Make comment sidebar appear from the side.
Set header's min-height to 92px to work with border-box box-sizing.
Hover effect on line numbers.
Comment out visited link colour.
Improve colour scheme, minor etc., and give background to selected line number.
Clicking line while another is already selected will just switch comment sidebar to that line.
Smoothen animation when revealing annotations.
Add note in JS to optimise by DRYing up code and using object orientation.