Tinymce bug ? WP 3.9

On one of my sites I have a problem with the content editor, it is no longer as before since I WordPress 3.9

Here are two screenshot:

http://img4.hostingpics.net/pics/86857820140419154713.png
http://img4.hostingpics.net/pics/60125920140419154627.png

I only have this problem on one of my sites in particular all the others are fine

I disabled all plugins, WordPress installed but nothing changed