I took the liberty of taking a look at this. It is caused by the Lato font which is being inserted by the “Fonts” theme component and applied to all elements (*). It is not a fixed width font and apparently the editor does not work well with that. Using /safe-mode works around the issue.
The actual cause is that you are applying that font to all elements by using the * selector. If you change this to html then you get the regular CSS inheritance and specificity back and your editor will get the correct font.