droppable: a small performance improvement by not using :visible, and…

… deferring to a simpler syntax of checking for diplay none

ui/ui.droppable.js

@@ -176,9 +176,10 @@ $.ui.ddmanager = {
 
 		var m = $.ui.ddmanager.droppables;
 		var type = e ? e.type : null; // workaround for #2317
+
 		for (var i = 0; i < m.length; i++) {
 			if(m[i].options.disabled || (t && !m[i].options.accept.call(m[i].element,(t.currentItem || t.element)))) continue;
-			m[i].visible = m[i].element.is(":visible"); if(!m[i].visible) continue; //If the element is not visible, continue
+			m[i].visible = m[i].element.css("display") == "none"; if(!m[i].visible) continue; //If the element is not visible, continue
 			m[i].offset = m[i].element.offset();
 			m[i].proportions = { width: m[i].element[0].offsetWidth, height: m[i].element[0].offsetHeight };
 
@@ -210,6 +211,7 @@ $.ui.ddmanager = {
 		if(draggable.options.refreshPositions) $.ui.ddmanager.prepareOffsets(draggable, e);
 
 		//Run through all droppables and check their positions based on specific tolerance options
+
 		$.each($.ui.ddmanager.droppables, function() {
 
 			if(this.options.disabled || this.greedyChild || !this.visible) return;