Immediate plans

Make gray background coloring convey more information
Add clear separators between change markers and text

Solve horizontal shifting problem:
Try putting all the change markers to the left, as suggested by users
Or have the text stay put and the change markers move

Improve blank-line-insertion algorithm
It doesn't always make the right choices
It is way too slow

Add more navigation features and flesh out the user interface

Then do another round of user testing