fix bug on wrong cursor coordinates in textarea case

 In textarea case, when user input is not at the bottom line , the position of menu is wrong like the picture below.

Now it's fixed , please accept this pr.  

![2014-12-18 3 37 12](https://cloud.githubusercontent.com/assets/1460411/5484973/29340594-86ce-11e4-925d-ccc62bc14a3c.png)