Codemirror editor fullscreen mode
From Joomla! Documentation
In Joomla 3.4.2 we improved the Codemirror Editor there we miss a one line change.
This pertains only to Joomla! version(s): 3.4.2 & 3.4.3
What is the cause
You can't use the Codemirror editor in fullscreen mode.
How to fix
Apply the patch https://github.com/joomla/joomla-cms/pull/7325.
This will be included in Joomla 3.4.4