I have tried it in both Firefox and Chrome and both show the message. It appears in the editor as the containing block for the editor is narrower than the content, hence there is a horizontal slider to show the content.
If I use the "View this online" link from the edit panel, the online display does not show the scroll message.
However, if I use the "send a test" email, the message appears.
Perhaps this will give you a clue as to what is causing the behavior and what i need to do to resolve the issue.
To facilitate the process, I have included a screen shot of the complete editor panel.
Thanks for your help!