Google Maps, Infobubble with MarkClusterer (zIndex fix)

Question

I want infobubble displaying next to the markers all the time. When I cluster them, infobubble appears at the front despite that markers are hidden... Is there a way to send it back within the markers?

Here's an image of what's happening

It might be because of "floatPane", please give some ideas on the fix

Code

  var marker =  new Marker({
              position: pt,                  
               map: main_map,
icon: icon_type
                });


  markers_array.push(marker);  
  main_map.addMarker(marker);  


     var popup_info = new InfoBubble({
           content:info,  
      padding: 0, shadowStyle: 0, 
      borderRadius: 5, minWidth:250, maxWidth:300, padding:10, 
      arrowSize: 7,
      borderWidth: 1,    disableAutoPan: false,
      backgroundClassName: 'transparent'

    });


google.maps.event.addListenerOnce(main_map, 'tilesloaded', function() { 

 marker_cluster = new MarkerClusterer(main_map, markers_array);

});