Page/Post Editor Visual editor content not showing in full 'Editor box'

Hello WordPress.
I was testing new beta version of WordPress. When I put dummy content in it. The content which I was putting in it not showing in full width. ”’FOR-EXAMPLE”’ if full box width is 100px, but when I put some long content in it, Its start showing that content in 60px width(EXAMPLE).
So that in Visual editor content is not showing in complete editor box, Some thing like there is width problem in a box.
– I am using this WordPress version with default theme.
– If you need to reproduce this bug, simply open up page editor in visual section and put some long dummy content you will that, the content starts showing in some like box which have low width then whole visual editor frame.
– For better understanding I have some screen shots, please have a look on mentioned link.
http://photontechs.com/imp.png
Thank you.
Daniyal