From https://github.com/jquery/jquery-ui/blob/44b2180782df6ef3324789324fcf3f98b85784a0/ui/jquery.ui.core.js#L93

jquery.selector.focusable.js

(function($) {
	function visible(element) {
		return $.expr.filters.visible(element) && !$(element).parents().addBack().filter(function() {
			return $.css(this, 'visibility') === 'hidden';
		}).length;
	}

	function focusable(element, isTabIndexNotNaN) {
		var map, mapName, img, nodeName = element.nodeName.toLowerCase();
		if ('area' === nodeName) {
			map = element.parentNode;
			mapName = map.name;
			if (!element.href || !mapName || map.nodeName.toLowerCase() !== 'map') {
				return false;
			}
			img = $('img[usemap=#' + mapName + ']')[0];
			return !!img && visible(img);
		}
		return (/input|select|textarea|button|object/.test(nodeName) ?
			!element.disabled :
			'a' === nodeName ?
				element.href || isTabIndexNotNaN :
				isTabIndexNotNaN) &&
			// the element and all of its ancestors must be visible
			visible(element);
	}

	$.extend($.expr[':'], {
		focusable: function(element) {
			return focusable(element, !isNaN($.attr(element, 'tabindex')));
		}
	});
})(jQuery);