The source window used to show which lines of a source file were executable by
showing a "-" sign to the right of the line number in the same column. This
does not seem to work anymore. The execution status of a line is stored in the
DOM, so I'm sure sure if the DOM is not getting updated properly and therefore
not getting displayed or if the DOM is right and the screen is not getting the info.
I've disabled these temporarily; whether or not they come back depends on what
gets decided about the margins in the source window. o.0