Selectize dropdown selection outside of bootstrap modal window causes the modal to close

This may not be a clean solutuion. But you can set the modal to static when your "select" is opened, and change it back when select is clicked or lost focus.

$("#select_customer_name").on("focus", function(e) {
    modal.data('bs.modal')._config.backdrop = 'static';
});

$("#select_customer_name").on("blur", function(e) {
    modal.data('bs.modal')._config.backdrop = true;
});

Note: This works on Chrome, not on Firefox.