Show missing content

[Floppy]

Currently, unchanged content hidden by github is invisible. Need to add a placeholder line. Tried this, but ran into possible coopy bugs.

[Floppy]

See example diff at datasets/s-and-p-500-companies@e64871e

[Floppy]

WIP is on the show-hidden-lines branch, but has a bug where the inserted placeholder lines are highlighted by coopy as changed.

[paulfitz]

@Floppy I think this has been fixed.