I've used window.onbeforeunload to display a custom message when a user attempts to leave a site.

Example:

window.onbeforeunload = function(){

  if(some_condition){

    return Are you sure you want to navigate away from this page?nAll unsaved changes will be lost.;

  }

};





+--------------------------------------------------------+

| Are you sure you want to navigate away from this page? |

| All unsaved changes will be lost.                      |

|                                                        |

|          [ Yes ]  [ Cancel ]                           |

+--------------------------------------------------------+

However, I'd like to enhance this a bit. If possible, I'd like to use a custom modal form instead of the generic popup.

Is there a way to do this?