Ciao wastzzz,

potresti cambiare il z-index all'onclick
codice:
<div style="position:absolute;top:20px;left:20px;background-color:red;width:100px;height:40px;z-index:100" onclick="this.style.zIndex+=2"></div>
<div style="position:absolute;top:30px;left:30px;background-color:green;width:100px;height:40px;z-index:101" onclick="this.style.zIndex+=2"></div>