Not sure the last patch was the correct solution, at least for the problem I'm seeing. Seems to me this isn't a CSS issue but a problem with codemirror not updating what's displayed when we transition to fullscreen.
What I observe:
1.) I click source button to view codemirror display.
2.) I then click the fullscreen button, page is maximized but the codemirror output is the same as it was before I hit fullscreen, not displaying all of the code.
3.) If I click on the editing area codemirror will do its update and the full source will be displayed.
If I look at the HTML source in the inspector at the div with the class CodeMirror-code before #3 the re will just be divs for the lines displayed, after the missing lines will be added to that div.
This is what seems to be currently working for me.