Sortable: Fixed jerkiness with nested-sortables. Fixes #4857 - Nested lists don't work properly with sortable.

(cherry picked from commit 2b899cc)

Author: Martin Hoch

ui/jquery.ui.sortable.js

@@ -296,7 +296,16 @@ $.widget("ui.sortable", $.ui.mouse, {
 			var item = this.items[i], itemElement = item.item[0], intersection = this._intersectsWithPointer(item);
 			if (!intersection) continue;
 
-			if(itemElement != this.currentItem[0] //cannot intersect with itself
+			// Only put the placeholder inside the current Container, skip all
+			// items form other containers. This works because when moving
+			// an item from one container to another the
+			// currentContainer is switched before the placeholder is moved.
+			//
+			// Without this moving items in "sub-sortables" can cause the placeholder to jitter
+			// beetween the outer and inner container.
+			if (item.instance !== this.currentContainer) continue;
+
+			if (itemElement != this.currentItem[0] //cannot intersect with itself
 				&&	this.placeholder[intersection == 1 ? "next" : "prev"]()[0] != itemElement //no useless actions that have been done before
 				&&	!$.ui.contains(this.placeholder[0], itemElement) //no action if the item moved is the parent of the item checked
 				&& (this.options.type == 'semi-dynamic' ? !$.ui.contains(this.element[0], itemElement) : true)