When using the draggable plugin you can set the stack option.

According to the documentation you set it like this:

Initialize a draggable with the  stack option specified.

$( ".selector" ).draggable( { stack: { group: 'products', min: 50 } } );

However when looking at the source there has been a change between 1.7.2 and 1.8.

in 1.7.2 the source is:

2. $.ui.plugin.add("draggable", "stack", {
3. start: function(event, ui) {
4. 
   
5. var o = $(this).data("draggable").options;
6. 
   
7. var group = $.makeArray($(o.stack.group)).sort(function(a,b) {
8. return (parseInt($(a).css("zIndex"),10) || o.stack.min) - (parseInt($(b).css("zIndex"),10) || o.stack.min);
9. });
10. 
    
11. $(group).each(function(i) {
12. this.style.zIndex = o.stack.min + i;
13. });
14. 
    
15. this[0].style.zIndex = o.stack.min + group.length;
16. 
    
17. }
18. });

in 1.8 it is:

2.     $.ui.plugin.add("draggable", "stack", {
3.         start: function(event, ui) {
4. 
   
5.             var o = $(this).data("draggable").options;
6. 
   
7.             var group = $.makeArray($(o.stack)).sort(function(a, b) {
8.                 return (parseInt($(a).css("zIndex"), 10) || 0) - (parseInt($(b).css("zIndex"), 10) || 0);
9.             });
10.             if (!group.length) { return; }
11. 
    
12. 
    
13.             var min = parseInt(group[0].style.zIndex) || 0;
14.             $(group).each(function(i) {
15.                 this.style.zIndex = min + i;
16.             });
17. 
    
18.             this[0].style.zIndex = min + group.length;
19. 
    
20.         }
21.     });

As you can see from the highlighted lines the change was from  "o.stack.group" to "o.stack"

This caused this line in the 1.8 version to fail.

1. var min = parseInt(group[0].style.zIndex) || 0;

an error like ' Cannot read property 'zIndex' of undefined' will result.

this was already noticed before and documented in ticket:

http://dev.jqueryui.com/ticket/5191

however this fix not works and the error remains the same when using stack as documented which is strange as you would expect group.length to be zero.

i changed my source using the draggable from "stack: {group: '.windows', min: 50}" to "stack: '.windows' to fix the problem.