Reweite Editor HTML and CSS
At the moment the html structure is sub optimal and kinda broken. Best seen when in distraction free mode. I spend already 3 hours trying to fix this with no luck. So I'll rewrite it with fullscreen in mind.
I think fullscreen mode using the inbuilt browser system isn't the best way to go. Far better to use native javascript and just hide lots of other elements on the page, expanding the text box size.
I might be wrong, but I think this is how the Wordpress.com system works: http://en.support.wordpress.com/distraction-free-writing/
DFW is likely to be dropped in the next version of WP core, or the one after, so this point may be redundant.