Line Numbers

Code editors need line numbers on the side. To get this right, we need to look at changing how the scrolling behaviour of the existing outer divs work on these. We need a structure like this:

div(Outer,
   div(LineNums, ...),
   div(Inner, ...)
)

The Outer div will need to have the vertical scroll bar to keep the line nums and inner content in-sync, but the inner div needs to be the one with the horizontal scroll bars, to ensure the line numbers are always visible.