When you scroll very quickly, CodeMirror shows big white "gaps" because it can't process what's supposed to be in your browser view quickly enough. This is distracting and will be annoying for power users.
Fill in the space by showing regular unhighlighted text as the screen goes by. To do this, we'll have to take out the larger font size and line spacing for headers, and make it uniform size and spacing.
I liked the treatment on headers, so this is a shame, but we can't have those "gaps"; people will hate it.