I've been thinking and trying out some solutions for this, and ended up customizing the PopUpRendererSkin. It works but I'm not really satisfied with the solution, what I would like is to use some kind of AppEvent for showing PopUp and closing/hidning PopUp. I've been looking through the documentation but I can't find anything like that, I've tried using SHOW_INFOWINDOW and some other events without success.I've not tried to create my own custom AppEvent, because I wouldn't know how to dispatch the events when PopUp shows or hides, if I knew that I would probably not even have to create any AppEvents.Anyone knows a way to listen to the events of showing and closing/hiding a PopUp? If anyone is interested in the solution I have so far, I got the PopUpRendererSkin.mxml from downloading the API and copied it to src/com/esri/viewer/skins/ and to the default.css I added:esri|PopUpRenderer
{
 skin-class: ClassReference("com.esri.viewer.skins.PopUpRendererSkin");
}All the code I put in PopUpRendererSkin.mxml, at the end of the commitProperties function add:// My addition for highlighting popup features...
    var glExists:Boolean = false;
    var glID:int;
    //Go through layerIds in map to check if the highlightGraphics graphiclayer exists and if so get its id.
    for (var i:int = 0; i < map.layerIds.length; i++)
    {
     if (map.layerIds== "highlightGraphics")
     {
      glID = i;
      glExists = true;
     }
    }
    //If highlightGraphics graphicslayer doesn't exist, create it and add it to the map.
    if(!glExists){
     var gl:GraphicsLayer = new GraphicsLayer();
     gl.id = "highlightGraphics";
     gl.visible = true;
     map.addLayer(gl,200);
     //Get the id of the graphicslayer
     for (var j:int = 0; j < map.layerIds.length; j++)
     {
      if (map.layerIds== "highlightGraphics")
      {
       glID = j;
       glExists = true;
      }
     }
    }
    //Set the highlight graphic symbol
    var hlGraphic:Graphic = new Graphic(graphic.geometry);
    switch(hlGraphic.geometry.type)
    {
     case Geometry.MAPPOINT:
     {
      hlGraphic.symbol = new SimpleMarkerSymbol("circle",22,0x000000,0,0,0,0,new SimpleLineSymbol("solid",0xFF0000,0.5,2));
      break;
     }
     case Geometry.POLYLINE:
     {
      hlGraphic.symbol = new SimpleLineSymbol("solid",0xFF0000,0.3,8);
      break;
     }
     default:
     {
      hlGraphic.symbol = new SimpleFillSymbol("solid",0x000000,0,new SimpleLineSymbol("solid",0xFF0000,0.4,3));
      break;
     }
    }
    //set a glow filter to the highlight graphic
    var gf:spark.filters.GlowFilter = new spark.filters.GlowFilter();
    gf.color = 0xFF0000;
    gf.alpha = 1;
    gf.strength = 2;
    gf.blurX = 8;
    gf.blurY = 8;
    hlGraphic.filters = [gf];
    //Clear the highlightGraphics graphiclayer and add the new highight graphic
    map.layers[glID].clear();
    map.layers[glID].add(hlGraphic);
    //add eventhandlers to "unhighlight" features
    map.infoWindow.closeButton.addEventListener(MouseEvent.CLICK, onInfoWindowClosed);
    map.addEventListener(MapMouseEvent.MAP_CLICK, onMapClick)
// --- end addition ---------------And then add a these two functions somewhere://My handler for clicking closebutton on a popup/infowindow
   private function onInfoWindowClosed(event:MouseEvent):void
   {
    if(hostComponent.map){
     for (var i:int = 0; i < hostComponent.map.layerIds.length; i++)
     {
      if (hostComponent.map.layerIds== "highlightGraphics")
      {
       hostComponent.map.layers.clear();
      }
     }
     hostComponent.map.infoWindow.closeButton.removeEventListener(MouseEvent.CLICK, onInfoWindowClosed);
    }    
   }
//My handler for a mapclick, mapclicking outside a feature with popup configuration will hide popup/infowindow and clear the highlight graphics.
   private function onMapClick(event:MapMouseEvent):void
   {
    for (var i:int = 0; i < event.map.layerIds.length; i++)
    {
     if (event.map.layerIds== "highlightGraphics")
     {
      event.map.layers.clear();
     }
    }
    event.map.infoWindow.hide();
    event.map.removeEventListener(MapMouseEvent.MAP_CLICK, onMapClick);
   }