I will second it. I would also move it on the bottom right, as I didn't look at it except with my peripheral vision and thought it was an X/close button. When instructions said to click it, I went from "why would I click the close button" to "oh, it's actually a fullscreen button" before I clicked it. I never would have discovered it.

Also, the "expand" button overlaps with the h-scroll bar when it's displayed.

http://i.imgur.com/4mwx7ty.png

Awesome project nevertheless.

This is a very nice implementation.

I did something similar myself django-writingfield[1], but it is Django specific.

I much prefer your UI and the switching of dark text/light background is really slick.

[1]: http://django-writingfield.obscuremetaphor.co.uk/