Fix html repr in untrusted notebooks (plain text fallback)

[benbovy]

• Closes expanded HTML repr when opening notebook #4041
• Tests added
• Passes isort -rc . && black . && mypy . && flake8
• Fully documented, including whats-new.rst for all changes and api.rst for new API

This is not very elegant (actually plain text repr is already included in the notebook as text/plain mime type but it is ignored when text/html mime type is present), but it seems to work. I haven't found a better workaround.

I don't really know if this can be properly tested (I only added a basic test).

Steps to test this fix:

• To "untrust" a notebook: open an existing notebook with a simple editor, manually edit one output cell with a xarray object repr, and save the ipynb file.
• Open this notebook with the Notebook app, you should see the plain text repr.

[shoyer]

I tested this out exporting a notebook to Github:
https://gist.github.com/shoyer/6ee36acd164116fee4681f6237ca5249

Here's what I see:

Somehow we get both text and malformed HTML? :(

It's definitely still an improvement over what we had before, though.

[benbovy]

Mmm it looks like Github's notebook renderer removes the hidden attribute of the xr-wrap div in addition to removing CSS. That's unfortunate.

I tested this workaround in jupyterlab, classic notebook and nbviewer. It works as expected. I haven't tested other front-ends (Colab, vscode, etc.).

[shoyer]

I sent a support request to GitHub asking for supporting the hidden attribute. We'll see if we get a response!

[shoyer]

Hopefully GitHub will fix their rendering issue, but this is a step in the right direction!

[dcherian]

Thanks @benbovy

[dcherian]

just confirmed that the fallback works perfectly. Thanks @benbovy !