Await existence of element before executing jQuery on it

somefile.js

// This can go into whatever file is enqueued for the given page, using the traditional admin_enqueue_scripts hook.
jQuery(document).ready(function($) {
	var onElementExists = function(element, callback) {
		if ($(element).length) {
			callback($(element));
		}
		function check_again(element, callback) {
			setTimeout(function() {
				onElementExists(element, callback);
			}, 100); // interval
		}
		check_again(element, callback);
	};
    // Do stuff when element exists.
    onElementExists('.mce-floatpanel', function(e) {
		$('.mce-floatpanel').css('background', '#f00'); // PoC: change modal background with jQuery
	});
});