Fix sortable for floating items

Fixed #8792:
http://jsfiddle.net/zhizhangchen/2HbZr/
http://jsfiddle.net/3JSd4/


Fixed #9041:
http://jsfiddle.net/A6QEn/
Fixed comment 5 of #9041
http://jsfiddle.net/bUFuK/1/


Regression tests:
http://jsfiddle.net/Y9Ned/1/
http://jsfiddle.net/YUqVL/2/
http://jsfiddle.net/3JdaT/1/