Investigate: Why do we have two tags `scroll-output` and `output_scroll` to do the same?

[bsipocz]

Noticed while reviewing a PR #683 (comment), (but didn't notice while reviewing the CSS PR 😱 ) 4b7a0ed

I would like to see having only one way to do the same things, but the tags have lived elsewhere before as I have notebooks that use the tags for a few years now, so it's not just a question of making the code here consistent, but also ensuring nothing is breaking.

[bsipocz]

For the tags I would tag @agoose77 as this is something that I suppose is expected to be consistent between the old and new stack.

(and I'm also tagging @dprada as the author of the PR that added all three tags).

[bsipocz]

OK, so after some digging it seems to be that output_scroll has been renamed to be scroll-output a while ago.

See jupyter-book/jupyter-book#1850 (comment) and linked issues and PRs.

[bsipocz]

Therefore a TODO item here is to cleanup the old one.