dropdown repostion fix when previous postion changes container position

Had some issues with dropdown position if showing it creates a scroll on the page.
so I changed the code so it will hide the dropdown before it recalculate its fixed position


![pos](https://f.cloud.github.com/assets/1829789/1149082/3f7aab5e-1ed4-11e3-9a51-a1a687e2edef.png)

example of the problem
http://jsfiddle.net/CGBRX/