forward/back buttons don't show confirm "are you sure you want to leave"

Hello!

A while back I asked if there were anyway to ask a user if he or she were sure they wanted to leave a page by showing an alert if anything but the confirm button were pressed. User leakim971  kindly posted this solution:

<input type="button" onclick="stopit=false;" value="Order" />

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<script language="JavaScript">
var stopit = true;
  window.onbeforeunload = confirmExit;
  function confirmExit()
  {
if( stopit ) {
    return "You have attempted to leave this page.  If you have made any changes to the fields without clicking the Save button, your changes will be lost.  Are you sure you want to exit this page?";
}
  }
</script>

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It works really well, but I wonder if there is anyway to prevent showing the alert on browser "forward" / "back" button presses (?)

Thank you very much!