I have the same issue. I'm using the jquery .mouseup

$('#myselect").on('mouseup', function(e){
 //your code
 });

It works with mouse selection over the multiple select list as you hold the mouse down and scroll to the end that you want. It will trigger multiple times if you click on the first, hold the ctrl key, and then scroll down and click on the next or just shift click on the individual options. If you want to avoid that I think you'll have to go with a button to refresh whatever field you want to fill from the selections however.