Source Button Problem

hello friends! As you can see in the following animation a part of the Q of Quellcode (Sourcecode) is covered... I believe from an image tag but I'm not sure!

By hovering this button the text is not covered anymore.



As you can see I made my own design, but the problem is not because of this selfmade design, it also appears by the fckeditor default design.

I need heeeeeeelp!