined in Yellow?

Thanks in advance for your help!

stop();

stage.displayState = StageDisplayState.FULL_SCREEN_INTERACTIVE;

import flash.events.MouseEvent;

stage.addEventListener(MouseEvent.MOUSE_WHEEL, myMouseWheel);

function myMouseWheel(event:MouseEvent):void {

stage.focus = MovieClip(event.target);
if (box_mc.y >= -709 && box_mc.y < 97) 
{
box_mc.y -= event.delta;
}

else if (box_mc.y < 97) {
box_mc.y = 97;
}

else if (box_mc.y >= -709) 
{
box_mc.y = -709;
}

}